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

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.