Inducing Dt in Plants Review

8102019 Inducing Dt in Plants Review

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Research review paper

Inducing drought tolerance in plants Recent advances

M Ashraf

Department of Botany University of Agriculture Faisalabad Pakistan

a b s t r a c ta r t i c l e i n f o

Article history

Received 21 September 2009

Received in revised form 6 November 2009

Accepted 9 November 2009

Available online 13 November 2009

Keywords

Drought tolerance

Conventional breeding

Marker assisted selection

Quantitative trait loci

Transgenic plants

Osmolytes

Hormones

Recombinant inbred lines

Undoubtedly drought is one of the prime abiotic stresses in the world Crop yield losses due to drought

stress are considerable Although a variety of approaches have been used to alleviate the problem of drought

plant breeding either conventional breeding or genetic engineering seems to be an ef 1047297cient and economic

means of tailoring crops to enable them to grow successfully in drought-prone environments During the lastcentury although plant breeders have made ample progress through conventional breeding in developing

drought tolerant linescultivars of some selected crops the approach is in fact highly time-consuming and

labor- and cost-intensive Alternatively marker-assisted breeding (MAB) is a more ef 1047297cient approach which

identi1047297es the usefulness of thousands of genomic regions of a crop under stress conditions which was in

reality previously not possible Quantitative trait loci (QTL) for drought tolerance have been identi 1047297ed for a

variety of traits in different crops With the development of comprehensive molecular linkage maps marker-

assisted selection procedures have led to pyramiding desirable traits to achieve improvements in crop

drought tolerance However the accuracy and preciseness in QTL identi1047297cation are problematic

Furthermore signi1047297cant genetictimesenvironment interaction large number of genes encoding yield and use

of wrong mapping populations have all harmed programs involved in mapping of QTL for high growth and

yield under water limited conditions Under such circumstances a transgenic approach to the problem

seems more convincing and practicable and it is being pursued vigorously to improve qualitative and

quantitative traits including tolerance to biotic and abiotic stresses in different crops Rapid advance in

knowledge on genomics and proteomics will certainly be bene1047297cial to 1047297ne-tune the molecular breeding and

transformation approaches so as to achieve a signi1047297cant progress in crop improvement in future Knowledge

of gene regulation and signal transduction to generate drought tolerant crop cultivarslines has beendiscussed in the present review In addition the advantages and disadvantages as well as future prospects of

each breeding approach have also been discussed

copy 2009 Elsevier Inc All rights reserved

Contents

1 Introduction 169

2 Conventional breeding for drought tolerance 170

3 Marker-assisted breeding (MAB) for drought tolerance 171

31 Identi1047297cation of QTL associated with drought tolerance 171

32 Manipulation of QTL for developing drought tolerant crop cultivarslines 173

4 Engineering crops for enhanced drought tolerancemdashTransgenic approach 176

5 Conclusions and future challenges 180References 181

1 Introduction

Plant breeding conventional breeding or genetic engineering is an

art through which crop varietiesof high quality and yield are developed

Breeding forany desired trait undoubtedly requires a signi1047297cant amount

of genetic variation at intra-speci1047297c inter-speci1047297c or inter-generic levels

Variability however can be achieved by new gene combinations

intercrossing those genotypes that hold desirable characteristics and

introducing new germplasm from other existing breeding programs

(Ashraf 1994 Flowers 2004 Ashraf and Akram 2009) The variability

achieved through these means canbe narrowed down by selectinga few

Biotechnology Advances 28 (2010) 169ndash183

E-mail address ashrafbotyahoocom

0734-9750$ ndash see front matter copy 2009 Elsevier Inc All rights reserved

doi101016jbiotechadv200911005

Contents lists available at ScienceDirect

Biotechnology Advances

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genotypes that excel in the target environment (Baumlnziger et al 2004)

Considerable improvement in a trait canbe made if thegenetic variance

among the genotypes of a crop selection intensity and heritability are

reasonably high (Falconer 1989)

Plant breeding has contributed to a large extent in tackling the chal-

lenges of food security at global level The contributions of plant breeding

to food production at global level have been enormous during the 20th

century There hasbeen most important plant breeding break-throughfor

almost all commercially important crops including major ones such asmaize wheat rice cotton etc The Green Revolution which started in the

1940sandmainlybasedon traditionalbreedingresultedin a phenomenal

increase in wheat and rice yield in many partsof the world and especially

in SouthAsia(Rajaram 2005) Dr Norman Borlaug (Founder of theGreen

Revolution) and his team spent almost two decades breeding high

yielding dwarf wheat that was able to resist plant pests and diseases The

dwarf wheat out-yielded thetraditional varietiesabout twoto threetimes

However relatively little breeding work has been carried out on

improving crops for drought tolerance The achievements made so far in

improving drought tolerance of different crops throughthe integration of

conventional breeding marker-assisted breeding (MAB) and genetic

engineering (transgenic approach) have been discussed in the present

review MAB and transgenic approach are diverse biotechnologies be-

cause through the earlier desirable genes can be tagged so they can be

easily selected within the breeding population whereas through the

latter desirable genes can be transferred from one species to another A

large number of genomic regions of a crop germplasm can be examined

fortheirbreeding value throughMABwhichfacilitates thebreeder to pool

genes of diverse origins (Vinh and Paterson 2005 Humphreys and

Humphreys 2005) In fact this was not possible before through classical

breeding In contrast through the transgenic approach speci1047297c cloned

genes can be incorporated into an organism by limiting the transfer of

undesirable genes from the donor organism Furthermore pyramiding of

genes with similar effects is possible through this approach (Ashraf et al

2008 Gosal et al 2009) However both MAB and transgenic approaches

are deemed ef 1047297cient and precise ways of improving a desired trait They

arebeing used widelythese daysto generate stresstolerant cultivarslines

of different crops Recent progress made in exploiting the knowledge of

gene regulation and the phenomena involved therein in developingdrought tolerant crop cultivarslines has also been discussed in the

present review

2 Conventional breeding for drought tolerance

Through conventional breeding genetic variability for drought

tolerance among cropscrop cultivars or among sexually compatible

plant species can be identi1047297ed and the genetic variation so identi1047297ed can

be introduced through different mating designs into cultivarslines with

good agronomic characteristics (Pocket) During the last century

conventional breeders at different renowned international research

centers have made considerable strides in developing drought tolerant

linescultivars of some important food crops For example breeding

approach started at the International Maize and Wheat ImprovementCenter (CIMMYT) Mexico in the 1970s for developing drought tolerant

maize is worth mentioning Based on theselection andbreeding program

maize yield improvement of 59 to 233 kg haminus1 cycleminus1 of recurrent full-

sib or S1 selection was recorded (Baumlnziger et al 2004) The CIMMYT

breeding programwas very outcome-oriented and multi-faceted because

it focused on a multitude of imperative problems including drought

low N and common leaf and ear diseases (Baumlnziger et al 2004) In 1997

CIMMYT spanned its breeding program to southern Africa aimed at

improving maize for the drought-hit areas A number of maize hybrids

developed by the CIMMYT scientists were found superior to all those

developed by private enterprisesin terms of growthand grain yield under

drought-proneenvironments (Baumlnziger et al2004) Plant breedersat the

Crops Research Institute (CRI) based at Kumasi Ghana have developed a

highly drought tolerant maize cultivar lsquoObatanpa GHrsquo

in 2006 in

collaboration with the International Institute of Tropical Agriculture

(IITA) Ibadan the CIMMYT Mexico and the Sasakawa Global 2000

(Published online April 25 2006) Similarly 16 early maturing maize

inbred lines (fromTZEI 1 to TZEI 16) resistant to a scrounging weed Striga

hermonthica (Del) Benth were produced by the IITA All these lines were

found to be highly resistant to water limited conditions (Published online

April 25 2006)

Wheat which is one of the important staple food crops of the world

is adversely affected by drought In view of a projection by Rajaram(2001) more than 50 of the 237 million ha area in the world under

wheat cultivation is affected by periodic drought As stated earlier im-

provementin drought toleranceof a cropthroughselectionand breeding

requires a substantial magnitude of heritablevariation If variationin the

existing germplasm of a crop is low then wild relatives may serve as a

rich sourceof appropriate genetic variation At CIMMYT a newsynthetic

hexaploid has been developed by crossing the diploid wild ancestor Aegilops tauschii (goat grass) with tetraploid durum wheat (Triticum

turgidum var durum) These hexaploid synthetics containinga complete

D-genome from A tauschii have provided a signi1047297cant new variation for

tolerance to both biotic and abiotic stresses (Villareal et al 1994

Valkoun 2001) At CIMMYT more than 1000 accessions of A tauschii

have been evaluated and new hexaploid lines developed A signi1047297cant

new genetic variation in these newly developed hexaploid wheat has

been observed for abiotic stresses including drought stress (Valkoun

2001) Useful variation for drought tolerance has also been identi1047297ed in

Triticum urartu T boeticum T dicoccoides (Valkoun 2001) and Aegilops

geniculata (Zaharieva et al 2001) However in view of Skovmand et al

(2001) A taushii is the predominant source of variation for drought

tolerance

The International Center for Agricultural Research in Dry Areas

(ICARDA) Aleppo Syria and the International Crops Research Institute

for the Semi-Arid Tropics (ICRISAT) Andhra Pradesh India have a

similar research mandate ie to improve the major staple food crops of

the dry regions particularly falling in Asia the Middle East and Africa

Plant breeders at these two sister institutes have focused mainly on the

premier dryland cereals such as barley millet sorghum and groundnut

and leguminous pulse crops such as lentil chickpea pigeonpea and faba

beans (Worlds Dryland Farmers New Agricultural Technology-GreenRevolution Never Reached Themmht) Although efforts are underwayat

both centers to develop drought tolerant varieties of different crops

mentioned earlier the plant breeders at ICARDA have recently de-

veloped a new variety of barley which they claim as the worlds most

drought tolerant barley variety (ICARDA News) In fact the empirical

breeding strategies were employed to develop the variety by crossing a

land race with a wild barleyline from Palestine It hasbeen reported that

the new drought-hardy barley variety produced 50 more grain yield

than that produced by ordinary barley linescultivars under dryland

conditions

The International Rice Research Institute (IRRI) based at Las Bantildeos

Philippines though is focusing primarily on quality and yield im-

provement in rice efforts are also currently underway to develop

drought tolerant rice keeping in view of the fact that over 50 of theworlds rice is cultivated in rain-fed areas where the crop experiences

intermittent drought (MacLean et al 2002)

A number of drought resistant cultivarslines of different crops

registered so far in Crop Science or reported in other sources have been

listed in Table 1 These cultivars undoubtedly have been developed

solely using different protocolsdesigns of the conventional breeding

approach These drought tolerant lines of different crops provide a

sound testament that conventional plant breeding played a consider-

able role during the last century not only for improving the quality and

yield of crops but also for improving abiotic stress tolerance including

drought tolerance However now there is a general consensus of the

plant breeders that empirical plant breeding is a highlytime-consuming

as well as a cost- and labor-intensive approach While transferring

desired genes from one plant to other through the conventional plant

170 M Ashraf Biotechnology Advances 28 (2010) 169ndash183



breeding a number of undesired genes are also transferred Further-

more to achieve a desired gain through traditional breeding a number

of selection and breeding cycles may be required However improve-

ment in a trait through conventional breeding is not possible if the

appropriate genetic variationin thegene poolof a crop is either very low

or absent The limited success in improving crop drought tolerance

could be due to the reason that the drought tolerance trait is controlled

by multiple genes having additive effect and a strong interaction exists

between the genes for drought tolerance and those involved in yieldpotential Thus there is a need to seek more ef 1047297cient approaches for

genetically tailoring crops for enhanced drought tolerance

3 Marker-assisted breeding (MAB) for drought tolerance

Through marker-assisted breeding (MAB) it is now possible to

examine the usefulness of thousands of genomic regions of a crop

germplasm under water limited regimes which was in fact

previously not possible By examining the breeding value of each of

the genomic regions the breeder can coalesce genes of multifarious

origins in novel ways which was not possible previously with

conventional breeding tools and protocols (Concept Note)

31 Identi 1047297cation of QTL associated with drought tolerance

Like tolerance to other abiotic stresses that to drought stress is

controlled by many minor genes (polygenes) that have additive

effects in their expression (Zhao 2002 Mohammadi et al 2005 Thi

Lang and Chi Buu 2008) Thus the loci on chromosomes housing such

types of genes are now referred to as quantitative trait loci (QTL)

Natural genetic variation of a crop can be exploited either via direct

selection under stressful conditions whether simulated or natural or

via mapping of QTL (polygenes) and subsequent marker-assisted

selection (Ashraf et al 2008) QTL mapping allows to assess the

locations numbers magnitude of phenotypic effects and pattern of

gene action (Vinh and Paterson 2005) The role of polygenes in

controlling a trait has been widely assessed by traditional means but

the use of DNA markers and QTL mapping has made it convenient todissect the complex traits (Humphreys and Humphreys 2005) For a

