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 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.