OsHKT2;2/1-mediated Na+ influx over K+ uptake in roots potentially increases toxic Na+ accumulation in a salt-tolerant landrace of rice Nona Bokra upon salinity stress
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HKT transporters are Na+-permeable membrane proteins, which mediate Na+ and K+ homeostasis in K+-depleted and saline environments in plants. Class II HKT transporters, a distinct subgroup found predominantly in monocots, are known to mediate Na+-K+ co-transport in principle. Here we report features of ion transport functions of No-OsHKT2;2/1, a class II transporter identified in a salt tolerant landrace of indica rice, Nona Bokra. We profiled No-OsHKT2;2/1 expression in organs of Nona Bokra plants with or without salinity stress. Dominant accumulation of the No-OsHKT2;2/1 transcript in K+-starved roots of Nona Bokra plants largely disappeared in response to 50 mM NaCl. We found that No-OsHKT2;2/1 expressed in the high-affinity K+ uptake deficient mutant of Saccharomyces cerevisiae and Xenopus laevis oocytes shows robust K+ selectivity even in the presence of a large amount of NaCl as reported previously. However, No-OsHKT2;2/1-expressing yeast cells exhibited Na+ hypersensitive growth under various concentrations of K+ and Na+ as the cells expressing Po-OsHKT2;2, a similar class II transporter from another salt tolerant indica rice Pokkali, when compared with the growth of cells harboring empty vector or cells expressing OsHKT2;4. The OsHKT2;4 protein expressed in Xenopus oocytes showed strong K+ selectivity in the presence of 50 mM NaCl in comparison with No-OsHKT2;2/1 and Po-OsHKT2;2. Together with apparent plasma membrane-localization of No-OsHKT2;2/1, these results point to possibilities that No-OsHKT2;2/1 could mediate destructive Na+ influx over K+ uptake in Nona Bokra plants upon salinity stress, and that a predominant physiological function of No-OsHKT2;2/1 might be the acquisition of Na+ and K+ in K+-limited environments.
KeywordsHKT K+ uptake Na+ transport Rice Salt stress
We would like to express our gratitude to Dr. Kazuya Yoshida and Prof. Yoshiyuki Murata (Okayama Univ.) for helpful discussions. We also would like to thank Prof. Jian Feng Ma (Okayama Univ.), Dr. Pulla Kaothien-Nakayama and Ms. Saori Okamura for the support of TEVC experiments, the comments on the manuscript and the assistance for this study, respectively. This work was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan 25119709 and the MEXT as part of Joint Research Program implemented at the Institute of Plant Science and Resources, Okayama University in Japan 2520, 2622, 2716 (to T.H.). The research in A.C. lab is supported by a grant from the Ministero dell’Istruzione, dell’Università e della Ricerca Fondo per gli Investimenti della Ricerca di Base (FIRB) 2010 RBFR10S1LJ_001.
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