QTLanalysis phenotypic evaluation is carried out of a large numberof

plants from a population segregating for a variety of genetic markers

then a part or the whole population is genotyped and 1047297nally

appropriate statistical analysis is performed to pinpoint the loci

controlling a trait (Asins 2002) Due to the intricacy of abiotic stress

tolerance and the problems encountered in phenotypic based

selection the QTL mapping has been considered as imperative to

the use of DNA markers for improving stress tolerance (Ashraf et al

2008) Ashraf et al (2008) have listed a variety of DNA markers such

as RFLPs RAPDs CAPS PCRindels AFLPs microsatellites (SSRs) SNPs

and DNA sequences being currently in use to examine the inheritance

of stress tolerance QTL mapping for the drought tolerance trait has

been done in different crops the most notable being maize wheatbarley cotton sorghum and rice (Quarrie et al 1994 Teulat et al

1997 Sari-Gorla et al 1999 Saranga et al 2001 Sanchez et al 2002

Bernier et al 2008)

In cotton using F3 families derived from the cross Gossypium

barbadense cv F-177 and Gossypium hirsutum cv Sivon Saranga et al

(2001) identi1047297ed a subset of 33 QTL under water limited regime ie 11

QTL for plant productivity 5 for some keyphysiological traits and 17 for

1047297ber quality Recently using marker-assisted selection near-isogenic

lines were produced through exchanging QTL for yield and some

drought-related traits between G barbadense cv F-177 and G hirsutum

cv Sivon (Levi et al 2009ab) For most of the traits studied the near-

isogenic lines showed a marked adaptation to drought but not for yield

In particular the G barbadense near-isogenic lines showed a steady

photosynthetic ef 1047297ciency under varying water limited regimes

In barley QTL involved in some key growth and water relation

attributes were detected using 187 recombinant inbred lines (RILs)

resulting froma cross between two Mediterranean cultivars Tadmor and

FrApm (Teulat et al 1997) QTL involved in variation in relative water

content (RWC) number of tillers (NL) and total shoot fresh mass were

found to be located on a RFLP-RAPD genetic map It was also found that

different DNA regions mediate in constitutive water stress responses

Under water limited regime although one region on chromosome 1 was

found to be mainly involved in variation for RWC and NL other maplocations were also found for RWC and NL It is imperative to note that

epistatic interactions among many QTL and between QTL and other

markers were observed only under drought conditions which suggest

that some chromosomal regions are de1047297nitely involved in controlling the

expression of the traits under water limited conditions

In sorghum (Sorghum bicolor ) several linkage maps have been

generated using RFLP and other known DNA markers (Xu et al 1994

Taramino et al 1997 Rami et al 1998 Kong et al 2000 Sanchez et al

2002)In viewof a report(Bowerset al 2003) over2400locihave been

mapped on an F2 population developed from an inter-speci1047297c cross S

bicolor timesS propinquum For 1047297ne mapping of genes and QTL the mean

markerdensityof 05 cM or 350 kb between DNAmarkersis appropriate

(Sanchez et al 2002) However using recombinant inbred lines (RILs)

and near-isogenic lines (NILs) Sanchez et al (2002) detected several

genomic regions linked to resistance to pre-1047298owering and post-1047298ower-

ing drought stress They found four distinct genomic regionsinvolved in

the stay-green trait using a RIL population derived from the cross

B35times Tx7000 Thesefour stay-green QTLexpressed repeatedly in all1047297eld

trials they conducted and explained 535 of the phenotypic variance

QTLstudiesfor thestay-greentrait proveduseful inthe identi1047297cationofa

number of genomic regions associated with drought resistance

In maizea linkage analysis between the manifestation of some key

characters like male and female 1047298owering time anthesis-silking

interval plant height and molecular markers [RFLP microsatellites

(SSR) andAFLP]was carried out under different water regimes using a

maize population consisting of 142 RILs derived from sel1047297ng the F1population from a cross B73timesH99 (Sari-Gorla et al 1999) Linkage

analysis showed that the QTL identi1047297ed for male 1047298owering time and

plant height were the same under well-watered and water-stressedconditions In contrast for female 1047298owering time and anthesis-silking

interval the expression of QTL was different under normal conditions

or under drought stress Feng-ling et al (2008) developed a maize

segregating population from the cross N87-1 (drought resis-

tant)times9526 (drought-sensitive) which was genotyped at 103 SSR

loci The resulting F24 families were tested under two water

treatments The authors identi1047297ed 12 QTL ie two for plant height

1047297ve for anthesis-silking interval four for root mass and one for grain

yield however most of them showed over-dominant gene action In

the same crop Guo et al (2008) characterized QTL for some key traits

such as 1047298ower time plant height yield and yield components using

recombinant inbredlinesdeveloped from thecross 5003times p138 under

both water de1047297cit and well-watered regimes They identi1047297ed 51 QTL

for 10 traits on 10 different chromosomes Under water de1047297citconditions 22 QTLwere found for 7 traits Phenotypic variation linked

to each QTL ranged from 168 to 133

In wheat the position of genes exhibiting a signi1047297cant effect on ABA

accumulation due to drought stress was identi1047297ed using a seriesof single

chromosome substitution lines and populations obtained from a cross

between a high-ABA-producing cv Ciano 67 and a low-ABA-producing

cv Chinese Spring (Quarrie et al 1994) They observed that chromo-

some 5A carries gene(s) for ABA accumulation MAPMAKER-QTL

showed that the ABA quantitative trait locus is located between the

two loci Xpsr575 and Xpsr426 approximately 8 cM from Xpsr426

In rice for example a number of drought-related QTL have been

identi1047297ed for different growth and physiological traits involved in

drought tolerance (La1047297tte et al 2004) For example Courtois et al

(2003) found 28 QTL responsible for various root characteristics

171M Ashraf Biotechnology Advances 28 (2010) 169ndash183



Table 1

Drought tolerant cultivarslines of different crops developed through conventional breeding at different centersinstitutions

Crop Cultivarline How developed Centersinstitutions involved Reference

Peanut ( Arachis hypogaea

L Fabaceae subsp

hypogaea var vulgaris )

ICGV 87354 Derived from a cross between

Argentine and PI259747 and

developed through nine

generations of bulk selection

Plant Materials Identi1047297cation

Committee of the International

Crops Research Institute for the

Semi-Arid Tropics (ICRISAT) India

Reddy et al (2001)

Common bean

(Phaseolus vulgaris L)

SEA 5 Dev eloped fro m the interraci al

double cross population BAT 477San

Cristobal 83Guanajuato 31Rio Tibagi

International Center for Tropical

Agriculture (CIAT) Cali Colombia

Singh et al (2001)

SEA 13 Derived from double cros s

population BAT 477San Cristobal

83BAT 93Jalo EEP 558



Singh et al (2001)

A 1 95 Developed f rom the single c ross Red

KloudtimesICA 10009

Centro Internacional de Agricultura

Tropical (CIAT) Palmira Colombia

Singh et al (2007)

Line CO46348 The complete pedigree of CO46348

was unknown however it was derived

from a single cross with the

pinto cultivar Othello

Colorado Agricultural Experiment

Station in cooperation with the

University of Idaho and USDA-ARS

Brick et al (2008)

Saf 1047298ower

(Carthamus tinctorius L

Morlin Derived from a single plant selection

from F11 population

Eastern Agricultural Research

Center and Montana Agricultural

Experiment Station Sidney

Bergman et al (2001)

Chickpea

(Cicer arietinum L)

FLIP 87-59C Developed by crossing ILC3843 with

FLIP87

International Center for Agricultural

Research in the Dry Areas (ICARDA)

Singh et al (1996)

Wheat

(Triticum aestivum L)

Willow Creek Through breeding in single replication

observation (SROB) nurseries

Montana Agricultural Experiment

Station Sydney

Cash et al (2009)

Ripper Deve lope d by using a modi1047297

ed bulkbreeding procedure Colorado Agricultural ExperimentStation USA Haley et al (2007)

NE01643 A bulk breeding procedure was used

and approximately 50 of F3population was visually selected on

the basis of agronomic appearance

Nebraska Agricultural Experiment

Station and the USDA-ARS

Baenziger et al (2008)

Prairie Red Derived from the crosses and

backcrosses of CO850034PI3721295

TAM 107


Station USA

Quick et al (2001)

Jinmai 50 Developed from the cross

Pingyang181timesQingfeng1

Wheat Breeding Innovation Group

(WBIG) in the Cotton Research

Institute of Shanxi Agri Sci Academy

Yuncheng China and

Testing and Appraising Committee

of Crop Cultivars of Shanxi

Province (TACCCSP)

Xinglai et al (2006)

Tall fescue

(Festuca arundinacea)

Nanryo First breeder seed was produced from

a breeders block composed of eight

clones

Kyushu Okinawa National Agricultural

Research Station (KONARC)

Kumamoto Japan and Japanese GrasslandFarming Forage Seed Association and

the USDA-ARS Grazinglands

Research Laboratory El Reno OK USA

Kindiger et al (2006)

Soybean

[Glycine max (L) Merr]

R01-416F and

R01-581F

Both lines were originated after

selection from F9 population

developed from a cross between

Jackson and KS4895

Arkansas Agricultural Experiment

Station USA

Chen et al (2007)

Wheatgrass

[Elymus trachycaulus

(Link) Gould ex Shinners]

FirstStrike The source plants of FirstStrike

originated from seeds of three

germplasm collections ie [(53ndash54

[NS] 15ndash16 [EW]) (71ndash72

[NS] 14ndash15 [EW]) and 10 mi

USDA-ARS Forage and Range Research

Laboratory at Utah State University

Logan UT in collaboration with

the US Army Engineer Research and

Development Center Hanover NH

Jenson et al (2007)

Barley

(Hordeum vulgare L)

Lenetah Developed using a pedigree selection

procedure with all early generation

population and selected from the

cross 94Ab12981times91Ab3148

Agricultural Research Service

Aberdeen ID in cooperation

with the Idaho Agricultural

Experimental Station

Obert et al (2008)

Giza 126 Selected for drought resistance inan F3 population received from

ICARDA initially originating from a

single cross Baladi BahteemSD729-

Por 12762BC

International Center for AgriculturalResearch in the Dry Areas (ICARDA)

Noaman et al (1995)

Giza 2000 The pedigree breeding method was

used for development and it was

originated from the cross between

the Egyptian local cultivar Giza 121

and the line 366131 (Giza 117

Bahteem 52Giza 118FAO 86)

Barley Research Department Agricultural

Research Center at Giza Egypt

Noaman et al (2007)

Giza 121 Line

366131

Plant selection within superior F4populations

Sakha Research Station Northern

Delta Region Egypt

Noaman et al (2007)

Giza 132 Derived from an F3 population The

pedigree method of breeding was

used and Giza 132 originated from

the cross Rihane-05As46Aths3Aths

Lignee 686


Research Center at Giza Egypt and

International Center for Agricultural Research

in the Dry Areas (ICARDA) Aleppo Syria

Noaman et al (2007)






resulted in pyramiding all 1047297ve segments It is imperative to note that

pyramiding of four root QTL was achieved after eight generations using

3000 marker assays in 323 lines The authors evaluated 22 near-isogenic

lines (NILs)for roottraitsin1047297ve different1047297eld trialsin Bangalore IndiaOf

the 1047297ve segments the target segment on chromosome 9 (RM242ndash

RM201) from cv Azucena markedly improved root length under both

well-irrigated and water limited conditions In a later study Steele et al

(2007) conducted a 1047297eld trial to test some key agronomic traits in near-

isogenic lines (NILs) derived from the previous study (Steele et al 2006)Four NILs were evaluated in1047297eld trials conducted in eastern and western

India for three years All four NILs excelled Kalinga III in terms of grainand

straw yield All these efforts using the marker-assisted breeding have

resulted in the release of a 1047297rst ever highly drought tolerant rice variety

Birsa Vikas Dhan 111 (PY 84) in the Indian state of Jharkhand (Steele

2009) Early maturity high drought tolerance and high grain yield with

good grain quality are the prominent characteristics of this novel variety

At IRRI efforts have also been made to improve drought tolerance of rice

using the marker- assisted breeding approach For example Bernier et al

(2007) while screening a population of 436 F3 lines derived from a cross

between two upland rice cultivars Vandana and Way Rarem selected

some linesTheselected lines were evaluated under water stressand non-

stress conditions in some 1047297eld trials conducted for two years to identify

QTL involved in drought resistance A QTL (qtl121) with a marked effect

on grain yield under drought stress was identi1047297ed on chromosome 12 in

both years which was derived from the susceptible parent Way Rarem

Under stress conditions the QTL (qtl121) also improved biomass

production harvest index and plant height while it showed reduced

number of days to 1047298owering However in contrast under well-watered

treatment the QTL did not affect any of the earlier mentioned characters

According to Bernier et al (2007) this is the 1047297rst QTL reported in rice

possessing a substantial and repeatable effect on grain yield production

under harsh1047297eld drought conditions To further con1047297rm the effectiveness

of this QTL in improving drought tolerance in rice Bernier et al (2009)

have recently conducted vast 1047297eld trials at different locations ie 10 at

IRRI and 11 in eastern India (Table 2) It is important to note that the

relative effect of the QTL on grain yield was augmented with the severity

of drought stress and had no effect under well-irrigated regimes This

con1047297rms that the QTLqtl121 has a pronounced andsteady effect on grainyield under upland water stress conditions in variable environments

