42K analysis of sodium-induced potassium efflux in barley: mechanism and relevance to salt tolerance.
*Stimulation of potassium (K(+)) efflux by sodium (Na(+)) has been the subject of much recent attention, and its mechanism has been attributed to the activities of specific classes of ion channels. *The short-lived radiotracer (42)K(+) was used to test this attribution, via unidirectional K(+)-flux analysis at the root plasma membrane of intact barley (Hordeum vulgare), in response to NaCl, KCl, NH(4)Cl and mannitol, and to channel inhibitors. *Unidirectional K(+) efflux was strongly stimulated by NaCl, and K(+) influx strongly suppressed. Both effects were ameliorated by elevated calcium (Ca(2+)). As well, K(+) efflux was strongly stimulated by KCl, NH(4)Cl and mannitol , and NaCl also stimulated (13)NH(4)(+) efflux. The Na(+)-stimulated K(+) efflux was insensitive to cesium (Cs(+)) and pH 4.2, weakly sensitive to the K(+)-channel blocker tetraethylammonium (TEA(+)) and quinine, and moderately sensitive to zinc (Zn(2+)) and lanthanum (La(3+)). *We conclude that the stimulated efflux is: specific neither to Na(+) as effector nor K(+) as target; composed of fluxes from both cytosol and vacuole; mediated neither by outwardly-rectifying K(+) channels nor nonselective cation channels; attributable, alternatively, to membrane disintegration brought about by ionic and osmotic components; of limited long-term significance, unlike the suppression of K(+) influx by Na(+), which is a greater threat to K(+) homeostasis under salt stress.
Britto
Dev T
DT
Department of Biological Sciences, University of Toronto, 1265 Military Trail, Toronto, ON, Canada, M1C 1A4.
Ebrahimi-Ardebili
Sasha
S
Hamam
Ahmed M
AM
Coskun
Devrim
D
Kronzucker
Herbert J
HJ
D000643:Ammonium Chloride / Q000494:pharmacology; D002586:Cesium / Q000494:pharmacology; D001467:Hordeum / Q000187:drug effects* / Q000378:metabolism*; D007700:Kinetics; D008353:Mannitol / Q000494:pharmacology; D009587:Nitrogen Isotopes; D018517:Plant Roots / Q000187:drug effects / Q000378:metabolism; D011188:Potassium / Q000378:metabolism*; D011189:Potassium Chloride / Q000494:pharmacology; D011197:Potassium Radioisotopes; D055049:Salt Tolerance / Q000187:drug effects*; D036226:Seedlings / Q000187:drug effects / Q000378:metabolism; D012964:Sodium / Q000494:pharmacology*; D012965:Sodium Chloride / Q000494:pharmacology; D019789:Tetraethylammonium / Q000494:pharmacology; D013997:Time Factors
D009587:Nitrogen Isotopes; D011197:Potassium Radioisotopes; D000643:Ammonium Chloride; D002586:Cesium; D008353:Mannitol; D012965:Sodium Chloride; D019789:Tetraethylammonium; D011189:Potassium Chloride; D012964:Sodium; D011188:Potassium