Inward-rectifying K+ channels in root hairs of wheat. A mechanism for aluminum-sensitive low-affinity K+ uptake and membrane potential control

K+ is the most abundant cation in cells of higher plants, and it plays vital roles in plant growth and development. Extensive studies on the kinetics of K+ uptake in roots have shown that K+ uptake is mediated by at least two transport mechanisms, one with a high and one with a low affinity for K+. However, the precise molecular mechanisms of K+ uptake from soils into root epidermal cells remain unknown. In the present study we have pursued the biophysical identification and characterization of mechanisms of K+ uptake into single root hairs of wheat (Triticum aestivum L.), since root hairs constitute an important site of nutrient uptake from the soil. These patch-clamp studies showed activation of a large inward current carried by K+ ions into root hairs at membrane potentials more negative than -75 mV. This K+ influx current was mediated by hyperpolarization-activated K+-selective ion channels, with a selectivity sequence for monovalent cations of K+ > Rb+ approximately equal to NH4+ >> Na+ approximately equal to Li+ > Cs+. Kinetic analysis of K+ channel currents yielded an apparent K+ equilibrium dissociation constant (Km) of approximately 8.8 mM, which closely correlates to the major component of low-affinity K+ uptake. These channels did not inactivate during prolonged stimulation and would thus enable long-term K+ uptake driven by the plasma membrane proton-extruding pump. Aluminum, which is known to inhibit cation uptake at the root epidermis, blocked these inward-rectifying K+ channels with half-maximal current inhibition at approximately 8 micromolar free Al3+. Aluminum block of K+ channels at these Al3+ concentrations correlates closely to Al3+ phytotoxicity. It is concluded that inward-rectifying K+ channels in root hairs can function as both a physiologically important mechanism for low-affinity K+ uptake and as regulators of membrane potential. The identification of this mechanism is a major step toward a detailed molecular characterization of the multiple components involved in K+ uptake, transport, and membrane potential control in root epidermal cells.