Regulation of K+ absorption in plant root cells by external K+: interplay of different plasma membrane K+ transporters.

Plant roots accumulate potassium from a wide range of soil concentrations, utilizing at least two distinct plasma membrane uptake systems with different affinities for the cation. Details on the structure and function of these K(+) transporters are accumulating, but many prominent questions remain regarding regulation of these uptake pathways in varying physiological conditions. Efficient use of the K(+) absorption capacity requires that the activity of all membrane K(+) conductances interact. In this paper, it is shown how intrinsic properties of the major K(+) transporters in the root plasma membrane generate sufficient inward K(+) flux at varying levels of external [K(+)]. In the high affinity range, uptake proceeds via K(+):H(+) symport and kinetic control prevents outward K(+) leakage through inward rectifying channels. Leakage through outward rectifying channels is minimized due to a combination of kinetic control and intrinsic open channel rectification as predicted by the constant field theory. At millimolar external K(+), symport activity is down regulated by the K(+) induced membrane depolarization. In these conditions, channel-mediated K(+) uptake can only explain the observed unidirectional fluxes in intact tissue if the cell switches from a state where the K(+) conductance dominates (K(+)-state) to one where the primary pumps dominate the membrane conductance (pump-state).