Contrasting roles in ion transport of two K(+)-channel types in root cells of Arabidopsis thaliana.

Plant roots accumulate K+ over a range of external concentrations. Root cells have evolved at least two parallel plasma-membrane K+ transporters which operate at millimolar and micromolar external [K+]: high-affinity K+ uptake is energised by symport with H+, while low-affinity uptake is assumed to occur via ion channels. To determine the role of ion channels in low-affinity K+ uptake, a characterisation of the principal K(+)-selective ion channels in the plasma membrane of Arabidopsis thaliana (L.) Heynh. cv. Columbia roots was undertaken. Two classes of K(+)-selective channels were frequently observed: one inward (IRC) and one outward (ORC) rectifying with unitary conductances of 5 pS, 20 pS (IRCs) and 15 pS (ORC), measured in symmetrical 10 mM KCl. The dominant IRC (5 pS) and ORC (15 pS) were highly cation-selective (PCl:PK < 0.025) but less selective amongst monovalent cations (PNa:PK approximately 0.17-0.3). Both the IRC and the ORC were blocked by Ba2+, Cs+ and tetra-ethyl-ammonium, whereas 4-aminopyridine and quinidine selectively inhibited the ORC. The ORC open probability was steeply voltage-dependent and ORC activation potentials were close to the potassium equilibrium potential (EK+), enabling ORCs to conduct mainly outward, but occasionally inward, K+ current. By contrast, gating of the 5-pS IRC was weakly voltage-ependent and IRC gating was invariably restricted to membrane potentials more negative than EK+, ensuring K+ transport was always inwardly directed.(ABSTRACT TRUNCATED AT 250 WORDS)