REGULATION OF K-ROOT CELLS BY EXTERNAL K+ - INTERPLAY OF DIFFERENT PLASMA-MEMBRANE K+ TRANSPORTERS( ABSORPTION IN PLANT)

Plant roots accumulate potassium from a wide range of soil concentrati ons, utilizing at least two distinct plasma membrane uptake systems wi th different affinities for the cation, Details on the structure and f unction of these K+ transporters are accumulating, but many prominent questions remain regarding regulation of these uptake pathways in vary ing physiological conditions. Efficient use of the K+ absorption capac ity requires that the activity of all membrane K+ conductances interac t. 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 outwar d K+ leakage through inward rectifying channels. Leakage through outwa rd rectifying channels is minimized due to a combination of kinetic co ntrol and intrinsic open channel rectification as predicted by the con stant field theory. At millimolar external K+, symport activity is dow n regulated by the K+ induced membrane depolarization. In these condit ions, channel-mediated K+ uptake can only explain the observed unidire ctional fluxes in intact tissue if the cell switches from a state wher e the K+ conductance dominates (K+-state) to one where the primary pum ps dominate the membrane conductance (pump-state).