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).