The Arabidopsis HKT1 gene homolog mediates inward Na+ currents in Xenopus laevis oocytes and Na+ uptake in Saccharomyces cerevisiae

The Na+-K+ co-transporter HKT1, first isolated from wheat, mediates high-af finity K+ uptake. The function of HKT1 in plants, however, remains to be el ucidated, and the isolation of HKT1 homologs from Arabidopsis would further studies of the roles of HKT1 genes in plants. We report here the isolation of a cDNA homologous to HKT1 from Arabidopsis (AtHKT1) and the characteriz ation of its mode of ion transport in heterologous systems. The deduced ami no acid sequence of AtHKT1 is 41% identical to that of HKT1, and the hydrop athy profiles are very similar. AtHKT1 is expressed in roots and, to a less er extent, in other tissues. Interestingly, we found that the ion transport properties of AtHKT1 are significantly different from the wheat counterpar t. As detected by electrophysiological measurements, AtHKT1 functioned as a selective Na+ uptake transporter in Xenopus laevis oocytes, and the presen ce of external K+ did not affect the AtHKT1-mediated ion conductance (unlik e that of HKT1). When expressed in Saccharomyces cerevisiae, AtHKT1 inhibit ed growth of the yeast in a medium containing high levels of Na+, which cor relates to the large inward Na+ currents found in the oocytes. Furthermore, in contrast to HKT1, AtHKT1 did not complement the growth of yeast cells d eficient in K+ uptake when cultured in K+-limiting medium. However, express ion of AtHKT1 did rescue Escherichia coli mutants carrying deletions in Ktransporters. The rescue was associated with a less than 2-fold stimulation of K+ uptake into K+-depleted cells. These data demonstrate that AtHKT1 di ffers in its transport properties from the wheat HKT1, and that AtHKT1 can mediate Na+ and, to a small degree, K+ transport in heterologous expression systems.