K⁺ transport by the oshkt2;4 transporter from rice with atypical Na⁺transport properties and competition in permeation of K⁺over Mg²⁺and Ca²⁺Ions

Members of class II of the HKT transporters, which have thus far only been isolated from grasses, were found to mediate Na⁺-K⁺ cotransport and at high Na⁺ concentrations preferred Na⁺-selective transport, depending on the ionic conditions. But the physiological functions of this K⁺-transporting class II of HKT transporters remain unknown in plants, with the exception of the unique class II Na⁺ transporter OsHKT2;1. The genetically tractable rice (Oryza sativa; background Nipponbare) possesses two predicted K⁺-transporting class II HKT transporter genes, OsHKT2;3 and OsHKT2;4. In this study, we have characterized the ion selectivity of the class II rice HKT transporter OsHKT2;4 in yeast and Xenopus laevis oocytes. OsHKT2;4 rescued the growth defect of a K⁺ ptake-deficient yeast mutant. Green fluorescent protein-OsHKT2;4 is targeted to the plasma membrane in transgenic plant cells. OsHKT2;4-expressing oocytes exhibited strong K⁺ permeability. Interestingly, however, K⁺ influx in OsHKT2;4-expressing oocytes did not require stimulation by extracellular Na⁺, in contrast to other class II HKT transporters. Furthermore, OsHKT2;4-mediated currents exhibited permeabilities to both Mg²⁺ and Ca²⁺ in the absence of competing K⁺ ions. Comparative analyses of Ca²⁺ and Mg²⁺ permeabilities in several HKT transporters, including Arabidopsis thaliana HKT1;1 (AtHKT1;1), Triticum aestivum HKT2;1 (TaHKT2;1), OsHKT2;1, OsHKT2;2, and OsHKT2;4, revealed that only OsHKT2;4 and to a lesser degree TaHKT2;1 mediate Mg²⁺ transport. Interestingly, cation competition analyses demonstrate that the selectivity of both of these class II HKT transporters for K⁺ is dominant over divalent cations, suggesting that Mg²⁺ and Ca²+ transport via OsHKT2;4 may be small and would depend on competing K⁺ concentrations in plants.