TY - JOUR AU - Sanders, Dale AB - 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≈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 voltageependent and IRC gating was invariably restricted to membrane potentials more negative than EK+, ensuring K+ transport was always inwardly directed. Studies on channel activity were conducted for a large number of root cells grown at two levels of external [K+], one where K+ uptake is likely to be principally through channels (6 mM K+) and one where it must be energised (100 μM K+). Shifting growth conditions from high to low K+ did not affect single-channel properties such as conductance and selectivity, nor the manifestation of the ORC and 20-pS IRC, but led to enhanced activity of the 5-pS IRC. The enhanced activity of the 5-pS IRC was mirrored by a parallel increase in unidirectional 86Rb+ influx after low-K+ growth, clearly indicating a dominant role of this particular channel in K+ uptake at supra millimolar external [K+]. TI - Contrasting roles in ion transport of two K+-channel types in root cells of Arabidopsis thaliana JF - Planta DO - 10.1007/BF00196667 DA - 2004-07-13 UR - https://www.deepdyve.com/lp/springer-journals/contrasting-roles-in-ion-transport-of-two-k-channel-types-in-root-7zriBlaoDB SP - 456 EP - 464 VL - 197 IS - 3 DP - DeepDyve ER -