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.