OsHKT1;5 mediates Na+ exclusion in the vasculature to protect leaf blades and reproductive tissues from salt toxicity in rice
Kobayashi, NI and Yamaji, N and Yamamoto, H and Okubo, K and Ueno, H and Costa, A and Tanoi, K and Matsumura, H and Fujii-Kashino, M and Horiuchi, T and Al Nayef, M and Shabala, S and An, G and Ma, JF and Horie, T, OsHKT1;5 mediates Na+ exclusion in the vasculature to protect leaf blades and reproductive tissues from salt toxicity in rice, Plant Journal, 91, (4) pp. 657-670. ISSN 0960-7412 (2017) [Refereed Article]
Salt tolerance quantitative trait loci analysis of rice has revealed that the SKC1 locus, which is involved in a higher K+/Na+ ratio in shoots, corresponds to the OsHKT1;5 gene encoding a Na+-selective transporter. However, physiological roles of OsHKT1;5 in rice exposed to salt stress remain elusive, and no OsHKT1;5 gene disruption mutants have been characterized to date. In this study, we dissected two independent T-DNA insertional OsHKT1;5 mutants. Measurements of ion contents in tissues and 22Na+ tracer imaging experiments showed that loss-of-function of OsHKT1;5 in salt-stressed rice roots triggers massive Na+ accumulation in shoots. Salt stress-induced increases in the OsHKT1;5 transcript were observed in roots and basal stems, including basal nodes. Immuno-staining using an anti-OsHKT1;5 peptide antibody indicated that OsHKT1;5 is localized in cells adjacent to the xylem in roots. Additionally, direct introduction of 22Na+ tracer to leaf sheaths also demonstrated the involvement of OsHKT1;5 in xylem Na+ unloading in leaf sheaths. Furthermore, OsHKT1;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 22Na+ allocation in the blade of the developing immature leaf in the mutants, these results suggest a novel function of OsHKT1;5 in mediating Na+ exclusion in the phloem to prevent Na+ transfer to young leaf blades. Our findings further demonstrate that the function of OsHKT1;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.
HKT, Na+ exclusion, Oryza sativa, phloem, salt tolerance, xylem