OsHKT1;5 mediates Na⁺ exclusion in the vasculature to protect leaf blades and reproductive tissues from salt toxicity in rice

Abstract

Summary Salt tolerance quantitative trait loci analysis of rice has revealed that the SKC 1 locus, which is involved in a higher K + /Na + ratio in shoots, corresponds to the Os HKT 1;5 gene encoding a Na + ‐selective transporter. However, physiological roles of Os HKT 1;5 in rice exposed to salt stress remain elusive, and no Os HKT 1;5 gene disruption mutants have been characterized to date. In this study, we dissected two independent T‐ DNA insertional Os HKT 1;5 mutants. Measurements of ion contents in tissues and 22 Na + tracer imaging experiments showed that loss‐of‐function of Os HKT 1;5 in salt‐stressed rice roots triggers massive Na + accumulation in shoots. Salt stress‐induced increases in the Os HKT 1;5 transcript were observed in roots and basal stems, including basal nodes. Immuno‐staining using an anti‐Os HKT 1;5 peptide antibody indicated that Os HKT 1;5 is localized in cells adjacent to the xylem in roots. Additionally, direct introduction of 22 Na + tracer to leaf sheaths also demonstrated the involvement of Os HKT 1;5 in xylem Na + unloading in leaf sheaths. Furthermore, Os HKT 1;5 was indicated to be present in the plasma membrane and found to localize also in the phloem of diffuse vascular bundles in basal nodes. Together with the characteristic 22 Na + allocation in the blade of the developing immature leaf in the mutants, these results suggest a novel function of Os HKT 1;5 in mediating Na + exclusion in the phloem to prevent Na + transfer to young leaf blades. Our findings further demonstrate that the function of Os HKT 1;5 is crucial over growth stages of rice, including the protection of the next generation seeds as well as of vital leaf blades under salt stress.