Breeding for Drought Resistance in Rice: Physiology and Molecular Genetics Considerations

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Abstract
Drought stress is a major constraint to rice (Oryza sativa L.) production and yield stability in many rainfed regions of Asia, Africa, and South America. The genetic improvement of adaptation to drought is addressed through the conventional approach by selecting for yield and its stability over locations and years. Because of low heritability of yield under stress and inherent variation in the field, such selection programs are expensive and slow in attaining progress. The objective of this paper is to review the current knowledge in physiology and molecular genetics which have implications in breeding for drought resistance in rice. The ability of root systems to provide for evapotranspirational demand from deep soil moisture and capacity for osmotic adjustment are considered major drought resistance traits in rice. However, selection for these traits still entail extensive investments in field nurseries or greenhouse facilities and is prone to problems of repeatability because of environmental variability. Recent development of molecular linkage maps of rice and other advances in molecular biology offer new opportunities for drought resistance breeding. Molecular markers linked to root traits and osmotic adjustment are being identified, which should lead to marker‐assisted selection. Transgenic rice plants having tolerance to water deficit and osmotic stresses have been reported. Work on genetic engineering of osmoprotectants, such as proline and glycine betaine, into the rice plant for drought tolerance improvement is in progress. Close collaboration between molecular geneticists, plant physiologists, and breeders is needed to critically assess the contribution of specific genes and application of molecular genetics to breeding for drought resistance in rice and other crops.