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) have been proposed to mediate Na⁺–K⁺ co-transport 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 up-regulated by NaCl treatment, while SvHKT2;1 expression was assumed to be up-regulated by K⁺ starvation and down-regulated 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 the K⁺ concentration in xylem saps of transformants was also higher than in those of wild-type plants. These results suggest transporter-enhanced K⁺ and Na⁺ uploading to the xylem from xylem parenchyma cells. Together, our data demonstrate that 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.