BY: MUHAMMAD FAROOQ
Plants tend to reduce heat-induced damage by leaf rolling, leaf shedding, reducing leaf size, thickening leaves, reducing growth duration, transpirational cooling and other adjustments in morphology and ontogeny (Wahid et al., 2007). Plant responses to heat stress are mediated by an intrinsic capacity to endure basal thermotolerance and, after acclimation, the ability to gain thermotolerance. The capability of crop plants to survive and produce good grain yield under heat stress is generally regarded as heat tolerance (Wahid et al., 2007).
A. Antioxidant Defense System
Reactive oxygen species (ROS),—superoxide radicals, hydroxyl radicals, and hydrogen peroxide—are produced in the cells in a natural fashion, but overproduction of these compounds can be harmful (Esfandiari et al., 2007). Heat stress triggers the production and accumulation of ROS (Almeselmani et al., 2009). Hence their detoxification by antioxidant systems is important for protecting plants against heat stress (Asada, 2006). The antioxidant defense system in plants involves both enzymatic and non-enzymatic antioxidant systems. The enzymatic antioxidant system includes ascorbate peroxidase, dehydroascorbate reductase, glutathione S-transferase, superoxide dismutase, catalase, guaiacol peroxidase, and glutathione. Non-enzymic antioxidants
include glutathione, ascorbate and tocopherols. Superoxide dismutase converts O− 2 to hydrogen peroxide, whereas catalase and peroxidases breakdown hydrogen peroxide. Catalase eliminates hydrogen peroxide by catalyzing its decomposition to H2O and O2. Both guaiacol peroxidase and ascorbate peroxidase can detoxify hydrogen peroxide, but both enzymes need hydrogen peroxide to be scavenged by reducing agents. Ascorbate helps scavenge OH, O− 2 and hydrogen peroxide for the ascorbate-peroxidase-mediated reactions, while guaiacol scavenges ROS for guaiacol peroxidise–mediated reactions (Goyal and Asthir, 2010). Balla et al. (2009) demonstrated that upon exposure to heat stress, during the reproductive phase, activities of enzymatic antioxidants were substantially increased in heat-tolerant genotypes of wheat. The activities of catalase and superoxide dismutase have been correlated with heat stress (34/22◦C) during the reproductive phase (Zhao et al., 2007), as well as the capacity to acquire thermotolerance (Almeselmani et al., 2009). Likewise, protection of wheat plants from heat induced oxidative damage during the reproductive phase has also been correlated with non enzymic antioxidants, such as ascorbate (Sairam et al., 2000).
B. Molecular Basis of Heat Tolerance
Expression of heat shock proteins (HSPs) is the most studied molecular response under heat stress. HSPs save proteins from heat-induced aggregation and thus during the recovery period, facilitates their re-folding ( Maestri et al., 2002). Expression of HSP genes is a fundamental response to heat stress (Rampino et al., 2009).When exposed to high temperature (>35◦C), normal protein synthesis in wheat is reduced, but HSPs are produced. In wheat genotypes grown at 32 to 35◦C, Nguyen et al. (1994) detected messenger RNAs encoding a major class of low molecular weight HSPs—HSP 16.9. In another study, where several wheat varieties were exposed to 3.2 to 3.6◦C higher than normal, HSP 18 accumulated in developing grains of heattolerant varieties more than susceptible types (Sharma-Natu et al., 2010). Sumesh et al. (2008) also observed higher HSP 100 content at elevated temperature in a relatively tolerant variety. Dehydrin proteins belong to Group 2 late embryogenesis abundant proteins (LEA). Dehydrins help to stabilize macromolecules against heat-induced damage (Brini et al., 2010). In wheat, for instance, DHN-5 protein helped protect and stabilize key enzymes to start metabolism (Brini et al., 2010).
Leaf senescence starts early in response to heat stress, particularly when these stresses occur during post-flowering stages of grain filling. Therefore, maintenance of leaf chlorophyll and photosynthetic capacity, called ‘stay-green,’ is considered an indicator of heat tolerance (Fokar et al., 1998). Because the loss of chlorophyll is associated with less assimilation of current carbon into grains (see above), stay-green genotypes should be better able to maintain grain filling under elevated temperatures. Certain stay-green sorghum genotypes have been found to contain higher specific leaf nitrogen contents, indicating that this trait is correlated with shoot nitrogen content (Borrell et al., 2001). The stay-green trait has been evaluated in several crops (Harris et al., 2007), but breeding for this trait has been limited in wheat. Although three Components—chlorophyll content at anthesis, duration of senescence, and rate of senescence determine the stay-green feature during heat stress, the rate of senescence, not the start of senescence, is important component of stay-green (Harris et al., 2007).
