Abstract Abiotic stresses including salinity are a major threat to agricultural pro-ductivity and hence global food security. Crop plants have adopted specialized strategies to reduce the impact of stress. The biogenic volatile organic compounds (VOCs) emitted from a wide range of plants help enable the buildup defense against biotic and abiotic stresses. Plant VOCs are comprised of different isoprene and monoterpene class of compounds in addition to alkanes, alkenes, carbonyls, alcohols, esters, ethers, and acids which have a demonstrated role against abiotic stress factors. Although it has been shown that several metabolic pathways may be involved in building up the defense, antioxidant route of alleviation is believed to be a common mechanism. The identification of the genes, transcriptomic profiling and proteins of the biosynthetic pathway has enabled ways to manipulate the syn-thesis of isoprenoid compounds. In recent years, there has been a growing interest in adopting VOC strategy to alleviate abiotic stresses in crop plants.
Environmental stress is a major threat to agricultural productivity and plants have adopted specialized strategies to reduce the impact of stress. The abiotic stresses include drought, salinity, cold and high temperature that affect the plant growth, de-velopment and yields of crop plants. Plants being sessile, experience multiple stress-es in their life cycle and hence the tolerance trait has become complex to be under-stood and managed. Among the different abiotic stresses, salinity stress is the most severe limiting crop productivity. Salinity interferes with the plants accessibility to
K. R. Hakeem et al. (eds.), Crop Improvement, DOI 10.1007/978-1-4614-7028-1_9, Springer Science+Business Media, LLC 2013
P. Suprasanna ()Nuclear Agriculture and Biotechnology Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, Indiae-mail: firstname.lastname@example.org
P. S. VariyarFood Technology Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, Indiae-mail: email@example.com
nutrients and water. Moreover, it induces osmotic stress; the physiological drought, which typically reduces the growth and photosynthesis in plants (Munns and Tester 2008). Salinity affects plant growth and development in two ways: through osmotic stress by reducing the soil water potential leading to limiting the water uptake and by causing uptake of Na+ and Cl which have an effect on plant metabolism. The mechanism by which plants perceive stress signals and relay their transmission to cellular machinery to trigger adaptive responses is crucial for the improvement of different strategies to impart stress tolerance in crops (Mantri et al. 2012).
The different abiotic stress factors result in the production of reactive oxygen species (ROS) that are extremely reactive and cause damage to biological macro-molecules like proteins, lipids, carbohydrates and DNA ultimately leading to oxi-dative stress. The ROS include, superoxide radicals, hydroxyl radical, perhydroxy radical, alkoxy radicals, hydrogen peroxide and singlet oxygen (Gill and Tuteja 2010). Under normal growth conditions, the ROS molecules are managed by effi-cient scavenging machinery consisting of various antioxidative defense mechanisms (Foyer and Noctor 2005). The production of ROS and their scavenging needs to be balanced under normal conditions of growth but, however the equilibrium is dis-turbed by abiotic stress factors including salinity (Tuteja 2007; Mantri et al. 2012).
Plants are sessile and have to encounter challenges imposed by other organisms and with the environment mainly by depending on their chemical repertoire. The signif-icance of natural products and their metabolic diversity contribute very much to the survival of the plant kingdom. The biogenic volatile organic compounds (VOCs) released from a wide range of plants help enable the buildup defense against insects, fungi, herbivores and environmental changes (Loreto and Schnitzler 2010; Holo-painen and Blande 2012). Plant VOCs are comprised of isoprenoids mainly isoprene and monoterpenes (Variyar et al. 2010). The function of isoprenoid compounds dur-ing environmental stress includes protection of the photosynthetic apparatus, de-toxification from free radicals and reactive oxygen species (ROS) (Munn-Bosch and Alegre 2000a; Spinelli et al. 2011). Although it has been shown that several metabolic pathways may be involved in building up the defense, antioxidant route is believed to be the common mechanism (Vickers et al. 2009a). The identification of genes in the biosynthetic pathway and transcriptomic profiling has enabled ways to manipulate the synthesis of isoprenoid compounds. Since chloroplasts are the sites of isoprene synthesis a possible relation may occur between isoprene production and environmental stresses affecting the photosynthetic apparatus (Velikova 2008; Loyola et al. 2012). It should thus be of interest to investigate isoprene synthesis in plants in relation to environmental chemistry. The emission of VOCs contributes to an appreciable quantity of photosynthetic carbon fixation under stress conditions, and hence VOCs could also play a significant role in the carbon exchange between the biosphere and the atmosphere (Guenther et al. 2011). Significant research prog-
P. Suprasanna and P. S. Variyar
ress has been made in the study of physiological mechanism(s) underlying iso-prenoid synthesis under abiotic stress conditions, especially high temperatures and oxidative stress conditions (Fineschi and Loreto 2012).
