A Single Amino-Acid Substitution in the Sodium Transporter HKT1 Associated with Plant Salt Tolerance

A crucial prerequisite for plant growth and survival is the maintenance of potassium uptake, especially when high sodium surrounds the root zone. The Arabidopsis HIGH-AFFINITY K + TRANSPORTER1 (HKT1), and its homologs in other salt-sensitive dicots, contributes to salinity tolerance by removing Na + from the transpiration stream. However, TsHKT1;2, one of three HKT1 copies in Thellungiella salsuginea , a halophytic Arabidopsis relative, acts as a K + transporter in the presence of Na + in yeast ( Saccharomyces cerevisiae ). Amino-acid sequence comparisons indicated differences between TsHKT1;2 and most other published HKT1 sequences with respect to an Asp residue (D207) in the second pore-loop domain. Two additional T . salsuginea and most other HKT1 sequences contain Asn (n) in this position. Wild-type TsHKT1;2 and altered AtHKT1 (AtHKT1 N-D ) complemented K + -uptake deficiency of yeast cells. Mutant hkt1 - 1 plants complemented with both AtHKT1 N - D and TsHKT1;2 showed higher tolerance to salt stress than lines complemented by the wild-type AtHKT1 . Electrophysiological analysis in Xenopus laevis oocytes confirmed the functional properties of these transporters and the differential selectivity for Na + and K + based on the n/d variance in the pore region. This change also dictated inward-rectification for Na + transport. Thus, the introduction of Asp, replacing Asn, in HKT1-type transporters established altered cation selectivity and uptake dynamics. We describe one way, based on a single change in a crucial protein that enabled some crucifer species to acquire improved salt tolerance, which over evolutionary time may have resulted in further changes that ultimately facilitated colonization of saline habitats.