Almeselmani, M., Deshmukh, P. S., and Sairam, R. K. 2009. High temperature stress tolerance in wheat genotypes: Role of antioxidant defence enzymes. Acta Agron. Hungar. 57: 1–14.
Asada, K. 2006. Production and scavenging of reactive oxygen species in chloroplasts and their functions. Plant Physiol. 141: 391–396.
Balla, K., Bencze, S., Janda T., and Veisz, O. 2009. Analysis of heat stress tolerance in winter wheat. Acta Agron. Hungar. 57: 437–444.
Borrell, A. K., Hammer, G., and van Oosterom, E. 2001. Stay-green: A consequence of the balance between supply and demand for nitrogen during grain filling. Ann. Appl. Biol. 138: 91–95.
Brini, F., Saibi,W., Amara, I., Gargouri, A., Masmoudi, K., and Hanin,M. 2010. Wheat dehydrin dhn-5 exerts a heat-protective effect on β-glucosidase and glucose oxidase activities. Biosci. Biotechnol. Biochem. 74: 1050–1054.
Esfandiari, E., Shekari, F., Shekari, F., and Esfandiar,M. 2007. The effect of salt stress on antioxidant enzymes activity and lipid peroxidation on the wheat seedlings. Not. Bot. Hort. Agrobot. Cluj. 35: 48–56.
Fokar, M., Blum, A., and Nguyen, H. T. 1998. Heat tolerance in spring wheat. II. Grain filling. Euphytica 104: 9–15.
Goyal, M. and Asthir, B. 2010. Polyamine catabolism influences antioxidative defense mechanism in shoots and roots of five wheat genotypes under high temperature stress. Plant Growth Regul. 60: 13–25.
Harris, K., Subudhi, P. K., Borrell, A., Jordan, D., Rosenow, D., Nguyen, H. T., Klein, P., Klein, R., and Mullet, J. 2007. Sorghum stay-green QTL individually reduce post-flowering drought-induced leaf senescence. J. Exp. Bot. 58: 327–338.
Wahid, A., Gelani, S., Ashraf, M., and Foolad, M. R. 2007. Heat tolerance in plants: an overview. Environ. Exp. Bot. 61: 199–223.
Maestri, E., Klueva, N., Perrotta, C., Gulli, M., Nguyen, H. T., and Marmiroli, N. 2002. Molecular genetics of heat tolerance and heat shock proteins in cereals. Plant Mol. Biol. 48: 667–681.
Nguyen, H. T., Joshi, C. P., Kluev, N., Weng, J., Hendershot K. L., and Blum, A. 1994. The heat-shock response and expression of heat-shock proteins in wheat under diurnal heat stress and field conditions. Aust. J. Plant Physiol. 21: 857–867.
Sairam, R. K., Srivastava, G. C., and Saxena, D. C. 2000. Increased antioxidant activity under elevated temperatures: a mechanism of heat stress tolerance in wheat genotypes. Biol. Plant. 43: 245–251.
Sharma-Natu, P., Sumesh, K. V., and Ghildiyal, M. C. 2010. Heat shock protein in developing grains in relation to thermotolerance for grain growth in wheat. J. Agron. Crop Sci. 196: 76–80.
Sumesh, K. V., Sharma-Natu, P., and Ghildiyal, M. C. 2008. Starch synthase activity and heat shock protein in relation to thermal tolerance of developing wheat grains. Biol. Plant. 52: 749–753.
Rampino, P., Mita, G., Pataleo, S., Pascali, M. D., Fonzo, N. D., and Perrotta, C. 2009. Acquisition of thermotolerance and HSP gene expression in durum wheat (Triticum durum Desf.) cultivars. Environ. Exper. Bot. 66: 257–264.
Zhao, H., Dai, T. B., Jing, Q., Jiang, D., and Cao, W. X. 2007. Leaf senescence and grain filling affected by post-anthesis high temperatures in two different wheat cultivars. Plant Growth Regul. 51: 149–158.