Isoprenoids protect plants against different abiotic stresses through improving the ability of plants to deal with cellular oxidative modifications, possibly through reaction of isoprenoids with the oxidizing species, or alteration of ROS signaling, or via membrane stabilization. It is postulated that dissolution of VOCs in membranes coupled to interactions with membrane proteins can lead to changes in transmem-brane potential and modulation of ion fluxes thereby inducing gene activity and a subsequent cellular response to stress (Vickers et al. 2009a). Plants have developed an efficient antioxidant mechanisms for ROS detoxification (Ahmad et al. 2008; Gill and Tuteja 2010; Ahmad and Umar 2011). Isoprenes can boost plants defense system not only by keeping the membrane integrity intact and making it less sensi-tive to denaturation, but also due to the fact that they have the capacity to quench ROS produced under oxidative stress. Vickers et al. (2009a) discussed the possible functions of isoprenes as natural antioxidant machinery in plants.
Plants are endowed with protective mechanisms to cope with a variety of abiotic stresses. When the stress impact goes beyond a certain threshold, plants normally experience stress, resulting in reduced growth and development. Most common and ensuing response, thus, is the production of reactive oxygen spices (ROS). The antioxidant effect of the isoprenoid compounds is mediated by their capacity to swiftly combine with different ROS such as singlet oxygen, superoxide, hydro-gen peroxide, hydroxyl radical that are released under stress regime (Holopainen 2004; Fineschi and Loreto 2012). Isoprenes are also known to alleviate visible dam-age (necrosis) of leaves exposed to ozone through a mechanism involving release of nitric oxide that interacts with increasing levels of ROS especially hydrogen peroxide. The occurrence of conjugated double bonds (delocalized -electrons) in the isoprene molecule may mediate electron and energy transfers, conferring ROS-scavenging ability (Vickers et al. 2009a). Considering chloroplast as the site of iso-prene biosynthesis (Logan et al. 2000), the ROS scavenging ability of isoprene molecule makes it important in plant defense against oxidative stress. Isoprenoids including terpenoids have also been shown to confer a protective effect on photo-synthetic process under heat and oxidative stress (Sharkey and Yeh 2001). Isoprenes have also been implicated to protect the photosynthetic system from thermal stress. The mechanism underlying such protective nature is attributed to the stabilization of membrane lipid bilayer by enhancing the hydrophobic (lipidlipid, lipidpro-tein and/or proteinprotein) interactions (Sharkey et al. 2008). Based on modeling studies with membranes, Siwko et al. (2007) demonstrated that isoprenes are able to partition into the phospholipid membrane enhancing membrane order without major alteration in the dynamic properties of the membrane.
Much less evidence has been accumulated so far on the role of volatile monoter-penes in alleviating oxidative stress. In plants that dont emit monoterpenes, it has been proved that photosynthesis becomes less sensitive to ozone that are externally supplied with volatile monoterpenes (Loreto and Fares 2007). In contrast, when monoterpene synthesis is blocked, ROS rapidly accumulate. The highly volatile
9 Coping Abiotic Stress with Plant Volatile Organic Chemicals (PVOCs)
Table9.1 List of cloned genes involved in the biosynthesis of monoterpenes
Gene Organism ReferencesLinalool synthase Clarkia breweri; Arte-
misia annua L.Dudareva et al. (1996); Cseke
et al. (1998); Jia et al. (1999)
(-)-Limonene synthase Abies grandis; Mentha spicata
Colby et al. (1993); Bohlmann et al. (1997)
( + )-Limonene synthase Agastache rugoa Maruyama et al. (2002)
(-)-Pinene synthase Abies grandis Bohlmann et al. (1997)Myrcene synthase Abies grandis, Quercus
ilex L.; Arabidopsis thaliana
Bohlmann et al. (1997); Bohlmann et al. (2000); Fischbach et al. (2001)
-Ocimene synthase Arabidopsis thaliana Bohlmann et al. (2000)( +)-Bornyl diphosphate synthase,
1,8 cineole synthase and ( + )-Sabinene synthase
Salvia offcinalis Wise et al. (1998)
(-)--Phellandrene synthase, (-)-camphene synthase, Terpi-nolene synthase and (-)- limo-nene/(-)--pinene synthase
Abies grandis Bohlmann et al. (1999)
(E)-Beta farnesene synthase Citrus junos Maruyama et al. (2001)
monoterpenes exhibit more effectiveness in scavenging antioxidants. On the other hand, the less volatile isoprenes pool up in membrane and intercellular spaces and thus become more effective antioxidants in the aqueous phase. Volatile sesqui-terpenes are produced in high levels in ozone-resistant tobacco upon exposure to ozone. It is thus possible that volatile isoprenoids constitute one of the non-enzy-matic oxidative defense systems thereby, reducing the oxidative damage caused by abiotic stresses.
