Breeding for drought resistance in rice: an integrated view from physiology to genomics

Drought is a major constraint affecting rice production, especially in rainfed areas across Asia, Africa and South America. Development of drought resistant varieties will considerably improve rainfed rice production. Rice breeding programs focusing on drought resistance by direct selection for yield under stress have made little progress to date. Incorporation of secondary traits contributing in drought resistance will hasten development of cultivars with improved performance under water-limiting environments. Root traits such as thickness, depth and penetration ability help to avoid drought by increased water uptake from deeper soils.Greater hydraulic conductance, xylem thickness and osmotic adjustment are secondary root traits enable better extraction of available soil moisture. However, use of most root traits as selection indices in breeding programs using phenotypic measurements still entails huge investments in field nurseries or green house facilities, demanding in labour and is prone to problems of repeatability due to environmental variability. One promising approach is to map genetic loci (quantitative trait loci/QTLs) linked to root traits and use marker-assisted breeding (MAB) strategy. QTLs for many drought resistance traits consistent across environments and genetic backgrounds have been mapped in rice. Some of these QTLs co-locate with QTLs for yield and yield components under stress and are introgressed into elite lines using MAB. The introgression lines performed better under drought are released as high yielding varieties suitable for rainfed ecosystems.