OsPIP2;1 impacts root hydraulic conductance and is a candidate gene for a drought avoidance QTL on rice chromosome 7

Mapping drought avoidance qualitative trait loci (QTLs) in the Bala × Azucena population has revealed a locus on chromosome 7 where the Azucena allele increases leaf rolling but reduces leaf drying. A cluster of four aquaporin genes OsPIP2;1, OsPIP2;4, OsPIP2;5, OsPIP2;9 co-locate with this QTL. The hypothesis that this gene cluster and the QTL are functionally related was tested, assuming that aquaporin would be linked to drought avoidance via an impact on root hydraulic conductance. Genetic differences in these genes was assessed bioinformatically. Physiological assessment of root hydraulic properties was carried out on near-isogenic lines and RNAi knockdowns of OsPIP2;1. For this, a two-plant pressure chamber was used and a novel technique for assessing root hydraulic flow using osmotic swelling of roots was developed. Bala has relatively rare haplotypes for these PIPs that are associated with the aus subgroup of rice, which it shares with the aus cultivar N22 from which it was directly derived in breeding. Allelic variation in root hydraulic flow and conductance was revealed. Notably, the allelic difference in hydraulic flow/conductance was only strongly evident when plants were droughted (e.g. with Azucena allele having an average 70% lower hydraulic flow than the Bala allele). RNAi lines with reduced expression of OsPIP2;1 had a commensurate reduction in root hydraulic flow and conductance. These studies confirm the role of OsPIP2;1 in root hydraulic flow in rice, and highlight the presence of aus-specific allelic variation that appears to impact root hydraulic conductance and drought avoidance in rice.