Monoterpenes have different effects on plant growth and development, depend-ing on their structure and the quantity. Thus -pinene exerts protective effect on the photosynthetic apparatus, while -terpinol shows toxicity. Monoterpenes exog-enously applied at levels of 0.5 g/l exhibited toxicity in plant cell cultures (Brown et al. 1987). Monoterpenes such as cineole, thymol, geraniol, menthol and camphor induced oxidative stress and lipid oxidation in maize roots (Zunino and Zygadlo 2004) while, -myrcene, limonene, -ocimene and -terpinene generated ROS and oxidative damage (Singh et al. 2009). Menthol has shown an increase in cytosolic free calcium ions which can generate signal transduction pathways in cucumber roots (Maffei et al. 2001). The aliphatic monoterpenes (ocimene and myrcene) in-duced considerable changes in the transcription of several hundred genes in Ara-bidopsis, many of them are designated as transcription factors, stress and defence genes (Godard et al. 2008). Several genes involved in the biosynthesis of terpenes have now been cloned in different plants (Phillips et al. 2006; Christianson 2006; Degenhardt et al. 2009). Table 9.1 lists some of the genes involved in the synthesis of monoterpenes.
P. Suprasanna and P. S. Variyar
Salt stress is known to mimic water stress limiting CO2 inflow by lowering con-ductance of stomata and mesophyll and by impairing carbon metabolism (Delfine et al. 1998, 1999). Loreto and Delfine (2000) tested whether revival from mod-est salt treatment could result in bursts of isoprene emission and concluded that the progression leading to isoprene release is resistant than photosynthesis to salt stress, and that a secondary source of isoprene, independent of photosynthesis, is induced by salt-stress. In case of short-term drought stress, significant reductions in photosynthesis were observed, whereas isoprene emission was either not repressed or became reduced in Quercus virginiana (Tingey et al. 1981) and Pueraria lo-bata (Sharkey and Loreto 1993). On the other hand, there was a good relationship between terpene emission and plant water status. The emission of several mono-terpenes and sesquiterpenes was studied in Mediterranean species (Rosmarinus of-ficinalis, Pinus halepensis, Cistus albidus and Quercus coccifera) upon subjecting them to long term water dehydration stress (Ormeno et al. 2007). There was a slow decrease of emissions in plants exposed to long term water deficit periods in P. halepensis and C. albidus as compared to decrease in sesquiterpene release of R. officinali. impraga et al. (2011) opined that drought stress can affect the VOC emissions in plants. In their experiments with young Common beech, the authors observed sudden burst of non-monoterpene class of VOCs during acute drought stress indicating opportunities for plant sensing using VOCs.
Isoprenoids have been demonstrated to confer defense against abiotic stress fac-tors, mainly thermal stress and oxidative stress conditions. A full understanding of the function of terpenes in plant defense process will require experiments at the molecular level, as terpenes may induce the expression of a number of stress-related genes. Studies in this direction by using inhibitors like fossidomycin that can inhibit the MEP pathway, fumigating non-isoprene synthesizing plants with exogenous iso-prenoid compounds and transgenic plants either expressing terpene synthesis genes or gene silencing, have yielded results supporting their protection against stresses (Dudareva and Pichersky 2008; Vickers et al. 2009a).
The enzymes leading to the production of monoterpene all appear to be active in the plastids, as all the genes in this pathway possess plastid-targeting signals (Haudenschild and Croteau 1998) and seems to be localized in chloroplasts (Bou-vier et al. 2000) and leucoplasts (Turner et al. 1999). The principal functional role of isoprene emission in plants is associated with the protection of leaf physiological processes against oxidative stress induced by heat (Sharkey and Yeh 2001). Behnke et al. (2007) analyzed this physiological role by testing transgenic Grey poplar plants in which expression of isoprene synthase (ISPS) was either silenced via RNA interference (RNAi) mechani...