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Course ; Principles of disease management

BREEDING FOR DISEASE RESISTANCE

SAJAD UN NABI

Disease resistance

Ability of a host to restrict or even prevent the production of disease symptoms on

invasion of pathogen due to presence of resistance gene.

Types of disease resistance

Vertical resistance:

Controlled by major genes (one or two)

Genes are readily transferred from one genotype to another

Presence of genes can be determined by exposing plants to particular races

Example:

Leaf Rust Resistance in Wheat

Horizontal resistance: Controlled by many genes each with minor effects

Can control a broad range of races due to the fact that many loci are involved

Difficult to transfer resistance from one genotype to another

Individual genes harder to identify

Example:

Fusarium Head Blight in Wheat

Steps in breeding for disease resistance

Source for resistance

Methods of breeding for disease resistance

Testing for disease resistance

Selection for resistant lines

• Known variety

• Wild relatives

• Related species

• Mutations

• Unrelated organisms

Selection

Introduction

Hybridization

• Pedigree method

• Backcross method

• Bulk method

Marker assisted selection

Genetic engineering

For identification of resistant plants from thesuceptible ones an artificial epidemics is created.

Soil borne diseases- sick plots are created fortesting resistance to such diseases.

Air borne diseases-by spraying a suspension ofspores.

Seed borne diseases-dry spores are dusted onseeds or seeds may be soaked in a suspension ofpathogen spores.

Advantages of breeding for disease resistance Resistant varieties offer the cheapest means of disease control. Resistant varieties obviate the use of fungicides ,thus reduce environmentalpollution Effectiveness of resistant varieties is not affected by environmentalconditions. It safeguards against the inadvertent release of such varieties that are mostsuceptible than earlier varieties. In rusts & smuts it is the only feasible approach for disease control.

Problems in breeding for disease resistance Resistance breakdown (vertifolia effect, boom & bust cycle). Horizontal resistance being durable but difficulty relates to an accurate &reliable assessment of the level of resistance. Sometimes there is negative correlation between yield & diseaseresistance.e.g wheat leaf rust gene Lr34 causes a 5% reduction in grain yield. For introgression of multiple resistance in varieties against several diseasesrequires meticulous planning and far greater effort than that required forsingle resistance.

Case study

ObjectiveTo introgress three major BB resistance genes (xa21, xa13, xa5) into

Samba mahsuri an indica rice variety from donor parent (SS1113) in which all three genes are present in a homozygous condition.

Materials and methods

Plant material

Marker assisted selection- Fore ground selection- (25 day old seedling) three STS markers Pta248

,RG136 ,RG556 closely linked to BB resistance genes Xa21,Xa13,Xa5 respectively were used to confirm resistance allele of each gene at each backcross generation.

Background selection-(60 day old seedling) Microsatellite loci that are polymorphic between donor and recurrent parents were identified by screening 139 microsatellite markers distributed through out rice genome.(table .1)

Assessment of BB resistance-Seven Xoo isolates were inoculated under green house conditions for assessment of BB resistance.

Result & discussion

• Total 11 plants out of145 in Bc1F1 werefound to beheterozygous for allthree ‘R’ gene linkedmarkers.These weresubjected to furtherbackgroundselection.

Selected plants having maximum recurrent parent genome werebackrossed to generate BC2F1 plants and process was continued toBC4F1. The number of triple heterozygotesamongst the backcross plants and the maximumamount of estimated recurrent parent genome contributionto the selected triple heterozygote plant at eachbackcross generation is provided in Table 2.

Three ‘R’ genes were inrogressed in Sambamahsuri to develop BB resistant lines that retain theyield & quality characteristics of Samba mahsuri . The derivative lines containing either of the singleresistance gene Xa21 or xa13 also exhibited a goodlevel of resistance, however the derivativescontaining only xa5 gene exhibited a marginal levelof resistance When the inoculated leaves were examined 21days after inoculation, the lesions had expanded inthe single gene (Xa21 or xa13) containing lines andthe two-gene pyramid lines but not in the three-genepyramid lines.

This suggests that there is some kind of quantitativecomplementation where in the presence of multipleresistance genes has an additive effect on the overalllevel of resistance. New set of markers, were identified a total of 122additional microsatellite markers that are polymorphicbetween SS1113 and Samba mahsuri, from which veryfew markers were used in the background selection. Improved Samba mahsuri was developed havingpresence all three resistant genes and became resistantto BLB.

Comparison

Susceptible Samba mahsuri