Two types of HKT transporters with different properties of Na⁺ and K⁺ transport in <i>Oryza sativa</i>

Abstract

Summary It is thought that Na + and K + homeostasis is crucial for salt‐tolerance in plants. To better understand the Na + and K + homeostasis in important crop rice ( Oryza sativa L.), a cDNA homologous to the wheat HKT1 encoding K + ‐Na + symporter was isolated from japonica rice, cv Nipponbare (Ni‐ OsHKT1 ). We also isolated two cDNAs homologous to Ni‐ OsHKT1 from salt‐tolerant indica rice, cv Pokkali (Po‐ OsHKT1 , Po‐ OsHKT2 ). The predicted amino acid sequence of Ni‐OsHKT1 shares 100% identity with Po‐OsHKT1 and 91% identity with Po‐OsHKT2, and they are 66–67% identical to wheat HKT1. Low‐K + conditions (less than 3 m m ) induced the expression of all three OsHKT genes in roots, but mRNA accumulation was inhibited by the presence of 30 m m Na + . We further characterized the ion‐transport properties of OsHKT1 and OsHKT2 using an expression system in the heterologous cells, yeast and Xenopus oocytes. OsHKT2 was capable of completely rescuing a K + ‐uptake deficiency mutation in yeast, whereas OsHKT1 was not under K + ‐limiting conditions. When OsHKTs were expressed in Na + ‐sensitive yeast, OsHKT1 rendered the cells more Na + ‐sensitive than did OsHKT2 in high NaCl conditions. The electrophysiological experiments for OsHKT1 expressed in Xenopus oocytes revealed that external Na + , but not K + , shifted the reversal potential toward depolarization. In contrast, for OsHKT2 either Na + or K + in the external solution shifted the reversal potential toward depolarization under the mixed Na + and K + containing solutions. These results suggest that two isoforms of HKT transporters, a Na + transporter (OsHKT1) and a Na + ‐ and K + ‐coupled transporter (OsHKT2), may act harmoniously in the salt tolerant indica rice.