Elucidating the physiological and molecular nature of drought resistance QTLs in rice
Drought is an important abiotic stress that has limited rice yield in rainfed drought-prone ecosystems. Its complexity has slowed progress in cultivar improvement. Recent advances in the identification of quantitative trait loci (QTLs) governing the complex mechanisms of drought resistance have been facilitated by the development of molecular markers in rice. The present study focused on elucidating the causative physiology and genetics of drought avoidance-related QTL on chromosome 7 and others, which have been previously revealed in the Bala x Azucena mapping population under field drought environments. Here the approaches conducted are (a) QTL mapping for morpho-physiological traits in controlled glasshouse conditions, (b) the development and testing of recombinant inbred near isogenic lines (RINILs) for the drought-avoidance QTL on chromosome 7 and (c) testing a positional candidate gene for that QTL using bioinformatics, physiological measurements and gene expression analysis. From the QTL data and comparative analysis of the results, regions on chromosomes 1, 3, 5 and 7 appear noteworthy in drought avoidance and can be considered candidates for marker-assisted selection. RINILs on chromosome 7 confirmed that the QTL improves plant water status (relative water content) under drought stress. Testing RINILs for a root growth QTL on chromosome 9 also confirmed original results of QTL mapping. Differences between parental varieties Bala and Azucena in root hydraulic flow and the expression of OsPIP2a, an aquaporin known to improve hydraulic conductivity under stress, together with evidence of non-synonymous sequence differences in the gene indicate that OsPP2a is a promising candidate gene for the chromosome 7 QTL. This can be tested in the future using RINILs developed here.