Na(+) Transporter SvHKT1;1 from a Halophytic Turf Grass Is Specifically Upregulated by High Na(+) Concentration and Regulates Shoot Na(+) Concentration

We characterized an Na(+) transporter SvHKT1;1 from a halophytic turf grass, Sporobolus virginicus. SvHKT1;1 mediated inward and outward Na(+) transport in Xenopus laevis oocytes and did not complement K(+) transporter-defective mutant yeast. SvHKT1;1 did not complement athkt1;1 mutant Arabidopsis, suggesting its distinguishable function from other typical HKT1 transporters. The transcript was abundant in the shoots compared with the roots in S. virginicus and was upregulated by severe salt stress (500 mM NaCl), but not by lower stress. SvHKT1;1-expressing Arabidopsis lines showed higher shoot Na(+) concentrations and lower salt tolerance than wild type (WT) plants under nonstress and salt stress conditions and showed higher Na(+) uptake rate in roots at the early stage of salt treatment. These results suggested that constitutive expression of SvHKT1;1 enhanced Na(+) uptake in root epidermal cells, followed by increased Na(+) transport to shoots, which led to reduced salt tolerance. However, Na(+) concentrations in phloem sap of the SvHKT1;1 lines were higher than those in WT plants under salt stress. Based on this result, together with the induction of the SvHKT1;1 transcription under high salinity stress, it was suggested that SvHKT1;1 plays a role in preventing excess shoot Na(+) accumulation in S. virginicus.