Mechanism of high-affinity potassium uptake in roots of Arabidopsis thaliana.
Potassium is a major nutrient in higher plants, where it plays a role in turgor regulation, charge balance, leaf movement, and protein synthesis. Terrestrial plants are able to sustain growth at micromolar external K+ concentrations, at which K+ uptake across the plasma membrane of root cells must be energized despite the presence of a highly negative membrane potential. However, the mechanism of energization has long remained obscure. Therefore, whole-cell mode patch clamping has been applied to root protoplasts from Arabidopsis thaliana to characterize membrane currents resulting from the application of micromolar K+. Analysis of whole cell current/voltage relationships in the presence and absence of micromolar K+ enabled direct testing of K+ transport for possible energization by cytoplasmic ATP and the respective trans-membrane gradients of Na+, Ca2+, and H+. Subtracted current/voltage relations for K(+)-dependent membrane currents are independent of ATP and reverse at potentials that imply H(+)-coupled K+ transport with a ratio of 1 H+:K+. Furthermore, the reversal potential of the K+ current shifts negative as external H+ activity is decreased. K(+)-dependent currents saturate in the micromolar concentration range with an apparent Km of 30 microM, a value in close agreement with previously reported Km values for high-affinity K+ uptake. We conclude that our results are consistent with the view that high-affinity K+ uptake in higher plants is mediated by a H+:K+ symport mechanism, competent in driving K+ accumulation to equilibrium ratios in excess of 10(6)-fold.
Maathuis
F J
FJ
Department of Biology, University of York, United Kingdom.
Sanders
D
D
D017360:Arabidopsis / Q000502:physiology*; D001692:Biological Transport; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D018517:Plant Roots / Q000502:physiology; D011188:Potassium / Q000378:metabolism* / Q000494:pharmacology; D011523:Protoplasts / Q000502:physiology
D011188:Potassium