Regulation and mechanism of potassium release from barley roots: an in planta 42K+ analysis.
Potassium (K(+) ) flux into plant cells is a well-characterized ion transport phenomenon. By contrast, little is known about the mechanisms and regulation of K(+) flux from the cell. Here, we present a radioisotopic analysis of K(+) fluxes from roots of intact barley (Hordeum vulgare), in the context of recent discoveries in the molecular biology and electrophysiology of this process. Plants were labelled with (42)K(+), and kinetics of its release from roots were monitored at low (0.1 mM) or high (1.0 mM) external K concentration, [K(+)](ext), and with the application of channel modulators and nutrient shifts. At 0.1 (but not 1.0) mM [K(+)], where K(+) efflux is thought to be mediated by K(+)-outward-rectifying channels, (42)K(+) efflux was inhibited by the channel blockers barium (Ba(2+)), caesium (Cs(+)), tetraethylammonium (TEA(+)), and lanthanum (La(3+)). Ammonium and nitrate (10 mM) stimulated and inhibited (42)K(+) efflux, respectively, while 10 mM [K(+)](ext) or [Rb(+) ](ext) decreased it. No evidence for the involvement of ATP-binding cassettes, nonselective cation channels, or active K(+)-efflux pumps was found. Our study provides new evidence for the thermodynamic transition between high- and low-affinity transport, from the efflux perspective, identifying the operation of channels at low [K(+)], and the cessation of transmembrane efflux at high [K(+)].
Coskun
Devrim
D
Department of Biological Sciences, University of Toronto, 1265 Military Trail, Toronto, ON, Canada.
Britto
Dev T
DT
Kronzucker
Herbert J
HJ
D001467:Hordeum / Q000187:drug effects / Q000378:metabolism*; D007553:Isotope Labeling / Q000379:methods*; D018517:Plant Roots / Q000187:drug effects / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism* / Q000494:pharmacology; D011197:Potassium Radioisotopes; D036226:Seedlings / Q000187:drug effects / Q000254:growth & development / Q000378:metabolism; D013997:Time Factors
D011197:Potassium Radioisotopes; D011188:Potassium