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(m) 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.