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.