The Ability to Regulate Transmembrane Potassium Transport in Root Is Critical for Drought Tolerance in Barley

In this work, the effect of drought on K⁺ uptake in root and its translocation from root to shoot was investigated using six barley genotypes contrasting in drought tolerance. Results showed that drought conditions caused significant changes in K⁺ uptake and translocation in a time- and genotype-specific manner, which consequently resulted in a significant difference in tissue K⁺ contents and drought tolerance levels between the contrasting barley genotypes. The role of K⁺ transporters and channels and plasma membrane (PM) H⁺-ATPase in barley's adaptive response to drought stress was further investigated at the transcript level. The expression of genes conferring K⁺ uptake (HvHAK1, HvHAK5, HvKUP1, HvKUP2 and HvAKT1) and xylem loading (HvSKOR) in roots were all affected by drought stress in a time- and genotype-specific manner, indicating that the regulation of these K⁺ transporters and channels is critical for root K⁺ uptake and root to shoot K⁺ translocation in barley under drought stress. Furthermore, the barley genotypes showed a strong correlation between H⁺ efflux and K⁺ influx under drought stress, which was further confirmed by the significant up-regulation of HvHA1 and HvHA2. These results suggested an important role of plasma membrane H⁺-ATPase activity and/or expression in regulating the activity of K⁺ transporters and channels under drought stress. Taken together, it may be concluded that the genotypic difference in drought stress tolerance in barley is conferred by the difference in the ability to regulate K⁺ transporters and channels in root epidermis and stele.