Sodium-Driven Potassium Uptake by the Plant Potassium Transporter HKT1 and Mutations Conferring Salt Tolerance

Abstract

Sodium (Na + ) at high millimolar concentrations in soils is toxic to most higher plants and severely reduces agricultural production worldwide. However, the molecular mechanisms for plant Na + uptake remain unknown. Here, the wheat root high-affinity potassium (K + ) uptake transporter HKT1 was shown to function as a high-affinity K + -Na + cotransporter. High-affinity K + uptake was activated by micromolar Na + concentrations; moreover, high-affinity Na + uptake was activated by K + (half-activation constant, 2.8 μM K + ). However, at physiologically detrimental concentrations of Na + , K + accumulation mediated by HKT1 was blocked and low-affinity Na + uptake occurred (Michaelis constant, ∼16 mM Na + ), which correlated to Na + toxicity in plants. Point mutations in the sixth putative transmembrane domain of HKT1 that increase Na + tolerance were isolated with the use of yeast as a screening system. Na + uptake and Na + inhibition of K + accumulation indicate a possible role for HKT1 in physiological Na + toxicity in plants.