High affinity Na⁺ transport by wheat HKT1;5 is blocked by K

The wheat sodium transporters TmHKT1;5-A and TaHKT1;5-D are encoded by genes underlying the major shoot Na⁺ exclusion loci Nax2 and Kna1 from Triticum monococcum (Tm) and Triticum aestivum (Ta), respectively. In contrast to HKT2 transporters that have been shown to exhibit high affinity K⁺-dependent Na⁺ transport, HKT1 proteins have, with one exception, only been shown to catalyze low affinity Na⁺ transport and no K⁺ transport. Here, using heterologous expression in Xenopus laevis oocytes we uncover a novel property of HKT1 proteins, that both TmHKT1;5-A and TaHKT1;5-D encode dual (high and low) affinity Na⁺-transporters with the high-affinity component being abolished when external K⁺ is in excess of external Na⁺. Three-dimensional structural modeling suggested that, compared to Na⁺, K⁺ is bound more tightly in the selectivity filter region by means of additional van der Waals forces, which is likely to explain the K⁺ block at the molecular level. The low-affinity component for Na⁺ transport of TmHKT1;5-A had a lower K than that of TaHKT1;5-D and was less sensitive to external K⁺. We propose that these properties contribute towards the improvements in shoot Na⁺-exclusion and crop plant salt tolerance following the introgression of TmHKT1;5-A into diverse wheat backgrounds.