A Weakly Voltage-Dependent, Nonselective Cation Channel Mediates Toxic Sodium Influx in Wheat

Abstract

Abstract 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 22Na+ influx into excised root segments. Na+ influx through the NSC channel resembled 22Na+ influx in its partial sensitivity to inhibition by Ca2+, Mg2+, and Gd3+, 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 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 the NSC channel identified in planar lipid bilayers.