Although pearl millet [Pennisetum glaucum (L) R Br] is known for its

high drought tolerance drought is a major constraint for its optimum

production in many areas of the world Thus breeding for water stress

tolerance in pearl millet is a major challenge for many research institutes

The breeding research at ICRISAT India has resulted in mapping several

QTL for stover and grain yield under terminal water de1047297cit conditions

(Serraj et al 2004) However a preliminary evaluation of a putative

drought resistance QTL on linkage group 2 (LG 2) of pearl millet was

carried out by evaluating hybrids generated through topcross pollinators

bred from progenies derived from the original mapping population

comprising thetolerant allele at thetarget QTL Thirtysixtopcross hybrids

were appraised in 21 different 1047297eld environments wherein they were

subjected to control and drought-stressed regimes during the reproduc-tive stages including 1047298owering and grain 1047297lling stages (Table 2) The

hybrids containing theQTLout-yieldedunder water stressregimes but at

the cost of reduced yield under well-watered conditions Subsequent

evaluations as reported by Serraj et al (2005) were based on testcross

hybrids of drought tolerance QTL introgression lines in the gene pool of

the drought-sensitive parent of the mapping population H 77833-2

These introgression lines were in fact developed by marker-assisted

backcrossing of a putative vital drought tolerance QTL into H 77833-2

from the mapping populations droughttolerant parentWhile evaluating

all QTL introgression lines under different moisture regimes it was found

that many of thelinesexcelledthe test crosshybrids interms of grain yield

under terminal drought stress

In anotherstudywith pearl millet line 863B hasbeen found to have a

superior general combining ability for the grain 1047297lling trait under

terminal drought conditions (Yadav et al 2004) A mapping population

developed from a cross 863BtimesICMB 841 was evaluated under early-

andlate-drought stressconditions whichled to theidenti1047297cationoftwo

genomic regions in line 863B associated with improved panicle harvest

index as well as with high drought tolerance (Yadav et al 2004) The

genomic regions Qgydticp-21 on LG 2 and Qgydticp-61 on LG 6

represent 236 and 144 of the total variation for panicle harvest

index respectively Subsequent appraisal of the mapping population

test crosses in different environments exhibited Qgydticp-21 as themajor QTL for marker-assisted selection program for drought tolerance

(Bidinger et al 2007)

In cotton QTL for yield and different drought-related secondary traits

such as carbon isotope ratio (δ13C) turgid solute potential and leaf

chlorophyll content were exchanged between the potential cultivars of

the two cotton species G barbadense (GB) cv F-177 and G hirsutum (GH)

cv Sivon through marker-assisted selection (Levi et al 2009ab) Several

of the resulting NILs out-performed in terms of the physiological traits for

which they were introgressedIn a subsequent study (Levi et al 2009ab)

photosynthetic ef 1047297ciency of two selected NILs and their recipient parents

were evaluated under water limited and well-watered 1047297eld conditions

The GBNIL showed a stable rate of net CO2assimilationrate undervarying

leaf water potentials with a signi1047297cant superiority over its recipient

parent F-177The highnet photosyntheticratein this NIL was foundto be

associated with lower stomatal limitation higher activity of Rubisco and

higher rate of electron transport In contrast the other NIL (GH NIL)

exhibited higher mesophyll conductance under water limited conditions

than its recipient parent Sivon but these genotypes had almost similar

values of net photosynthetic rate However both types of NILs did not

perform well in terms of yield relative to the recipient parents under

drought stress conditions

A marker-assisted backcross (MABC) selection program meant for

improving grain yield under water limited conditions in tropical

maize was conducted at CIMMYT Mexico (Ribaut and Ragot 2006)

which involved the crossing of drought resistant line Ac7643 with a

drought susceptible line CML247 Marker-based selection was carried

out stepwise on all four generations (from BC1F1 to BC2F3) After the

four consecutive MABC cycles the 70 BC2F3 individuals exhibiting the

closest allelic composition at target and non-target loci were bredwith two CIMMYT testers (CML254 and CML274) Thirty genotypes

were selected on the basis of their performance in terms of grain yield

and some key agronomic traits However the best 1047297ve MABC-derived

hybrids produced yield about 50 more than that of control hybrids

but in contrast under mild water stress there was no difference

between MABC-derived hybrids and the control plants This con1047297rms

that the expression of genetic variation for drought tolerance mainly

depends on the severity of drought stress

In barley an attempt has been made to improve yield under

dryland conditions using wild barley (Hordeum spontaneum) as a

potential source of alleles for drought tolerance (Baum et al 2003) A

population developed by backcrossing cultivated barley (Hordeum

vulgare) with H spontaneum was evaluated in three Mediterranean

countries under rain-fed conditions to detect wild barley allelesinvolved in producing high yield under drought conditions Six QTL

from the wild barley were found to be responsible for enhanced yield

under water limited conditions These results exhibit that identi1047297ca-

tion of new alleles from wild relatives is a useful means of improving

drought tolerance in different potential crops

Marker-assisted selection was also employed to improve the stay-

green trait involved in the drought tolerance of sorghum (Harris et al

2007) Four major QTL (Stg1 to Stg4) contributing to the stay-green trait

were mapped using a population derived from BTtimes642 and RTtimes7000

The genotype BTtimes 642 is a potential source of stay-green trait

Physiological evaluation of four RTtimes7000 NILs comprising Stg1 Stg2Stg3 or Stg4 showed that BTtimes642 alleles in each of these loci could

substantially contribute to the stay-green trait However RTtimes7000 NILs

having BTtimes 642 DNA relating to Stg2 showed higher stay-green






characteristic at maturity than that in RTtimes7000 or the other RTtimes7000

NILs under terminal drought stress These NILs also had markedly lower

rates of leaf senescence with respect to that in RTtimes7000 These results

clearly show that improvement in drought tolerance of sorghum is

possible through map-based cloning of the genes responsible for the vital

secondary traits such as stay-green or delayed leaf senescence

All the above-mentioned reports clearly show that considerable

improvement in plant drought tolerance is possible through marker-

assisted selection Marker-assisted selection undoubtedly allows topyramid genes at two or more loci to improve drought tolerance The

identi1047297cation of QTL for yield or secondary traits plays a key role in

improving drought tolerance in different crops through MAS In fact

when a marker-trait association has been found clearly MAS can

minimize to a great extent the dependence on particular environmental

conditions during the selection procedure one of the main barriers

encountered in the traditional breeding of traits affected by drought

stress (Tuberosa and Salvi 2006) Although the achievements made so

far through MAS in improving drought tolerance seem to be simple and

straightforward the main problem being faced by the breeders em-

ploying MAS is the challenge of accuracy and preciseness in QTL iden-

ti1047297cation as well as the application of the knowledge resulting to a

successful MAS program Considerable genetictimesenvironment interac-

tion the large number of genes controlling yield and erroneous use of

mapping populations resultingfrom parents thathavenarrow difference

in drought tolerance has adversely affected the programs entailing

mapping of QTL for high yield under drought stress Restrictions of

molecular markers encountered previously have been exceeded with

the advent of gene-based numerous SNP markers Using SNP and other

markers high density genetic maps can be constructed for the detection

and characterization of QTLgenes responsible for drought tolerance In

fact with theaccessibility of genomesequenceinformation of each crop

integrated genetic and physical maps and SNPmarkers for speci1047297c traits

will lead to a substantial role in molecular breeding for abiotic stress

toleranceincludingdrought tolerance In 2005 a 1047297rstattempt wasmade

to clone QTL (Salvi and Tuberosa 2005) which is indeed an important

milestonein molecularbreeding having a substantial roleto understand

and manipulate the traits responsible for drought tolerance (Tuberosa

and Salvi 2006 Tondelli et al 2006 Cattivelli et al 2008)

4 Engineering crops for enhanced drought tolerancemdash Transgenic

approach

Thegreat challenge of food securitybeing faced thesedays theworld-

over has directed plant scientists towards gene revolution after green

revolutiondue to advancesin biotechnologyThe generevolution in fact

involves modi1047297cation of qualitative and quantitative traits in an

organism by transferring desired genes from one species to another

This strategy is referred to as the transgenic approach In contrast to

classical breeding the transgenic approach allows the incorporation of

only thespeci1047297c cloned genesinto an organism and restricts thetransfer

of undesirable genes from donor organism Through this approach

pyramiding of genes with similar effects can also be achieved Rapidadvance in recombinant-DNA technology and development of precise

and ef 1047297cient gene-transfer protocols have resulted in ef 1047297cient transfor-

mation and generation of transgenic lines in a number of crop species

(Gosal et al 2009)

Transgenic approach is being pursued actively throughout the world

to improve traits including tolerance to biotic and abiotic stresses in a

numberof crops(Ashrafet al2008) Aswithsaltstress plant responsesto

droughtstress are complex because it involves many genes with additive

effects so theprospects of improving drought tolerance in crops seem not

to be very bright Despite this efforts have been made during the last few

decades to generate transgenic lines of different crops which have shown

improved tolerance to drought stress Some of the transgenic lines so

produced fortheover-expressionof speci1047297c traits are listed in Table 3 The

major emphasis of bioengineers has been on engineering genes that

encode compatible organic osmolytes plant growth regulators antiox-

idants heat-shock and late embryogenesis abundant proteins and

transcription factors involved in gene expression

It is now well established that compatible organic solutes play a

central role in plant drought tolerance (Ashraf and Foolad 2007)

However overproduction of compatible organic osmotica is one of the

prominent responses of plants exposed to osmotic stress (Serraj and

Sinclair 2002 Ashraf et al 2008) and the genes encoding the synthesis

of such organic solutes can be engineered to overproduce these solutesin transgenic plants For example among the many organic osmolytes

known to play a substantial role in stresstolerance glycine betaine (GB)

a quaternary ammonium compound occurs richly in response to

dehydration stress (Mansour 2000 Mohanty et al 2002 Yang et al

2003 Ashraf and Foolad 2007) However for the biosynthesis of GB in

higher plants choline monooxygenase (CMO) and betaine aldehyde

dehydrogenase (BADH) are two key enzymes In some independent

studies with different crops genes encoding these two enzymes have

been engineered (Table 3) For example transgenic tobacco lines over-

expressing CMO have been produced (Shen et al 2002 Zhang et al

2008) These transgenic lines showed higher accumulation of glycine

betaine under water limited conditions and hence enhanced drought

tolerance Similarly a potential maize inbred line DH4866 was

transformed with the E coli betA gene encoding choline dehydrogenase

(Quan et al 2004) The transformed maize plants contained higher

levels of glycine betaine and showed higher tolerance to drought as

compared to wild-type plants when tested at the initial growth stages

Like GB proline is also an important compatible organic osmolyte

that plays a key role in stress tolerance Pyrroline-5-carboxylate

synthetase (P5CR) is the key enzyme for proline biosynthesis The

gene for this enzyme has been engineered in soybean (Ronde et al

2004) petunia (Yamadaet al 2005)and tobacco(Gubis et al 2007)All

these transgenic lines showed enhanced accumulationof proline as well

as high drought tolerance (Table 3)

