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

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 mm) induced the expression of all three OsHKT genes in roots, but mRNA accumulation was inhibited by the presence of 30 mm 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.