HKT1;4 was linked to a salt tolerance QTL in wheat. Here, two wheat HKT1;4-type transporters were functionally characterised in Xenopus oocytes. Beside shared properties, differences in Na+ transport affinity were evidenced. Such functional diversity sheds light on the QTL bases
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.