Trehalose a nonreducing sugar is also a potential organic osmoticum

which has a substantial role in the protection of plants against stresses

However transgenic linesof differentcropshave been generatedusing the

genes of some key enzymes involved in trehalose biosynthesis For

example enhanced drought tolerance has been achieved by transformingthe gene TPS1 for trehalose-6-phosphate synthase in tobacco (Romero

et al 1997 Karim et al 2007) Enhanced drought tolerance has also been

observed in transformed rice plants expressing chimeric gene Ubi1TPSP

due to increased accumulation of trehalose ( Jang et al 2003) In these

studies and some other reported in theliterature engineering constitutive

over-expression of genes encoding TPS andor TPP (trehalose-6-phos-

phate phosphatase) resulted in enhanced trehalose accumulation as well

as drought tolerance However the main problem with such transforma-

tion had been that it led to abnormal plant development under normal

growth conditions because the gene transformed remained turned on all

the time To resolve this problem Wu and Garg (2003) alternatively

adopted another way to engineer enhanced trehalose accumulation in

such a manner that trehalose biosynthesistook place only when the plant

encountered abiotic stress Theyemployed a stress-inducible promoter forthe over-expression of E coli trehalose biosynthesis genes (otsA and otsB)

as fusion gene (TPSP trehalose-6-phosphate synthase phosphatase) for

developing abiotic stress tolerance in rice It is pertinent to note here that

the TPSP fusion gene transformation resulted in normal growth under

non-stress conditions but the expression of the fusion gene occurred only

under stress conditions In another study a TPS1ndashTPS2 fusion gene

construct was incorporated into Arabidopsis thaliana through Agrobacter-

ium using either the 35S or the stress regulated rd29A promoter (Miranda

et al 2007) The lines over-expressing the TPS1ndashTPS2 construct showed

normal growth as well as enhanced tolerance to multiple stresses such as

salinity drought freezing and high temperature However in contrast

the plants over-expressing TPS1 alone under the operation of 35S

promoter exhibited aberrant growth and form From all these reports it

is obvious that a substantial improvement in drought tolerance of plants




can be achieved through engineering the genes involved in trehalose

metabolism without the occurrence of any cost in the form of abnormal

growth and development on growing the transgenic lines under normal

well-irrigated conditions

Mannitol a polyol is one of the most important osmoprotectants that

play a vital role in plant stress tolerance However attempts have been

made to achieve improved drought tolerance by the over-expression of

mannitol in plants by engineering genes involved in the biosynthesis of

mannitol For example ecotypic expressionof themt1D gene (involved inthe biosynthesis of mannitol) in wheat plants increased tolerance to both

drought and salt stresses (Abebe et al 2003) In contrast tobacco plants

transformed with a mannitol-1-phosphate dehydrogenase gene resulted

in enhanced mannitol accumulation but enhanced mannitol accumula-

tion did not affect osmotic adjustment or drought tolerance in the

transformed plants as compared to those in the untransformed plants

(Karakas et al 1997) In view of these contrasting reports there is a need

to ascertainwhether or notmannitolover-expressionis relatedto drought

tolerance in different species In case there is a lack of relationship of

drought tolerance with mannitol accumulation the efforts to engineer

crops for enhanced overproduction of mannitol and hence enhanced

drought tolerance would be futile

Like other stresses drought stress leads to increased accumulation of

reactive oxygen species (ROS) in plants thus causing an oxidative stress

To counteract these ROS plants canintrinsically developdifferenttypes of

antioxidants Overproduction of antioxidants in response to drought-

induced oxidative stresshasbeen found to be associatedwith thedrought

stresstolerance of different plant species (Pastoriand Foyer2002 Sunkar

et al 2006) Furthermore genes encoding different types of antioxidants

have been engineered in different plants for achieving enhanced drought

tolerance For example engineering of the gene SOD encoding superoxide

dismutase caused enhanced drought tolerance in alfalfa (McKersie et al

1996 1997) potato (Perl et al 1993) and rice (Wang et al 2005)

Likewise the gene for another potential antioxidant enzyme ascorbate

peroxidase from Arabidopsis was over-expressed in tobacco chloroplasts

(Badawi et al 2004) The transgenic lines so produced exhibited

enhanced tolerance to osmotic stress created by polyethylene glycol

(PEG) Transgenic lines of tobacco produced by over-expressing mono-

dehydroascorbate reductase (MDAR) gene from Arabidopsis showed a21-fold higher MDAR activity and 22-fold higher level of reduced

ascorbic acid than that in non-transformed plants (Eltayeb et al 2007)

Transgenic plantsshowedenhanced resistance to ozonesalt and PEG Liu

et al (2008) generated transgenic tobacco plants over-expressing VTE1

gene encoding tocopherol cyclase (VTE1) a key enzyme of tocopherol

biosynthesis The transformed plants exhibited enhanced drought

tolerance which was associated with decreased electrolyte leakage lipid

peroxidation and H2O2 content but increased chlorophyll content

compared with the non-transformed plants

Helicases which catalyze the unwinding of DNARNA double helical

structures are distributed in yeast animals and plants (Luo et al 2009)

These helicases comprising nine speci1047297c motifs are also referred to as

DEAD-box helicases depending on their highly conserved amino acid

sequence (Asp(D)ndashGlu(E)ndashAla(A)ndashAsp(D) in motif II (Gorbalenya andKoonin 1993 Tanner et al 2003 Luo et al 2009) Some earlier studies

have shownthe putative role of helicases in plant abiotic stress tolerance

(Owttrim 2006 Vashisht and Tuteja 2006) Recently Luo et al (2009)

have isolated a DEAD-box-containing cDNA sequence from alfalfa

(Medicago sativa) and it was designated as M sativa helicase 1 (MH1)

The ectopic expression of MH1 in Arabidopsis led to improved seed

germination and plant growth under drought salinity and oxidative

stress which was found to be associated with enhanced capacity of

osmotic adjustment activities ascorbate peroxidase and superoxide

dismutase and proline content in the transgenic Arabidopsis plants This

study clearly shows the signi1047297cant role of the over-expression of the

helicase MH1 in droughttoleranceby enhancing thecapacity of plants to

counteract thereactive oxygen species (ROS) as well as adjust themselves

osmotically

Late embryogenesis abundant (LEA) proteins may accumulate in

responseto droughtstressin plantsand playa vital role in plant protection

against the adverse effects caused by drought stress (Hong et al 2005

Gosal et al 2009) The putative role of LEA proteins in plant drought

tolerance has been suggested to be due to their involvement in the

maintenance of cell membrane structure and ion balance binding of

water and their action as molecular chaperones (Close 1997 Browne

etal2002 Babu etal2004)Howevereffortshavebeen made duringthe

last two decades to engineer LEA genes for enhanced plant droughttolerance For example engineering the LEA genes PMA1959 and PMA80

(encoding a group 1 LEA protein and a group 2 LEA protein respectively

both from wheat) in rice resulted in enhanced drought tolerance (Cheng

et al 2002) Similarly a LEA gene HVA1 (which encodes a group 3 LEA

protein) from barley was engineered in rice (Xu et al 1996) and wheat

(Sivamani et al 2000)Bothriceand wheat transformed lines soproduced

showed enhanced tolerance to drought stress In two independent

studies a LEA protein gene ME-lea N4 from Brassica napus was transferred

through Agrobacterium to lettuce (Lactuca sativa L Park et al 2005a) and

Chinese cabbage (Brassica campestris Pekinensis Park et al 2005b) using

the CaMV 35S promoter The transgenic lines of both crops showed

enhanced tolerance to both salinity and drought Recently Dalal et al

(2009) have assessed the role of a group 4 LEA protein LEA4-1 from B

napus in stress tolerance Expression analysis showed that expression of

LEA4-1 gene in leaf tissues in Brassica species was induced by multiple

stresses including ABA salinity low temperature and drought However

over-expression of BnLEA4-1 in Arabidopsis driven by the constitutive

CaMV 35S or stress-inducible RD29A promoter resulted in improved

tolerance of transgenic Arabidopsis to salinity and drought stresses

Another LEA protein gene OsLEA 3-1 has been incorporated into rice via

Agrobacterium under the operation of different promoters (Xiao et al

2007) The rice transgenics developed particularly under the control of

constitutive CaMV 35S and stress-inducible HVA1 promoters showed

enhanced drought tolerance when tested under natural 1047297eld conditions

(Xiao et al 2007) In view of all the above reports the prospective role of

LEA genes in protecting the plants from drought stress seems plausible

Abscisic acid (ABA) a well known growth inhibitor modulates a

number of key growth and physiological processes in plants including

suppression in seed germination maintenance of seed dormancy byinhibiting cell growth induction of stomatal closure thereby minimizing

transpiration to prevent water loss and acceleration of abscission and

senescence (Finkelstein et al 2002 Fujita et al 2005) It is now evident

that ABA production is enhanced under water limited conditions and it

can effectively protect plants against drought stress (Shinozaki and

Yamaguchi-Shinozaki 2000 Finkelstein et al 2002 Xiong et al 2002

Fujitaet al 2005) Inviewof someearlier reports it isapparentthat many

of the drought stress-inducible genes detected in plants are activated by

ABA (Ingram and Bartels 1996 Seki et al 2002) A detailed examination

of the promoters of ABA-regulated genes has revealed a highly conserved

cis-acting ABA-responsive element (ABRE) (Giraudat et al 1994 Busk

andPages1998) Fujitaet al(2005)clonedthreediverse cDNAsencoding

ABRE binding proteins (AREB1 AREB2 and AREB3) in Arabidopsis They

also reported that the expression of AREB1 and AREB2 was up-regulatedby ABA drought and salinity They also showed that of the nine AREB

homologs reported in Arabidopsis expression of the three members

AREB1ABF2 AREB2ABF4 and ABF3DPBF5 was stimulated by drought

ABA and high salinity in vegetative tissues Furthermore they showed

that the expression of only AREB1 gene was not suf 1047297cient to direct the

expression of downstream genes under non-stress conditions However

they overcame this problem by creating an activated form of AREB1

(AREB1DQT) It was interesting to note that AREB1DQT-over-expressing

Arabidopsis plants showed ABA hypersensitivity and improved drought

tolerance However AREB1 was found to be the most effective positive

regulator of ABAsignalingin Arabidopsis under water de1047297citconditions In

other studies over-expression of speci1047297c transcription factors such as the

dehydration-responsive element-binding protein 1A (DREB 1A) ABF3

and ABF4 caused enhanced drought tolerance genes in Arabidopsis plants








which was found to be related to partial stomatal closure and decreased

transpiration (Kasuga et al 1999 Joung-youn et al 2002) Luchi et al

(2001) have shown that the increase in one of the limiting reactions of

ABA biosynthesis with the precise gene paralog encoding At-NCED3 also

induced enhanced drought tolerance in Arabidopsis In another study

Jeanneau et al(2002)examined the role of an ABA anddroughtregulated

maize gene ASR1 (Vienne et al 1999) coding for an ABA stress ripening

protein and the effect of photosynthesis regulation through the ectopic

expression of the S bicolor C4-phosphoenolpyruvate carboxylase geneC4-PEPC in transgenic maize The transgenic maize lines so produced

showed enhanced photosynthetic capacity and water use ef 1047297ciency as

well as high biomass production under mild water de1047297cit conditions

Recently Ding et al (2009) have shown that transgenic expression of

MYB15 encoding a transcriptionfactor R2R3 MYB in Arabidopsis showed

considerable sensitivity to exogenous ABA and enhanced tolerance to

both drought and salinity The transgenic lines over-expressing MYB15

showed over-expression of genes involved in ABA biosynthesis ( ABA1

ABA2) signaling ( ABI3) and responsive genes ( AtADH1 RD22 RD29B

AtEM6 ) after application of ABA suggestingthat improved stresstolerance

in the MYB15 transgenic lines is associated with enhanced expression of

the genes involved in ABAbiosyntheticand signaling pathways as well as

those encoding the proteins involved in stress protection

Transcription factors are speci1047297c types of proteins that bind DNA

and are involved in the regulation of gene transcription hence gene

regulation Since regulation of genes involved in stress tolerance is

important for improving this trait in plants strenuous efforts are

being made these days to identify and characterize transcription

factors (regulatory proteins) involved in stress-speci1047297c gene regula-

tion However several transcription factors have been identi1047297ed

which are involved in gene regulation in plants under water limited

conditions (Bartels and Sunkar 2005 Vinocur and Altman 2005)Ofa

number of transcription factors listed elsewhere (Gosal et al 2009)

dehydration-responsive element-binding factors (DREB) have

attracted the attention of many scientists since Jaglo-Ottosen et al

(1998) and Liu et al (1998) 1047297rst reported the up-regulation of many

genes in DREB1CBF transgenic Arabidopsis involved in tolerance to a

variety of stresses including drought salinity freezing etc Similarly

transgenic Arabidopsis plants over-expressing DREB1CBF3 operatedby the constitutive promoter CaMV 35S also exhibited improved

tolerance to salinity drought and freezing (Kasuga et al 1999)

Introduction of DREB1A into wheat driven by rd29A promoter resulted

in enhanced drought tolerance (Pellegrineschi et al 2004) Ecotypic

expression of Arabidopsis DREB1A (CBF3) into transformed rice plants

under the operation of constitutive promoter CaMV 35S resulted in

improved tolerance to drought and salinity (Oh et al 2005)

Dubouzet et al (2003) isolated four rice CBFDREB1A orthologs Os-

DREB1A OsDREB1B OsDREB1C and OsDREB1D However the trans-

genic rice plants over-expressing OsDREB1 exhibited improved

tolerance to drought salinity and freezing In maize over-expression

of ZmDREB2A under the control of constitutive or stress-inducible

promoter resulted in enhanced drought tolerance in plants (Qin et al

2007) Similarly peanut plants transformed with rd29ADREB1A hadhigher transpiration ef 1047297ciency than the wild type under drought

stress (Bhatnagar-Mathur et al 2007) In a recent study Bhatnagar-

Mathur et al (2009) have generated transgenic plants of peanut over-

expressing Arabidopsis AtDREB1A driven by a stress-inducible promot-

er Atrd29A Although the transgenic peanut plants accumulated consid-

erably higher levels of some key antioxidant enzymes (superoxide

dismutase ascorbate peroxidase and glutathione reductase) and proline

content andlower levelsof lipid peroxidation as compared to those in the

wild-type plants under moisture stress conditions all these antioxidant

and biochemical indicators had no signi1047297cant relationship with transpi-

ration ef 1047297ciency of the transgenic plants over-expressing AtDREB1A

Despite DREB other transcription factors are also known to be

involved in plant stress responses One such type is that with APETELA2

(AP2)-domain Recently Oh et al (2009) have identi1047297ed42 AP2 genes in

rice which are triggered by various stresses including salinity drought

freezing and ABA While carrying out the phylogenic analysis of these 42

stress-inducible AP2 genes they have identi1047297ed 6 subgroups (IndashVI) with

conspicuous signature motifs andtwogenes AP37 and AP59 belongingto

subgroupsI andII respectivelywere triggered just after 2 h of exposure to

water de1047297cit and high saline conditions but they differed in their

expression pro1047297le particularly under ABA and low temperature The

transformed rice plants over-expressing AP37 and AP59 under the

operation of the constitutivepromoter OsCc1 showedenhancedresistanceto high drought and saline conditions at the vegetative growth stage

