High-affinity K+ transporters from a halophyte, Sporobolus virginicus, mediate both K+ and Na+ transport in transgenic Arabidopsis, X. laevis oocytes, and yeast.

Class II high-affinity potassium transporters (HKTs) proposed to mediate Na+-K+ cotransport in plants, as well as Na+ and K+ homeostasis under K+-starved and saline environments. We identified class II HKTs, namely SvHKT2;1 and SvHKT2;2 (SvHKTs), from the halophytic turf grass, Sporobolus virginicus. SvHKT2;2 expression in S. virginicus was upregulated by NaCl treatment, while SvHKT2;1 expression was assumed to be upregulated by K+ starvation and downregulated by NaCl treatment. Localization analysis revealed SvHKTs predominantly targeted the plasma membrane. SvHKTs complemented K+-uptake deficiency in mutant yeast, and showed both inward and outward K+ and Na+ transport activity in Xenopus laevis oocytes. When constitutively expressed in Arabidopsis, SvHKTs mediated K+ and Na+ accumulation in shoots under K+-starved conditions, and K+ concentration in xylem saps of transformants was also higher than those of wild type plants. These results suggest transporter-enhanced K+ and Na+ uploading to xylem from xylem parenchyma cells. Together, our data demonstrate SvHKTs mediate both outward and inward K+ and Na+ transport in X. laevis oocytes, and possibly in plant and yeast cells, depending on the ionic conditions.