A weakly voltage-dependent, nonselective cation channel mediates toxic sodium influx in wheat

To determine the transporters responsible for toxic Na+ influx in wheat (Tr iticum aestivum), root plasma membrane preparations were screened using the planar lipid bilayer technique as an assay for Na+-permeable ion channel a ctivity. The predominant channel in the bilayer was a 44-pS channel that we called the nonselective cation (NSC) channel, which was nonselective for m onovalent cations and weakly voltage dependent. Single channel characterist ics of the NSC channel were compared with Na-22(+) influx into excised root segments. Na+ influx through the NSC channel resembled Na-22(+) influx in its partial sensitivity to inhibition by Ca2+, Mg2+, and Gd3+, and its inse nsitivity to all other inhibitors tested (tetraethylammonium, quinine, Cs+, tetrodotoxin, verapamil, amiloride, and flufenamate). Na+ influx through t he NSC channel also closely resembled an instantaneous current in wheat roo t protoplasts (S.D. Tyerman, M. Skerrett, A. Garill, G. P. Findlay, R. Leig h [1997] J Exp hot 48: 459-480) in its permeability sequence, selectivity f or K+ over Na+ (approximately 1.25), insensitivity to tetraethylammonium, v oltage independence, and partial sensitivity to Ca2+. 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 t he NSC channel identified in planar lipid bilayers.