Over-expression of an Na+- and K+-permeable HKT transporter in barley improves salt tolerance

Soil salinity is an increasing menace that affects agriculture across the globe. Plant adaptation to high salt concentrations involves integrated functions, including control of Na+ uptake, translocation and compartmentalization. Na+ transporters belonging to the HKT family have been shown to be involved in tolerance to mild salt stress in glycophytes such as Arabidopsis, wheat and rice by contributing to Na+ exclusion from aerial tissues. Here, we have analysed the role of the HKT transporter HKT2;1, which is permeable to K+ and Na+, in barley, a relatively salt-tolerant crop that displays a salt-including behaviour. In Xenopus oocytes, HvHKT2;1 co-transports Na+ and K+ over a large range of concentrations, displaying low affinity for Na+, variable affinity for K+ depending on external Na+ concentration, and inhibition by K+ (K-i approximately 5 mM). HvHKT2;1 is predominantly expressed in the root cortex. Transcript levels are up-regulated in both roots and shoots by low K+ growth conditions, and in shoots by high Na+ growth conditions. Over-expression of HvHKT2;1 led to enhanced Na+ uptake, higher Na+ concentrations in the xylem sap, and enhanced translocation of Na+ to leaves when plants were grown in the presence of 50 or 100 mM NaCl. Interestingly, these responses were correlated with increased barley salt tolerance. This suggests that one of the factors that limits barley salt tolerance is the capacity to translocate Na+ to the shoot rather than accumulation or compartmentalization of this cation in leaf tissues. Thus, over-expression of HvHKT2;1 leads to increased salt tolerance by reinforcing the salt- including behaviour of barley.