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 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 catalyse low affinity Na+ transport and no K+ transport. Here, using heterologous expression in Xenopus laevis oocytes we show 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+. Based on 3-D structural modelling we propose that tighter binding of K+, compared to that of Na+ in the selectivity filter region by means of additional van der Waals forces, explains the K+ block at the molecular level. The low-affinity component for Na+ transport of TmHKT1;5-A had a lower Km than that of TaHKT1;5-D and was less sensitive to external K+. We propose that these properties underpin the improvements in shoot Na+-exclusion and crop plant salt tolerance following the introgression of TmHKT1;5-A into diverse wheat backgrounds.