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. [ABSTRACT FROM AUTHOR]