However as compared with OsCc1AP59 plants the OsCc1AP37 plants

showed considerably higher tolerance to drought by producing 16ndash57

more grain yield over non-transgenic controls under severe drought

conditions of the 1047297eld These 1047297ndings suggest the potential role of the

AP37 gene to improve drought tolerance in rice

From the whole preceding discussion it is not hard to infer that

considerable progress can be made within the shortest possible time in

improving plant drought tolerance by engineering the genes involved in

the synthesis of organic osmolytes plant growth regulators antioxidants

late embryogenesis abundant proteins and transcription factors (regula-

tory proteins) involved in gene expression However most of the

transgeniclines of differentcrops were tested under controlledlaboratory

or glasshouse conditions wherein they have shown a remarkable per-

formance under simulated stress conditions With the exception of only

two studies (rice transgenics Xiaoet al2007Oh etal 2009) noneof the

transgenic linesproduced throughgenetic engineeringin different studies

has been tested in natural1047297eld conditions Thus it is not knownhow they

would perform during 1047297eld testing because a natural 1047297eld encounters a

myriad of environmental factors other than the drought stress

The other key issue is that like salt tolerance the degree of drought

tolerance varies with growth and development in most plant species (El-

Far and Allan 1995 Reddy et al 2004 Rassaa et al 2008 ) Thus the

degree of drought tolerance observed in the transgenic lines at one

particular stage particularly at theinitialgrowth stage may not re1047298ect the

same when tested at other growth stages or as adult So there is a need to

ensure the overall drought tolerance of a plant species for farmers

standpoint

Like in the case of plant salt tolerance (Ashraf and Akram 2009) mostof the drought tolerant transgenic lines of different crops developed are

based on only a single gene transformation whereas the claims of the

scientists regarding the performance of the lines with respect to drought

tolerance seem to be overstated as earlier reported in the case of salt

tolerance (Flowers 2004 Ashrafand Akram2009) Thus manipulation of

a number of genes predominantly involved in stress tolerance to

transgenic plants seems to be a plausible approach This will certainly

allow pyramiding of desirable traits to achieve considerable advance in

crop drought tolerance

5 Conclusions and future challenges

The main focus of the present review has been on three prominent

plantbreedingapproachesforachievingenhancedcropdrought toleranceie conventional breeding marker-assisted breeding and genetic

engineering Despite the fact that conventional breeding has many

limitations as listed elsewhere (Ashraf 1994 Flowers 2004 Ashraf and

Akram 2009) a reasonable number of cultivarslines tolerant to drought

stress have so farbeen developedsomeof which have notonly performed

well under controlled environmental conditions but also under natural

drought-prone environments Certainly the traditional protocols em-

ployed in developing such drought tolerant cultivarslines have not been

very cost-intensive if we compare the cost being incurred these days on

modern plant breeding approaches including marker-assisted breeding

and genetic engineering However the main debacle with traditional

plant breeding approach is that it does not offer labor and time savings

Marker-assisted breeding approach is a prospective alternative to

traditional breeding because of being less time-consuming and labor-




and cost-effective Molecular mapping and analysis of QTL have been

carried out for a number of qualitative and quantitative traits including

stress tolerance which has undoubtedly resulted in a great magnitude

of knowledge and better understanding of the causal genetic phenom-

ena that regulate these traits However limited success has resulted by

using this knowledge to manipulate genes in an effective way for the

improvement of a speci1047297c trait in crops although some crop cultivars

developed through using these tools have performed extremely well

under1047297

eld stressconditions However there are a variety of reasons forthe limited success in terms of achieving enhanced drought tolerance

using the marker-assisted breeding approach For example a major

dif 1047297culty confronting the scientists is the challenge of precise QTL

identi1047297cation In addition a substantial genetictimes environment interac-

tion inconsistent repeatability large number of genes regulating yield

and invalid use of mapping populations have hampered the pursuits

involving mapping of QTL for enhanced drought stress tolerance

Despite these other factors also impede the application of QTL for

genetic improvement of a trait For example due to unfavorable

epistatic interaction it is hard to transfer the effects of a desired allele to

an elite background material (Podlich et al 2004 Collins et al 2008)

Furthermore in some cases QTL from a speci1047297c background do not

show signi1047297cant effects or cease completely in different backgrounds

even under analogous growth conditions (Cho and Hong 2006 Collins

et al 2008) This makes the utilization of QTL more intricate

Transformation of the knowledge acquired from QTL-oriented molec-

ular studies into a well-de1047297ned upshot for the stakeholders is one of the

key challenges confronting the breeders Certainly a multidisciplinary

approach including more speci1047297cally the identi1047297cation of QTL mediated

signal transduction in response to stresses needs to be adopted

Furthermore although QTL cloning procedures are unwieldy they can

lead to a meaningful outcome because a cloned QTL can offer a reliable

markerfor MABas well as furnish an outlinefor thedetection of superior

allelic variants in crop species (Till et al 2007)

Genetic engineering (Transgenic approach) offers a promise whereby

one can expect a substantial improvement in a desired trait within the

shortest stretch of time Engineering genes encoding organic osmolytes

plant growth regulators antioxidants late embryogenesis abundant pro-

teins and transcription factors has resulted into transgenic lines whichhave out-performed under controlled stress conditions In most cases the

claims made by the researchers in terms of performance of transgenic

lines tested under controlled conditions are considerably in1047298ated

However with the exception of a very few notable cases most of the

transgenic lines of differentcrops have been rarely1047297eld-tested Thushow

far these transgenic lines perform under 1047297eld stress conditions is not

known because under natural1047297eld conditions a linecultivar has to face a

multitude of environmental factors other than the drought stress Thus

appraisal of performance of a transgenic line under natural 1047297eld con-

ditionsis mandatory forthe stakeholdersperspective It is also imperative

to note that most of the drought tolerant transgenic lines have been

developed using a single gene transformation which may not be as

productive as if it had been developed using transformation of many

genes Thus transferring a number of prominent genes effectively in-volved in stress tolerance to transgenic plants seems to be a logical

approach Although a large number of genes appear to be involved in

stress tolerance and most of them have been fully characterized the

function of many of them in the mechanism of stress tolerance is yet to be

investigated

References

Abebe T Guenzi AC Martin B Cushman JC Tolerance of mannitol-accumulatingtransgenic wheat to water stress and salinity Plant Physiol 20031311748ndash55

Ashraf M Breeding for salinity tolerance in plants Crit Rev Plant Sci 19941317 ndash42Ashraf M Akram NA Improving salinity tolerance of plants through conventional

breeding and genetic engineering an analytical comparison Biotechnol Adv200927744ndash52 doi101016jbiotechadv200905026

Ashraf M Foolad MR Roles of glycinebetaine and proline in improving plant abioticstress resistance Environ Exp Bot 200759206ndash16

Ashraf M Athar HR Harris PJC Kwon TR Some prospective strategies for improvingcrop salt tolerance Adv Agron 20089745-110

Asins MJ Present and future of quantitative trait locus analysis in plant breeding PlantBreed 2002121281ndash91

Babu RC Nguyen BD Chamarerk V Shanmugasundaram P Chezhian P Jeyaprakash P et alGeneticanalysis of droughtresistance in riceby molecular markers association betweensecondary traits and 1047297eld performance Crop Sci 2003431457ndash69

Babu RC Zhang J Blum A Ho THD Wu R Nguyen HT HVA1 a LEA gene from barleyconfers dehydration tolerance in transgenic rice( Oryzasativa L)via cell membraneprotection Plant Sci 2004166855ndash62

Badawi GH Kawano N Yamauchi Y Over-expression of ascorbate peroxidase in

tobacco chloroplasts enhances the tolerance to salt stress and water de1047297cit PhysiolPlant 2004121231ndash8Badu-Apraku B Yallou CG Registration of striga-resistant and drought tolerant tropical

early maize populations TZE-W Pop DT STR C4 and TZE-Y Pop DT STR C4 J PlantRegistr 20093(1)86ndash90

Badu-Apraku B Menkir A Kling JG Fakorede MAB Registration of 16 striga resistantearly maturing tropical maize inbred lines Crop Sci 2006461410ndash1

Baenziger PS Beecher B Graybosch RA Ibrahim AMH Baltensperger DD Nelson LA et alRegistration of lsquoNEO1643rsquo wheat J Plant Registr 20082(1)36ndash42

Baumlnziger M Setimela PS Hodson D Vivek B Breeding for improved drought tolerance inmaize adapted to southern Africa Proceedings of the 4th International Crop ScienceCongress Brisbane Australia Published on CDROM 2004 26 Sep ndash 1 Oct

Bartels D Sunkar R Drought and salt tolerance in plants Crit Rev Plant Sci 20052423 ndash58Baum M Grandol S Backes G Jahoor A Sabbagh A Ceccarelli S QTLs for agronomic

traits in the Mediterranean environment identi1047297ed in recombinant inbred lines of the cross lsquoArtarsquo H spontaneum 41-1 Theor Appl Genet 20031071215ndash25

Bergman JW Riveland NR Flynn CR Carlson GR Wichman DM Registration of lsquoMorlinrsquo

saf 1047298ower Crop Sci 2001411640Bernier J Kumar A Venuprasad R Spaner D Atlin G A large-effect QTL for

grain yield under reproductive-stage drought stress in upland rice Crop Sci200747507ndash18

BernierJ Kumar A SerrajR SpanerD Atlin G Review breeding uplandrice fordroughtresistance J Sci Food Agric 200888927ndash39

Bernier J Serraj R Kumar A Venuprasad R Impa S Gowdaa RPV et al The large-effectdrought-resistance QTL qtl121 increases water uptake in upland rice Field CropsRes 200911039ndash46

Bhatnagar-Mathur P ReddyDS Lavanya M Yamaguchi-Shinozaki K Sharma KK Stress-inducible expression of Arabidopsis thaliana DREB1A in transgenic peanut ( Arachishypogaea L) increases transpiration ef 1047297ciency under water-limiting conditionsPlant Cell Rep 2007262071ndash82

Bhatnagar-Mathur P Devi MJ Vadez V Sharma KK Differential antioxidative responses intransgenic peanut bear no relationship to their superior transpiration ef 1047297ciency underdrought stress J Plant Physiol 2009166(11)1207ndash17 doi101016jjplph200901001

Bidinger FR Serraj R Rizvi SMH Howarth C Yadav RS Hash CT Field evaluation of droughttolerance QTL effects on phenotype and adaptation in pearl millet [ Pennisetum glaucum(L) R Br] topcross hybrids Field Crops Res 200594(1)14ndash32

Bidinger FR Nepolean T Hash CT Yadav RS Howarth CJ Identi1047297cation of QTLs for grainyield of pearl millet (Pennisetum glaucum (L) R Br) in environments with variablemoisture during grain 1047297lling Crop Sci 200747969ndash80

Bowers JE Abbey C Anderson S Chang C Draye X A high-density geneticrecombination map of sequence-tagged sites for Sorghum as a framework forcomparative structural and evolutionary genomics of tropical grains and grassesGenetics 2003165367ndash86

Brick MA Ogg JB Singh SP Schwartz HF Johnson JJ Pastor-Corrales MA Registration of drought-tolerant rust-resistant high-yielding pinto bean germplasm lineCO46348 J Plant Registr 20082(2)120ndash4

Browne J Tunnacliffe A Burnell A Anhydrobiosis-plant desiccation gene found in anematode Nature 200241638

Busk PK Pages M Regulation of abscisic acid induced transcription Plant Mol Biol199837425ndash35

Carena MJ Wanner DW Development of genetically broad-based inbred lines of maizefor early-maturing (70-80RM) hybrids J Plant Registr 20093107ndash11

Cash SDBrucknerPL Wichman DMKephart KD Berg JEBoynerR et alRegistration of Willow Creek forage wheat J Plant Registr 20093(2)185ndash90

Cattivelli L Rizza F Badeck FW Mazzucotelli E Francia AMEM Mare AT et al Droughttolerance improvement in crop plants an integrated view from breeding to genomics

Field Crops Res 20081051-14Chen M Wang QY Cheng XG Xu ZS Li LC Ye XG et al GmDREB2 a soybean DRE-

binding transcription factor conferred drought and high-salt tolerance intransgenic plants Biochem Biophys Res Commun 2007353299ndash305

Cheng Z Targolli J Huang X Wu R Wheat LEA genes PMA80 and PMA1959 enhancedehydration tolerance of transgenic rice (Oryza sativa L) Mol Breed 20021071ndash82

ChoEK Hong ChB Over-expression of tobacco NtHSP70-1 contributes to drought-stresstolerance in plants Plant Cell Reports 200625349ndash58

