Recent advances in molecular tool development for drought tolerance breeding in cereal crops: a review

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Abstract Drought stress is one of the major yield constraints for cereal crops. Traditionally, for developing drought tolerant cultivars, selection either direct or indirect is practiced. Although this approach is effective, yet time consuming and labour intensive. Identification of drought related quantitative trait loci (QTLs) coupled with marker assisted selection has shown some positive results. Transgenic and “omics” technologies promise to make progress in breeding for drought tolerance through a more fundamental understanding of underlying mechanisms of drought tolerance and identifying potential candidate genes. These new approaches provide opportunities to direct the continued breeding of genotypes giving stable yields under drought stress. Key words: drought tolerance, marker assisted selection, omics, quantitative trait loci, transgenics. Introduction Cereal crops have been predominantly used as staple food around the globe since the time immemorial. However, our efforts in meeting the food requirements on sustainable basis for ever increasing population are seriously being hampered due to biotic and abiotic stresses. Abiotic factors (like drought, salinity and temperature), are most important accounting for about 70% reductions in yield of our cereal crops (Boyer, 1982). Drought is a major yield limiting factor badly affecting the crop productivity worldwide (Hussain et al., 2011). The simplest definition of drought in the context of agriculture is a situation when the water availability to plant is less than what is required to sustain its growth and development (Deikman et al., 2011). Drought tolerance is the ability of the plant to survive in water limited conditions (Turner, 1979). However, inducing drought tolerance in crop plants is not a simple task rather one of the most difficult challenges currently the breeders face. This is due to its polygenic nature with low heritability and high G × E interactions (Fleury et al., 2010). This complex nature and also the lack of proper understanding of the underlying mechanisms of drought tolerance explain the slow progress in improving the yield of crops in drought prone environments (Tuberosa, Salvi, 2006; Cattivelli et al., 2008). Plants use different mechanisms to cope with the drought. Like they use the escape strategy through accelerating flowering before the onset of drought season (Deikman et al., 2011), or improve the water use efficiency (WUE) through closing stomata and increasing root development (Gowda et al., 2011) or can also use some tolerance mechanisms such as osmotic adjustment and production of antioxidants (Umezawa etal., 2006), yet there is no unified drought mechanism at the whole plant or gene level (Blum, 2004). This review encompasses an overview on the current work reported in inducing drought tolerance in cereal crops mainly wheat, rice and maize, highlighting the role of conventional breeding in conjunction with marker assisted selection, transgenic approaches and modern genomic tools.