title: How high do ion fluxes go? A re-evaluation of the two-mechanism model of K(+) transport in plant roots.
abstract: Potassium (K(+)) acquisition in roots is generally described by a two-mechanism model, consisting of a saturable, high-affinity transport system (HATS) operating via H(+)/K(+) symport at low (<1mM) external [K(+)] ([K(+)]ext), and a linear, low-affinity system (LATS) operating via ion channels at high (>1mM) [K(+)]ext. Radiotracer measurements in the LATS range indicate that the linear rise in influx continues well beyond nutritionally relevant concentrations (>10mM), suggesting K(+) transport may be pushed to extraordinary, and seemingly limitless, capacity. Here, we assess this rise, asking whether LATS measurements faithfully report transmembrane fluxes. Using (42)K(+)-isotope and electrophysiological methods in barley, we show that this flux is part of a K(+)-transport cycle through the apoplast, and masks a genuine plasma-membrane influx that displays Michaelis-Menten kinetics. Rapid apoplastic cycling of K(+) is corroborated by an absence of transmembrane (42)K(+) efflux above 1mM, and by the efflux kinetics of PTS, an apoplastic tracer. A linear apoplastic influx, masking a saturating transmembrane influx, was also found in Arabidopsis mutants lacking the K(+) transporters AtHAK5 and AtAKT1. Our work significantly revises the model of K(+) transport by demonstrating a surprisingly modest upper limit for plasma-membrane influx, and offers insight into sodium transport under salt stress.
authors.affiliation: Department of Biological Sciences & Canadian Centre for World Hunger Research (CCWHR), University of Toronto, Toronto, Ontario M1C 1A4, Canada. Electronic address: devrim.coskun@mail.utoronto.ca.
authors.forename: Devrim
authors.initials: D
authors.lastname: Coskun
authors.affiliation: Department of Biological Sciences & Canadian Centre for World Hunger Research (CCWHR), University of Toronto, Toronto, Ontario M1C 1A4, Canada. Electronic address: britto@utsc.utoronto.ca.
authors.forename: Dev T
authors.initials: DT
authors.lastname: Britto
authors.affiliation: Robert W. Holley Center for Agriculture and Health, USDA-ARS, Cornell University, Ithaca, New York 14853, USA. Electronic address: lvk1@cornell.edu.
authors.forename: Leon V
authors.initials: LV
authors.lastname: Kochian
authors.affiliation: Department of Biological Sciences & Canadian Centre for World Hunger Research (CCWHR), University of Toronto, Toronto, Ontario M1C 1A4, Canada. Electronic address: herbert.kronzucker@utoronto.ca.
authors.forename: Herbert J
authors.initials: HJ
authors.lastname: Kronzucker
mesh_terms: D017360:Arabidopsis / Q000378:metabolism*; D001692:Biological Transport; D001467:Hordeum / Q000378:metabolism*; D008954:Models, Biological; D018517:Plant Roots / Q000378:metabolism; D011188:Potassium / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*
chemical_list: D012964:Sodium; D011188:Potassium
keywords: Apoplast; Channels; Efflux; Influx; Radiotracer; Salinity; Transporters