Close TJ Dehydrins a commonality in the response of plants to dehydration and lowtemperature Physiol Plant 1997100291ndash6

Collins NC Tardieu F Tuberosa R Quantitative trait loci and crop performance underabiotic stress where do we stand Plant Physiol 2008147469ndash86

Concept Note Combining breeding and biotechnology to develop water ef 1047297cient maizefor Africa (WEMA) Afr Agric Technol Foundation

Courtois BShen L Petalcorin WCarandang S MauleonR LiZ Locating QTLs controllingconstitutive root traits in the rice population IAC 165-Co39 Euphytica 2003134335ndash45

Da-hong L Hui L Yang YL Ping-ping Z Jian-sheng L Down-regulated expression of RACK1geneby RNAinterference enhancesdroughttolerancein rice RiceSci 200916(1)14ndash20




Dalal M Tayal D Chinnusamy V Bansala KC Abiotic stress and ABA-inducible group 4 LEAfrom Brassicanapus plays a key rolein saltand droughttolerance J Biotechnol 2009139137ndash45

Ding Z Li S An X Liu X Qin H Wang D Transgenic expression of MYB15 confers enhancedsensitivity to abscisic acid and improved drought tolerance in Arabidopsis thaliana

J Genet Genomics 20093617ndash29Dubouzet JG Sakuma Y Ito Y Kasuga M Dubouzet EG Miura S et al OsDREB genes in

rice Oryza sativa L encode transcription activators that function in drought high-salt- and cold-responsive gene expression Plant J 200333751ndash63

El-Far IA Allan AY Responses of some wheat cultivars to sowing methods and droughtat different stages of growth Assuit J Agric Sci 199526(1)267ndash77

Eltayeb AE Kawano N Badawi GH Kaminaka H Sanekata T Shibahara T et alOverexpression of monodehydroascorbate reductase in transgenic tobacco confersenhanced tolerance to ozonesalt and polyethyleneglycol stresses Planta 2007225(5)1255ndash64

Falconer DS Introduction to quantitative genetics London New York Longman 1989Feng-ling FU Zhi-Lei F Shi-bing G Shu-feng Z Wan-chen L Evaluation and quantitative

inheritanceof several drought-relativetraits in maize Agric SciChina 20087(3)280ndash90Finkelstein R Gampala S Rock C Abscisic acid signaling in seeds and seedlings Plant

Cell 20021415ndash45Flowers TJ Improving crop salt tolerance J Exp Bot 200455307ndash19Fujita Y Fujita M Satoh R Maruyama K Parvez MM Seki M et al AREB1 is a transcription

activator of novel ABREdependent ABAsignaling thatenhances drought stress tolerancein Arabidopsis Plant Cell 2005173470ndash88

Giraudat J Parcy F Bertauche N Gosti F Leung J Morris PC et al Current advances inabscisic acid action and signalling Plant Mol Biol 1994261557ndash77

Gorbalenya AE Koonin EV Helicases amino acid sequence comparisons and structurendashfunction relationships Curr Opin Struct Biol 19933419ndash29

Gosal SS Wani SH Kang MS Biotechnology and drought tolerance J Crop Improvement20092319ndash54

Gubis J Vaňkovaacute R Červenaacute V Draguacuteňovaacute M Hudcovicovaacute M Lichtnerovaacute H et alTransformedtobacco plants with increased toleranceto droughtSouthAfr J Bot200773505ndash11

GuoO Zhang J GaoQ Xing SLi F Wang WDrought tolerancethrough over-expressionof mono ubiquitin in transgenic tobacco J Plant Physiol 20081651745 ndash55

Haley SD Johnson JJ Peairs FB Quick JS Stromberger JA Clayshulte SR et al Registration of lsquoRipperrsquo wheat J Plant Registr 200711ndash6

Han SE Park SR Kwon HB Yi BY Lee GB Byun MO Genetic engineering of drought-resistant tobacco plants by introducingthe trehalose phosphorylase (TP) gene fromPleurotus sajor-caju Plant Cell Tissue Organ Cult 200582151ndash8

Harris K Klein R Mullet J Sorghum stay-green QTL individually reduces post-1047298oweringdrought-induced leaf senescence J Exp Bot 200758327ndash38

Hong BS Zong-Suo L Ming-An S LEA proteins in higher plants structure functiongene expression and regulation Colloids Surf B Biointerf 200545131ndash5

Howarth CJ Yadav RS Successful marker assisted selection for drought tolerance anddisease resistance in pearl milletIGER Innovations 2002

Humphreys MO Humphreys MW Breeding for stress resistance general principles InAshraf M Harris PJC editors Abiotic stresses plant resistance through breedingand molecular approaches 2005 p 19ndash46

Ingram J Bartels D The molecular basis of dehydration tolerance in plants Ann RevPlant Physiol Plant Mol Biol 199647377ndash403

Jaglo-Ottosen KR Gilmour SJ Zarka DG Schabenberger O Thomashow MF ArabidopsisCBF1 overexpression induces COR genes and enhances freezing tolerance Science1998280104ndash6

Jang IC Oh SJ Seo JS Choi WB Song SI Kim CH et al Expression of a bifunctional fusion of the Escherichia coli genes for trehalose-6-phosphate synthase and trehalose-6-phosphate phosphatase in transgenic rice plants increases trehalose accumulationandabiotic stresstolerancewithoutstuntinggrowth Plant Physiol2003131516ndash24

Jeanneau M Gerentes D Foueillassar X Zivy M Vidal J Toppan A et al Improvement of drought tolerance in maize towards the functional validation of the Zm-Asr1 gene andincrease of water use ef 1047297ciency by over-expressing C4-PEPC Biochimie 2002841127ndash35

Jenson KB Palazzo AJ Waldron BL Bushman BS Registration of lsquoFirstStrikersquo Slenderwheatgrass J Plant Registr 2007124ndash5

Joung-youn K Hyung-in C Min-young I Soo-young K Arabidopsis basic leucine zipperproteins that mediate stress-responsive abscisic acid signaling Plant Cell 20024343ndash57

Juskiw PE Helm JH Oro M Nyachiro JM Salmon DF Registration of lsquoBentleyrsquo barley

J Plant Registr 20093(2)119ndash23Kalamaki MS Alexandrou D Lazari D Merkouropoulos G Fotopoulos V Pateraki I et al

Over-expression of a tomato N-acetyl-L -glutamate synthase gene (SlNAGS1) in Arabidopsis thaliana results in high ornithine levels and increased tolerance in saltand drought stresses J Exp Bot 200960(6)1859ndash71

Kamoshita A Babu CR Boopathi NM Fukai S Phenotypic and genotypic analysis of drought-resistance traits for development of rice cultivars adapted to rainfedenvironments Field Crops Res 2008109(103)1-23 doi101016jfcr200806010

KarakasB Ozias-AkinsP Stushnoff C SuefferheldM Rieger M Salinityand drought toleranceof mannitol-accumulating transgenic tobacco Plant Cell Environ 199720609ndash16

Karim S Aronsson H Ericson H Pirhonen M Leyman B Welin B et al Improved droughttolerance without undesired side effects in transgenic plants producing trehalose PlantMol Biol 200764371ndash86

Kasuga M Liu Q Miura S Yamaguchi-Shinozaki K Shinozaki K Improving plant droughtsaltand freezing tolerance by gene transfer of a single stress-inducible transcription factorNat Biotechnol 199917287ndash91

Kindiger M Gaub H Hasegawac M Katsurab Y Ueyamad K Gotob S et al Registrationof lsquoNanryorsquo tall fescue Crop Sci 2006461815ndash6

Kong I Dong J HartGE Characteristics linkage mappositionsand allelic differentiationof Sorghum bicolour (L) Moench by DNA simple-sequence repeats (SSRs) TheorAppl Genet 2000101438ndash48

Kumar R Venuprasad R Atlin GN Genetic analysis of rainfed lowland rice droughttolerance under naturally-occurring stress in eastern India heritability and QTL effects Field Crops Res 200710342ndash52

La1047297tte HR Price AH Courtois B Yield response to water de1047297cit in an upland ricemapping population associations among traits and genetic markers Theor ApplGenet 20041091237ndash46

Lal S Gulyani V Khurana P Overexpressionof HVA1 gene frombarley generates tolerance tosalinity and water stress in transgenic mulberry (Morus indica) Transgenic Res 200817

(4)651ndash

63Lanceras J Pantuwan G Jongdee B Toojinda T Quantitative trait loci associated withdrought tolerance at reproductive stage in rice Plant Physiol 2004135384ndash99

Levi A Ovnat L Paterson AH Saranga Y Photosynthesis of cotton near-isogenic linesintrogressed with QTLs for productivity and drought related traits Plant Sci 2009a17788ndash96

Levi A Paterson AH Barak V Yakir D Wang B Chee PW et al Field evaluation of cottonnear-isogenic lines introgressed with QTLs for productivity and drought relatedtraits Mol Breed 2009b23179ndash95

Liu Q Kasuga M Sakuma Y Abe H Miura S Yamaguchi- Shinozaki K et al Twotranscription factors DREB1 and DREB2 with an EREBPAP2 DNA binding domainseparate two cellular signal transduction pathways in drought-and low-temper-ature-responsive gene expression respectively in Arabidopsis Plant Cell 1998101391ndash406

Liu X Hua X Guo J Qi D Wang L Liu Z et al Enhanced tolerance to drought stress intransgenic tobacco plants overexpressing Biotechnol Lett 2008301275ndash80

Liu X Wanga Z Wanga L Wua R Phillips J Deng X LEA 4 group genes from theresurrection plant Boea hygrometrica confer dehydration tolerance in transgenictobacco Plant Sci 200917690ndash8

Luchi S Kobayashi M Taji T Naramoto M Seki M Kato T et al Regulation of droughttolerance by gene manipulation of 9-cis-epoxycarotenoid dioxygenase a keyenzyme in abscisic acid biosynthesis in Arabidopsis Plant J 200127325ndash33

Luo Y Liu YB Dong YX Gao XQ Zhang YS Expression of a putative alfalfa helicase increasestolerance to abioticstress in Arabidopsis by enhancing the capacities for ROS scavengingand osmotic adjustment J Plant Physiol 2009166385ndash94

MacLean JLDaweDC Hardy B Hettel GPRiceAlmanac sourcebook for themostimportanteconomic activity on Earth3rd ed Wallingford England CABI Publishing 2002

Mansour MMF Nitrogen containing compounds and adaptation of plants to salinitystress Biol Plant 200043491ndash500

McKersie BD Bowley SR Harjanto E Leprince O Water-de1047297cit tolerance and 1047297eldperformance of transgenic alfalfa overexpressing superoxide dismutase Plant Physiol19961111177ndash81

McKersie BD Murnaghan J Bowley SR Manipulating freezing tolerance in transgenicplants Acta Physiol Plant 199719485ndash95

Miranda JA Avonce N Suaacuterez R Thevelein JM Dijck PV Iturriaga G A bifunctional TPS-TPP enzyme from yeast confers tolerance to multiple and extreme abiotic-stressconditions in transgenic Arabidopsis Planta 2007226(6)1411ndash21

Mohammadi M Taleei A Zeinali H Naghavi MR Ceccarelli S Grando Baum M QTL analysis for phenologic traits in doubled haploid population of barley Int J AgricBiol 20057(5)820ndash3

Mohanty A Kathuria H Ferjani A Sakamoto A Mohanty P Murata N et al Transgenicsof an elite indica rice variety Pusa Basmati 1 harbouring the codA gene are highlytolerant to salt stress Theor Appl Genet 200210651ndash7

Noaman MM El Sayad AA Asaad FA El Sherbini AM El Bawab AO El Moselhi MA et alRegistration of lsquoGiza 126rsquo barley Crop Sci 199535(6)1710

Noaman MM Ahmed IA El-Sayed AA Abo-El-Enin RA El-Gamal AS El-Sherbiny AM et alRegistration of lsquoGiza 2000rsquo drought-tolerant six-rowed barley for rainfed and newreclaimed areas in Egypt Crop Sci 200747440

Obert DE Evans CP Wesenberg DM Windes JM Erickson CA Jackson EW et alRegistration of lsquoLenetahrsquo spring barley J Plant Registr 20082(2)85ndash7

Oh SJ Jeong JS Kim EH Yi NR Yi SI Jang IC et al Matrix attachment region fromthe chicken lysozyme locus reduces variability in transgene expression andconfers copy number-dependence in transgenic rice plants Plant Cell Rep20054145ndash54

Oh SJ Kim YS Kwon C Park HK Jeong JS Kim JK Overexpression of the transcriptionfactor AP37 in rice improves grain yield under drought conditions Plant Physiol

20092191ndash

200 doi101104pp109137554Owttrim GW RNA helicases and abiotic stress Nucleic AcidsRes 200634(11)3220ndash30Park BJ Liu Z Kanno A Kameya T Increased tolerance to salt and water de1047297cit stress in

transgenic lettuce (Lactuca sativa L) by constitutive expression of LEA PlantGrowth Regul 2005a45165ndash71

