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

Authors

  • Tomoaki Horie,

    1. Graduate School of Biological Sciences, Nara Institute of Science and Technology, 8916-5 Takayama-cho, Ikoma-shi, Nara 630-0101, Japan, and
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  • Kazuya Yoshida,

    Corresponding author
    1. Graduate School of Biological Sciences, Nara Institute of Science and Technology, 8916-5 Takayama-cho, Ikoma-shi, Nara 630-0101, Japan, and
      For correspondence (fax +81 743 72 5469; e-mail kazz@bs.aist-nara.ac.jp
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  • Hideki Nakayama,

    1. Graduate School of Biological Sciences, Nara Institute of Science and Technology, 8916-5 Takayama-cho, Ikoma-shi, Nara 630-0101, Japan, and
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  • Katsuyuki Yamada,

    1. Department of Physiology, Fukui Medical University, Matsuoka-cho, Yoshida-gun, Fukui 910-1193, Japan
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  • Shigetoshi Oiki,

    1. Department of Physiology, Fukui Medical University, Matsuoka-cho, Yoshida-gun, Fukui 910-1193, Japan
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  • Atsuhiko Shinmyo

    1. Graduate School of Biological Sciences, Nara Institute of Science and Technology, 8916-5 Takayama-cho, Ikoma-shi, Nara 630-0101, Japan, and
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For correspondence (fax +81 743 72 5469; e-mail kazz@bs.aist-nara.ac.jp).

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.

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