The Voltage-Independent Cation Channel in the Plasma Membrane of Wheat Roots Is Permeable to Divalent Cations and May Be Involved in Cytosolic Ca2+ Homeostasis

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Abstract
A voltage-independent cation (VIC) channel has been identified in the plasma membrane of wheat (Triticum aestivum) root cells (P.J. White [1999] Trends Plant Sci 4: 245–246). Several physiological functions have been proposed for this channel, including roles in cation nutrition, osmotic adjustment, and charge compensation. Here, we observe that Ca2+ permeates this VIC channel when assayed in artificial, planar lipid bilayers, and, using an energy barrier model to describe cation fluxes, predict that it catalyzes Ca2+ influx under physiological ionic conditions. Thus, this channel could participate in Ca2+ signaling or cytosolic Ca2+ homeostasis. The pharmacology of45Ca2+ influx to excised wheat roots and inward cation currents through the VIC channel are similar: Both are insensitive to 20 μm verapamil or 1 mmtetraethylammonium, but inhibited by 0.5 mmBa2+ or 0.5 mm Gd3+. The weak voltage dependency of the VIC channel (and its lack of modulation by physiological effectors) suggest that it will provide perpetual Ca2+ influx to root cells. Thus, it may effect cytosolic Ca2+ homeostasis by contributing to the basal Ca2+ influx required to balance Ca2+ efflux from the cytoplasm through ATP- and proton-coupled Ca2+transporters under steady-state conditions.