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

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.