Characterisation of two HKT1;4 Transporters from Triticum monococcum to elucidate the Determinants of the Wheat Salt Tolerance Nax1 QTL.

TmHKT1;4-A1 and TmHKT1;4-A2 are two Na(+) transporter genes that have been identified as associated to the salt tolerance Nax1 locus found in a durum wheat (Triticum turgidum L. subsp. durum) line issued from a cross with T. monococcum In the present study, we were interested in getting clues on the molecular mechanisms underpinning this salt tolerance QTL. By analysing the phylogenetic relationships between wheat and T. monococcum HKT1;4-type genes, we found that durum and bread wheat genomes possess a close homologue of TmHKT1;4-A1, but no functional close homologue of TmHKT1;4-A2 Furthermore, performing real-time RT-PCR experiments, we showed that TmHKT1;4-A1 and TmHKT1;4-A2 are similarly expressed in the leaves but that TmHKT1;4-A2 is strongly more expressed in the roots, which would enable it to contribute more to the prevention of Na(+) transfer to the shoots upon salt stress. We also functionally characterised the TmHKT1;4-A1 and TmHKT1;4-A2 transporters by expressing them in Xenopus oocytes. The two transporters displayed close functional properties (high Na(+)/K(+) selectivity, low affinity for Na(+), stimulation by external K(+) of Na(+) transport), but differed in some quantitative parameters: Na(+) affinity was 3-fold lower and the maximal inward conductance was 3-fold higher in TmHKT1;4-A2 than in TmHKT1;4-A1. The conductance of TmHKT1;4-A2 at high Na(+) concentration (>10 mM) was also shown to be higher than that of the two durum wheat HKT1;4-type transporters so far characterised. Altogether, these data support the hypothesis that TmHKT1;4-A2 is responsible for the Nax1 trait and provide new insight into the understanding of this QTL