Functional characterization in Xenopus oocytes of Na transport systems from durum wheat reveals diversity among two HKT1;4 transporters

Plant tolerance to salinity constraint involves complex and integrated functions including control of Na uptake, translocation, and compartmentalization. Several members of the high-affinity K transporter (HKT) family, which comprises plasma-membrane transporters permeable to K and Na or to Na only, have been shown to play major roles in plant Na and K homeostasis. Among them, HKT1;4 has been identified as corresponding to a quantitative trait locus (QTL) of salt tolerance in wheat but was not functionally characterized. Here, we isolated two HKT1;4-type cDNAs from a salt-tolerant durum wheat (Triticum turgidum L. subsp. durum) cultivar, Om Rabia3, and investigated the functional properties of the encoded transporters using a two-electrode voltage-clamp technique, after expression in Xenopus oocytes. Both transporters displayed high selectivity for Na, their permeability to other monovalent cations (K, Li, Cs, and Rb) being ten times lower than that to Na. Both TdHKT1;4-1 and TdHKT1;4-2 transported Na with low affinity, although the half-saturation of the conductance was observed at a Na concentration four times lower in TdHKT1;4-1 than in TdHKT1;4-2. External K did not inhibit Na transport through these transporters. Quinine slightly inhibited TdHKT1;4-2 but not TdHKT1;4-1. Overall, these data identified TdHKT1;4 transporters as new Na-selective transporters within the HKT family, displaying their own functional features. Furthermore, they showed that important differences in affinity exist among durum wheat HKT1;4 transporters. This suggests that the salt tolerance QTL involving HKT1;4 may be at least in part explained by functional variability among wheat HKT1;4-type transporters.