Structural variations in wheat HKT1;5 underpin differences in Na transport capacity.

An important trait associated with the salt tolerance of wheat is the exclusion of sodium ions (Na) from the shoot. We have previously shown that the sodium transporters TmHKT1;5-A and TaHKT1;5-D, from Triticum monoccocum (Tm) and Triticum aestivum (Ta), are encoded by genes underlying the major shoot Na-exclusion loci Nax1 and Kna1, respectively. Here, using heterologous expression, we show that the affinity (K ) for the Na transport of TmHKT1;5-A, at 2.66 mM, is higher than that of TaHKT1;5-D at 7.50 mM. Through 3D structural modelling, we identify residues D/a gap and D/G that contribute to this property. We identify four additional mutations in amino acid residues that inhibit the transport activity of TmHKT1;5-A, which are predicted to be the result of an occlusion of the pore. We propose that the underlying transport properties of TmHKT1;5-A and TaHKT1;5-D contribute to their unique ability to improve Na exclusion in wheat that leads to an improved salinity tolerance in the field.