In higher plants, the mechanism by which potassium is transported across the plasma membrane is uncertain. Results from electrophysiological and biochemical studies suggest a model in which a proton pump (H+ -ATPase) generates a proton electrochemical gradient which drives potassium uptake through potassium carriers and channels. By isolating and characterizing mutants of Arabidopsis thaliana with altered potassium transport, this model is being tested. Preliminary experiments indicate that cesium is a phytotoxic competitive inhibitor of potassium uptake in A. thaliana seedlings. From 100,000 M2 seed, several dozen cesium-resistant A. thaliana mutant plants were isolated and many display a defect in potassium transport. The mutants showed two broad phenotypic groupings, one which is pleiotropic in that potassium and phosphate transport are both affected, and a second in which a specific alteration has occurred in the affinity of the transport system for potassium. The second set of mutants is being characterized genetically. In order to clone and sequence genes encoding plasma membrane potassium transporters, a chromosome walk to loci of uptake mutants with altered affinities for potassium is being performed. Potassium plays a special role in the life of plant cells. The accumulation of high concentrations of this ion drives the uptake of water without an increase in cell volume because the plasma membrane is restrained in its expansion by a rigid cell wall. Thus, the main function for accumulated potassium appears to be the generation and maintenance of a large turgor pressure. In higher plants this pressure plays an especially important role in providing the driving force for erect plant growth through cell elongation. The concentration of potassium in the cytoplasm of cells is maintained even when the extracellular concentration is reduced to extremely low levels. It has long been recognized that the generation and maintenance of this large potassium gradient requires metabolic energy. This research explores the molecular mechanism for potassium transport across the plasma membrane of plants in order to gain a better understanding of how this important plant mineral functions in plant growth and development.***//