Park BJ Liu Z Kanno A Kameya T Genetic improvement of Chinese cabbage for salt anddroughttolerance by constitutiveexpression of a B napus LEAgenePlantSci 2005b169553ndash8

Pastori GM Foyer CH Common componentsnetworks and pathways of crosstoleranceto stress The central role of ldquoredoxrdquo and abscisic acid-mediated controls PlantPhysiol 2002129460ndash8

PellegrineschiA ReynoldsM PachecoM Brito RMAlmeraya R Yamaguchi-ShinozakiK et alStress-induced expression in wheat of the Arabidopsis thaliana DREB1Agenedelayswaterstress symptoms under greenhouse conditions Genome 200447493ndash500

Perl A Perl-Treves R Galili S Aviv D Shalgi E Malkin S et al Enhanced oxidative-stressdefense in transgenic potato overexpressing tomato Cu Zn superoxide dismutaseTheor Appl Genet 199385568ndash76

Podlich DW Winkler CR Cooper M Mapping as you go an effective approach formarker-assisted selection of complex traits Crop Sci 2004441560ndash71




Pratt RC Casey MA Registration of maize germplasm line Oh605 Crop Sci 2006461004ndash5Qin F Kakimoto M Sakuma Y Maruyama K Osakabe Y Tran LSP et al Regulation and

functional analysis of ZmDREB2A in response to drought and heat stress in Zea mays LPlant J 20075054ndash69

Quan R Shang M Zhang H Zhao Y Zhang J Engineering of enhanced glycinebetainesynthesis improves drought tolerance in maize Plant Biotechnol J 20042477ndash86

Quarrie SA Gulli M Calestani C Steed A Marmiroli N Location of a gene regulatingdrought-induced abscisic acid production on the long arm of chromosome 5A of wheat Theor Appl Genet 199489794ndash800

QuickJS Stromberger JA Clayshulte S Clifford B Johnson JJ Peairs FB et al Registrationof lsquoPrairie Redrsquo wheat Crop Sci 2001411362ndash3

RajaramS Prospects andpromise ofwheatbreedingin the21stcentury Euphytica 20011193-15Rajaram S Role of conventional plant breeding and biotechnology in future wheat

production Turk J Agric Forest 200529105ndash11Rami JF Dufour P Trouche G Fliedel G Mestres C Davrieux F et al Quantitative trait

loci for grain quality productivity morphological and agronomical traits insorghum (Sorghum bicolor L Moench) Theor Appl Genet 199897605ndash16

Rassaa N Salahb HBH Latiri K Thermal responses of durum wheat Triticum durum toearly water stress consequence on leaf and 1047298ower development Plant Biol Pathol2008331(5)363ndash71

Reddy LJ Nigam SN Rao RCN Reddy NS Registration of ICGV 87354 peanut germplasmwith drought tolerance and rust resistance Crop Sci 200141274ndash5

Reddy ARChaitanyaKV VivekanandanM Drought-inducedresponsesof photosynthesis andantioxidant metabolism in higher plants J Plant Physiol 20041611189ndash202

Ribaut JM Ragot M Marker-assisted selection to improve drought adaptation in maize thebackcross approach perspectives limitations and alternatives J Exp Bot 200658351ndash60

Robin S Pathan MS Courtois B La1047297tte R Carandang S Lanceras S et al Mappingosmotic adjustment in an advanced back-cross inbred population of rice Theor

Appl Genet 2003107(7)1288ndash96Romero C Belles JM Vaya JL Serrano R Culianez-Macia FA Expression of the yeast

trehalose-6-phosphate synthase gene in transgenic tobacco plants pleiotropicphenotypes include drought tolerance Planta 1997201293ndash7

Ronde JAD Cress WA Krugerd GHJ Strasserd RJ Van Staden J Photosynthetic responseof transgenic soybean plants containing an Arabidopsis P5CR gene during heat anddrought stress J Plant Physiol 20041611211ndash24

Salvi S Tuberosa R To clone or not to clone plant QTLs present and future challengesTrends Plant Sci 200510297ndash304

Sanchez AC Subudhi PK Rosenow DT Nguyen HT Mapping QTLs associated with droughtresistance in sorghum (Sorghum bicolor L Moench) Plant Mol Biol 200248713ndash26

Saranga Y Menz M Jiang CX Wright RJ Yakir D Paterson AH Genomic dissection of genotypetimesenvironment interactions conferring adaptation of cotton to aridconditions Genome Res 2001111988ndash95

Sari-Gorla M Krajewski P Di Fonzo N Villa M Frova C Genetic analysis of droughttolerance in maize by molecular markers II Plant height and 1047298owering Theor ApplGenet 199999289ndash95

Seki M Narusaka M Ishida J Nanjo T Fujita M Oono Y et al Monitoring the expressionpro1047297les of 7000 Arabidopsis genes under drought cold and high-salinity stressesusing a full-length cDNA microarray Plant J 200231279ndash92

Serraj R Sinclair TR Osmolyte accumulation can it really increase crop yield underdrought conditions Plant Cell Environ 200225333ndash41

Serraj R Krishnamurthy L Kashiwagi J Kumar J Chandra S Crouch JH Variation in roottraits of chickpea (Cicer arietinum L) grown under terminal drought Field CropsRes 200488115ndash27

Serraj R Hash CT Rizvi MHS Sharma A Yadav RS Bidinger FR Recent advances in marker-assisted selection for drought tolerance in pearl millet Plant Prod Sci 20058(3)334ndash7

Shen YG Du BX Zhang WK Zhang JS Chen SY AhCMO regulated by stresses in Atriplexhortensis can improve drought tolerance in transgenic tobacco Theor Appl Genet2002105815ndash21

Shinozaki K Yamaguchi-Shinozaki K Molecular responses to dehydration and lowtemperature differences and cross-talk between two stress signaling pathwaysCurr Opin Plant Biol 20003217ndash23

Singh KB Omar M Saxena MC Johansen C Registration of FLIP 87-59C a drought-tolerant chickpea germplasm line Crop Sci 199636(2)1ndash2

Singh SP Teran H Gutierrez JA Registration of SEA 5 and SEA 13 drought tolerant drybean germplasm Crop Sci 200141276ndash7

Singh SP Teran H Lema M Schwartz HF Miklas PN Registration of white moldresistant dry bean germplasm line A 195 J Plant Registr 2007162ndash3

Sivamani E Bahieldin A Wraith JM Improved biomass productivity and water useef 1047297ciency under water de1047297cit conditions in transgenic wheat constitutivelyexpressing the barley HVA1 gene Plant Sci 20001551ndash9

Skovmand B Reynolds MP DeLacy IH Searching genetic resources for physiologicaltraits with potential for increasing yield In Reynolds MP Ortiz-Monasterio IMcNab A editors Application of Physiology in Wheat Breeding 2001 p 17ndash28

Steele K Novel upland rice variety bred using marker-assisted selection and client-oriented breeding released in Jharkhand India Bangor University 2009

Steele KA Price AH Shashidar HE Witcombe JR Marker-assistedselection to introgressrice QTLs controlling root traitsinto an Indianupland rice variety Theor Appl Genet2006112208ndash21

Steele KA Virk DS Kumar R Prasad SC Witcombe JR Field evaluation of upland ricelines selected for QTLs controlling root traits Field Crops Res 2007101180 ndash6

Sunkar R Kapoor A Zhu JK Post transcriptional induction of two CuZn superoxidedismutase genes in Arabidopsis is mediated by down regulation of miR398 andimportant for oxidative stress tolerance Plant Cell 2006182051ndash65

Talame V Sanguineti MC Chiapparino E Bahri H Ben Salem M Forster BP et alIdenti1047297cation of Hordeum spontaneum QTL alleles improving 1047297eld performance of barley grown under rainfed conditions Ann Appl Bot 2004144309ndash20

Tanner NKCordinO BanroquesJ DoereM Linder PThe Q Motif a newlyidenti1047297ed motif in DEAD box helicases may regulate ATP binding and hydrolysis Mol Cell 200311127ndash38

Taramino G Tarchini R Ferrario S Lee M Pe ME Characterization and mapping of simplesequence repeats (SSRs) in Sorghum bicolor Theor Appl Genet 19979566ndash72

Teulat B Monneveux P Wery J Borriegraves C Souyris I Charrier A et al Relationshipsbetween relative water content andgrowth parameters in barleya QTLstudy NewPhytol 199713799-107

Thi Lang N Chi Buu B Fine mapping for drought tolerance in rice ( Oryza sativa L)Omonrice 2008169-15Till BJ Comai L Henikoff S Tillering and ecotillering for crop improvement In

Varshney RK Tuberosa R editors Genomics-assisted crop improvement genomicsapproaches and platforms vol 1 2007 p 333ndash50

Tondelli A Francia E Barabaschi D Aprile A Skinner JS Stockinger EJ et al Mappingregulatory genes as candidates for cold and drought stress tolerance in barleyTheor Appl Genet 2006112445ndash54

Tuberosa R Salvi S Genomics approaches to improve drought tolerance in cropsTrends Plant Sci 200611405ndash12

Valkoun JJ Wheat pre-breeding using wild progenitors Euphytica 200111917ndash23Vashisht AA Tuteja N Stress responsive DEAD-box helicases a new pathway to

engineer plant stress tolerance J Photochem Photobiol B Biol 200684150 ndash60Vendruscolo ECG Schuster I Pileggi M Scapim CA Molinari HBC Marur CJ et al Stress-

induced synthesis of proline confers tolerance to water de1047297cit in transgenic wheat J Plant Physiol 20071641367ndash76

Vienne D Leonardi A Damerval C Zivy M Genetics of proteome variation for QTL characterization application to drought stress responses in maize J Exp Bot 199950303ndash9

Villareal RL Mujeeb-Kazi A Rajaram S Toro ED Morphological variability in somesynthetic hexaploid wheats derived from Triticum turgidum times T tauschii J GenetBreed 1994487-16

Vinh NT Paterson AH Genome mapping and its implication for stress resistance inplants In Ashraf M Harris PJC editors Abiotic stresses plant resistance throughbreeding and molecular approaches 2005

Vinocur B Altman A Recent advances in engineering plant tolerance to abiotic stressachievements and limitations Curr Opin Biotechnol 200516123ndash32

Wang YJ Hao YJ Zhang ZG Chen T Zhang JS Chen SY Isolation of trehalose-6-phosphate phosphatase gene from tobacco and its functional analysis in yeast cells

J Plant Physiol 2005162215ndash23Wu R Garg A Engineering rice plants with trehalose-producing genes improves

tolerance to drought salt and low temperature ISB News Report 2003Xiao B Huang Y Tang N Xiong L Overexpression of LEA gene in rice improves drought

resistance under 1047297eld conditions Theor Appl Genet 200711535ndash46Xinglai P Sangang X Qiannying P Yinhong S Registration of lsquo Jinmai 50rsquo wheat Crop Sci

200646983ndash5Xiong L Lee H Ishitani M Zhu JK Regulation of osmotic stress-responsive gene

expression by the LOS6ABA1 locus in Arabidopsis J Biol Chem 20022778588ndash96Xu GW Magill CW Shertz KF Hart GE A RFLP linkage map of Sorghum bicolor (L)

Moench Theor Appl Genet 199489139ndash45Xu D Duan X Wang B Hong BHo THD Wu RExpression of a late embryogenesis abundant

protein gene HVA1 from barley confers tolerance to water de1047297cit and salt stress intransgenic rice Plant Physiol 1996110249ndash57

Yadav RS Hash CT Bidinger FR Devos KM Howarth CJ Genomic regions associatedwith grain yield and aspects of post-1047298owering drought tolerance in pearl milletacross stress environments and testers background Euphytica 2004136265ndash77

YamadaM MorishitaH Urano K Shiozaki N Yamaguchi-Shinozaki K ShinozakiK et alEffects of free proline accumulation in petunias under drought stress J Exp Bot2005561975ndash81

Yang WJ Rich PJ Axtell JD Wood KV Bonham CC Ejeta G et al Genotypic variation forglycinebetaine in sorghum Crop Sci 200343162ndash9

Yan-Ying QU Ping MU Xue-Qin L Yu-Xiu T Feng W Hong-Liang Z et al QTL mappingand correlations between leaf water potential and drought resistance in rice underupland and lowland environments Acta Agron Sin 200834(2)198ndash206

Ye XDWu XLZhaoH Frehner MNoumlsberger J Potrykus Iet al Altered fructan accumulationin transgenic Lolium multi 1047298orum plants expressing a Bacillus subtilis sacB gene Plant Cell

Rep 200120205ndash

12Zaharieva M Gaulin E Havaux M Acevedo E Monneveux P Drought and heat responses in

the wild wheat relative Aegilops geniculata Roth potential interest for wheatimprovement Crop Sci 2001411321ndash9

Zhang J Zheng HGAartiA Pantuwan G NguyenTT Tripathy JNet alLocatinggenomicregions associated with components of drought resistance in rice comparativemapping within and across species Theor Appl Genet 200110319ndash29

Zhang GH Su Q An LJ Wu S Characterization and expression of a vacuolar Na+H+

antiporter gene from the monocot halophyte Aeluropus littoralis Plant PhysiolBiochem 200846117ndash26

