A weakly voltage-dependent, nonselective cation channel mediates toxic sodium influx in wheat.
To determine the transporters responsible for toxic Na(+) influx in wheat (Triticum aestivum), root plasma membrane preparations were screened using the planar lipid bilayer technique as an assay for Na(+)-permeable ion channel activity. The predominant channel in the bilayer was a 44-pS channel that we called the nonselective cation (NSC) channel, which was nonselective for monovalent cations and weakly voltage dependent. Single channel characteristics of the NSC channel were compared with (22)Na(+) influx into excised root segments. Na(+) influx through the NSC channel resembled (22)Na(+) influx in its partial sensitivity to inhibition by Ca(2+), Mg(2+), and Gd(3+), and its insensitivity to all other inhibitors tested (tetraethylammonium, quinine, Cs(+), tetrodotoxin, verapamil, amiloride, and flufenamate). Na(+) influx through the NSC channel also closely resembled an instantaneous current in wheat root protoplasts (S.D. Tyerman, M. Skerrett, A. Garill, G.P. Findlay, R. Leigh [1997] J Exp Bot 48: 459-480) in its permeability sequence, selectivity for K(+) over Na(+) (approximately 1.25), insensitivity to tetraethylammonium, voltage independence, and partial sensitivity to Ca(2+). Comparison of tissue, protoplast (S.D. Tyerman, M. Skerrett, A. Garill, G.P. Findlay, R. Leigh [1997] J Exp Bot 48: 459-480), and single- channel data indicate that toxic Na(+) influx is catalyzed by a single transporter, and this is likely to be the NSC channel identified in planar lipid bilayers.
Davenport
R J
RJ
Department of Plant Sciences, Downing Street, Cambridge CB2 3EA, United Kingdom. rjd23@cus.cam.ac.uk
Tester
M
M
D002412:Cations / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D007473:Ion Channels / Q000378:metabolism*; D017136:Ion Transport; D008051:Lipid Bilayers / Q000378:metabolism; D008564:Membrane Potentials; D018517:Plant Roots / Q000378:metabolism; D012964:Sodium / Q000378:metabolism* / Q000633:toxicity; D014908:Triticum / Q000187:drug effects / Q000378:metabolism*
D002412:Cations; D007473:Ion Channels; D008051:Lipid Bilayers; D012964:Sodium