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

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 r ice (Oryza sativa L.), a cDNA homologous to the wheat HKT1 encoding K+-Nasymporter was isolated from japonica rice, ev Nipponbare (Ni-OsHKT1). We al so isolated two cDNAs homologous to Ni-OsHKT1 from salt-tolerant indica ric e, 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-OsHK T2, and they are 66-67% identical to wheat HKT1. Low-K+ conditions (less th an 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 charac terized the ion-transport properties of OsHKT1 and OsHKT2 using an expressi on system in the heterologous cells, yeast and Xenopus oocytes. OsHKT2 was capable of completely rescuing a K+-uptake 'deficiency mutation in yeast, w hereas OsHKT1 was not under K+-limiting conditions. When OsHKTs were expres sed in Na+-sensitive yeast, OsHKT1 rendered the cells more Na+-sensitive th an did OsHKT2 in high NaCl conditions. The electrophysiological experiments for OsHKT1 expressed in Xenopus oocytes revealed that external Na+, but no t K+, shifted the reversal potential toward depolarization. In contrast, fo r OsHKT2 either Na+ or K+ in the external solution shifted the reversal pot ential toward depolarization under the mixed Na+ and K+ containing solution s. These results suggest that two isoforms of HKT transporters, a Na+ trans porter (OsHKT1) and a Na+- and K+-coupled transporter (OsHKT2), may act har moniously in the salt tolerant indica rice.