Zhao J QTLs for oil content and their relationships to other agronomic traits in anEuropeantimesChinese oilseed rape population Germany Diss Grorg-Agust Univer-sity of Goettingen 2002

Zhao J Ren W Zhi D Wang L Xia G Arabidopsis DREB1ACBF3 bestowed transgenic tallfescue increased tolerance to drought stress Plant Cell Rep 2007261521ndash8

Zhou W Li Y Zhao BC Ge RC Shen YZ Wang G Huang ZJ Over-expression of TaSTRG gene improves salt and drought tolerance in rice J Plant Physiol 200915(166)1660ndash70




genotypes that excel in the target environment (Baumlnziger et al 2004)

Considerable improvement in a trait canbe made if thegenetic variance

among the genotypes of a crop selection intensity and heritability are

reasonably high (Falconer 1989)

Plant breeding has contributed to a large extent in tackling the chal-

lenges of food security at global level The contributions of plant breeding

to food production at global level have been enormous during the 20th

century There hasbeen most important plant breeding break-throughfor

almost all commercially important crops including major ones such asmaize wheat rice cotton etc The Green Revolution which started in the

1940sandmainlybasedon traditionalbreedingresultedin a phenomenal

increase in wheat and rice yield in many partsof the world and especially

in SouthAsia(Rajaram 2005) Dr Norman Borlaug (Founder of theGreen

Revolution) and his team spent almost two decades breeding high

yielding dwarf wheat that was able to resist plant pests and diseases The

dwarf wheat out-yielded thetraditional varietiesabout twoto threetimes

However relatively little breeding work has been carried out on

improving crops for drought tolerance The achievements made so far in

improving drought tolerance of different crops throughthe integration of

conventional breeding marker-assisted breeding (MAB) and genetic

engineering (transgenic approach) have been discussed in the present

review MAB and transgenic approach are diverse biotechnologies be-

cause through the earlier desirable genes can be tagged so they can be

easily selected within the breeding population whereas through the

latter desirable genes can be transferred from one species to another A

large number of genomic regions of a crop germplasm can be examined

fortheirbreeding value throughMABwhichfacilitates thebreeder to pool

genes of diverse origins (Vinh and Paterson 2005 Humphreys and

Humphreys 2005) In fact this was not possible before through classical

breeding In contrast through the transgenic approach speci1047297c cloned

genes can be incorporated into an organism by limiting the transfer of

undesirable genes from the donor organism Furthermore pyramiding of

genes with similar effects is possible through this approach (Ashraf et al

2008 Gosal et al 2009) However both MAB and transgenic approaches

are deemed ef 1047297cient and precise ways of improving a desired trait They

arebeing used widelythese daysto generate stresstolerant cultivarslines

of different crops Recent progress made in exploiting the knowledge of

gene regulation and the phenomena involved therein in developingdrought tolerant crop cultivarslines has also been discussed in the

present review

2 Conventional breeding for drought tolerance

Through conventional breeding genetic variability for drought

tolerance among cropscrop cultivars or among sexually compatible

plant species can be identi1047297ed and the genetic variation so identi1047297ed can

be introduced through different mating designs into cultivarslines with

good agronomic characteristics (Pocket) During the last century

conventional breeders at different renowned international research

centers have made considerable strides in developing drought tolerant

linescultivars of some important food crops For example breeding

approach started at the International Maize and Wheat ImprovementCenter (CIMMYT) Mexico in the 1970s for developing drought tolerant

maize is worth mentioning Based on theselection andbreeding program

maize yield improvement of 59 to 233 kg haminus1 cycleminus1 of recurrent full-

sib or S1 selection was recorded (Baumlnziger et al 2004) The CIMMYT

breeding programwas very outcome-oriented and multi-faceted because

it focused on a multitude of imperative problems including drought

low N and common leaf and ear diseases (Baumlnziger et al 2004) In 1997

CIMMYT spanned its breeding program to southern Africa aimed at

improving maize for the drought-hit areas A number of maize hybrids

developed by the CIMMYT scientists were found superior to all those

developed by private enterprisesin terms of growthand grain yield under

drought-proneenvironments (Baumlnziger et al2004) Plant breedersat the

Crops Research Institute (CRI) based at Kumasi Ghana have developed a

highly drought tolerant maize cultivar lsquoObatanpa GHrsquo

in 2006 in

collaboration with the International Institute of Tropical Agriculture

(IITA) Ibadan the CIMMYT Mexico and the Sasakawa Global 2000

(Published online April 25 2006) Similarly 16 early maturing maize

inbred lines (fromTZEI 1 to TZEI 16) resistant to a scrounging weed Striga

hermonthica (Del) Benth were produced by the IITA All these lines were

found to be highly resistant to water limited conditions (Published online

April 25 2006)

Wheat which is one of the important staple food crops of the world

is adversely affected by drought In view of a projection by Rajaram(2001) more than 50 of the 237 million ha area in the world under

wheat cultivation is affected by periodic drought As stated earlier im-

provementin drought toleranceof a cropthroughselectionand breeding

requires a substantial magnitude of heritablevariation If variationin the

existing germplasm of a crop is low then wild relatives may serve as a

rich sourceof appropriate genetic variation At CIMMYT a newsynthetic

hexaploid has been developed by crossing the diploid wild ancestor Aegilops tauschii (goat grass) with tetraploid durum wheat (Triticum

turgidum var durum) These hexaploid synthetics containinga complete

D-genome from A tauschii have provided a signi1047297cant new variation for

tolerance to both biotic and abiotic stresses (Villareal et al 1994

Valkoun 2001) At CIMMYT more than 1000 accessions of A tauschii

have been evaluated and new hexaploid lines developed A signi1047297cant

new genetic variation in these newly developed hexaploid wheat has

been observed for abiotic stresses including drought stress (Valkoun

2001) Useful variation for drought tolerance has also been identi1047297ed in

Triticum urartu T boeticum T dicoccoides (Valkoun 2001) and Aegilops

geniculata (Zaharieva et al 2001) However in view of Skovmand et al

(2001) A taushii is the predominant source of variation for drought

tolerance

The International Center for Agricultural Research in Dry Areas

(ICARDA) Aleppo Syria and the International Crops Research Institute

for the Semi-Arid Tropics (ICRISAT) Andhra Pradesh India have a

similar research mandate ie to improve the major staple food crops of

the dry regions particularly falling in Asia the Middle East and Africa

Plant breeders at these two sister institutes have focused mainly on the

premier dryland cereals such as barley millet sorghum and groundnut

and leguminous pulse crops such as lentil chickpea pigeonpea and faba

beans (Worlds Dryland Farmers New Agricultural Technology-GreenRevolution Never Reached Themmht) Although efforts are underwayat

both centers to develop drought tolerant varieties of different crops

mentioned earlier the plant breeders at ICARDA have recently de-

veloped a new variety of barley which they claim as the worlds most

drought tolerant barley variety (ICARDA News) In fact the empirical

breeding strategies were employed to develop the variety by crossing a

land race with a wild barleyline from Palestine It hasbeen reported that

the new drought-hardy barley variety produced 50 more grain yield

than that produced by ordinary barley linescultivars under dryland

conditions

The International Rice Research Institute (IRRI) based at Las Bantildeos

Philippines though is focusing primarily on quality and yield im-

provement in rice efforts are also currently underway to develop

drought tolerant rice keeping in view of the fact that over 50 of theworlds rice is cultivated in rain-fed areas where the crop experiences

intermittent drought (MacLean et al 2002)

A number of drought resistant cultivarslines of different crops

registered so far in Crop Science or reported in other sources have been

listed in Table 1 These cultivars undoubtedly have been developed

solely using different protocolsdesigns of the conventional breeding

approach These drought tolerant lines of different crops provide a

sound testament that conventional plant breeding played a consider-

able role during the last century not only for improving the quality and

yield of crops but also for improving abiotic stress tolerance including

drought tolerance However now there is a general consensus of the

plant breeders that empirical plant breeding is a highlytime-consuming

as well as a cost- and labor-intensive approach While transferring

desired genes from one plant to other through the conventional plant


















































Bernier et al 2008)













































































Table 1




L Fabaceae subsp










Reddy et al (2001)

Common bean







Singh et al (2001)






Singh et al (2001)


KloudtimesICA 10009



Singh et al (2007)








Brick et al (2008)

Saf 1047298ower



from F11 population





Chickpea

(Cicer arietinum L)


FLIP87



Singh et al (1996)

Wheat





Station Sydney

Cash et al (2009)











TAM 107


Station USA

Quick et al (2001)






Yuncheng China and



Province (TACCCSP)


Tall fescue




clones







Soybean


R01-416F and

R01-581F




Jackson and KS4895


Station USA

Chen et al (2007)

Wheatgrass













Jenson et al (2007)

Barley

(Hordeum vulgare L)









Obert et al (2008)




Por 12762BC


Noaman et al (1995)









Noaman et al (2007)

Giza 121 Line

366131



Delta Region Egypt

Noaman et al (2007)





Lignee 686





Noaman et al (2007)














































































































































































approach













(Gosal et al 2009)











































































































































osmotically









































































































































































standpoint





































under1047297














































investigated

References




















































































(4)651ndash





















20092191ndash










































































Rep 200120205ndash


























































Bernier et al 2008)













































































Table 1




L Fabaceae subsp










Reddy et al (2001)

Common bean







Singh et al (2001)






Singh et al (2001)


KloudtimesICA 10009



Singh et al (2007)








Brick et al (2008)

Saf 1047298ower



from F11 population





Chickpea

(Cicer arietinum L)


FLIP87



Singh et al (1996)

Wheat





Station Sydney

Cash et al (2009)











TAM 107


Station USA

Quick et al (2001)






Yuncheng China and



Province (TACCCSP)


Tall fescue




clones







Soybean


R01-416F and

R01-581F




Jackson and KS4895


Station USA

Chen et al (2007)

Wheatgrass













Jenson et al (2007)

Barley

(Hordeum vulgare L)









Obert et al (2008)




Por 12762BC


Noaman et al (1995)









Noaman et al (2007)

Giza 121 Line

366131



Delta Region Egypt

Noaman et al (2007)





Lignee 686





Noaman et al (2007)














































































































































































approach













(Gosal et al 2009)











































































































































osmotically









































































































































































standpoint





































under1047297














































investigated

References




















































































(4)651ndash





















20092191ndash










































































Rep 200120205ndash












Table 1




L Fabaceae subsp










Reddy et al (2001)

Common bean







Singh et al (2001)






Singh et al (2001)


KloudtimesICA 10009



Singh et al (2007)








Brick et al (2008)

Saf 1047298ower



from F11 population





Chickpea

(Cicer arietinum L)


FLIP87



Singh et al (1996)

Wheat





Station Sydney

Cash et al (2009)











TAM 107


Station USA

Quick et al (2001)






Yuncheng China and



Province (TACCCSP)


Tall fescue




clones







Soybean


R01-416F and

R01-581F




Jackson and KS4895


Station USA

Chen et al (2007)

Wheatgrass













Jenson et al (2007)

Barley

(Hordeum vulgare L)









Obert et al (2008)




Por 12762BC


Noaman et al (1995)









Noaman et al (2007)

Giza 121 Line

366131



Delta Region Egypt

Noaman et al (2007)





Lignee 686





Noaman et al (2007)














































































































































































approach













(Gosal et al 2009)











































































































































osmotically









































































































































































standpoint





































under1047297














































investigated

References




















































































(4)651ndash





















20092191ndash










































































Rep 200120205ndash






















































































































































































approach













(Gosal et al 2009)











































































































































osmotically









































































































































































standpoint





































under1047297














































investigated

References




















































































(4)651ndash





















20092191ndash










































































Rep 200120205ndash




















































































































































































approach













(Gosal et al 2009)











































































































































osmotically









































































































































































standpoint





































under1047297














































investigated

References




















































































(4)651ndash





















20092191ndash










































































Rep 200120205ndash




















































approach













(Gosal et al 2009)











































































































































osmotically









































































































































































standpoint





































under1047297














































investigated

References




















































































(4)651ndash





















20092191ndash










































































Rep 200120205ndash


















































approach













(Gosal et al 2009)











































































































































osmotically









































































































































































standpoint





































under1047297














































investigated

References




















































































(4)651ndash





















20092191ndash










































































Rep 200120205ndash










































































osmotically









































































































































































standpoint





































under1047297














































investigated

References




















































































(4)651ndash





















20092191ndash










































































Rep 200120205ndash


















































































































standpoint





































under1047297














































investigated

References




















































































(4)651ndash





















20092191ndash










































































Rep 200120205ndash
















































































































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under1047297














































investigated

References




















































































(4)651ndash





















20092191ndash










































































Rep 200120205ndash














































































































standpoint





































under1047297














































investigated

References




















































































(4)651ndash





















20092191ndash










































































Rep 200120205ndash




















under1047297














































investigated

References




















































































(4)651ndash





















20092191ndash










































































Rep 200120205ndash



















































(4)651ndash





















20092191ndash










































































Rep 200120205ndash











































































Rep 200120205ndash










Documents

Inducing Dt in Plants Review