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.
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 Ca(2+) 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 Ca(2+) influx under physiological ionic conditions. Thus, this channel could participate in Ca(2+) signaling or cytosolic Ca(2+) homeostasis. The pharmacology of (45)Ca(2+) influx to excised wheat roots and inward cation currents through the VIC channel are similar: Both are insensitive to 20 microM verapamil or 1 mM tetraethylammonium, but inhibited by 0.5 mM Ba(2+) or 0.5 mM Gd(3+). The weak voltage dependency of the VIC channel (and its lack of modulation by physiological effectors) suggest that it will provide perpetual Ca(2+) influx to root cells. Thus, it may effect cytosolic Ca(2+) homeostasis by contributing to the basal Ca(2+) influx required to balance Ca(2+) efflux from the cytoplasm through ATP- and proton-coupled Ca(2+) transporters under steady-state conditions.
White
Philip J
PJ
Department of Plant Genetics and Biotechnology, Horticulture Research International, Wellesbourne, Warwick, United Kingdom. philip-j.white@hri.ac.uk
Davenport
Romola J
RJ
D001464:Barium / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000502:physiology*; D002132:Calcium Radioisotopes; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology*; D003600:Cytosol / Q000378:metabolism; D005682:Gadolinium / Q000494:pharmacology; D006706:Homeostasis; D018517:Plant Roots / Q000187:drug effects / Q000502:physiology*; D011188:Potassium / Q000378:metabolism; D024642:Potassium Channels, Voltage-Gated / Q000502:physiology; D015398:Signal Transduction / Q000187:drug effects; D012964:Sodium / Q000378:metabolism; D015222:Sodium Channels / Q000502:physiology; D019789:Tetraethylammonium / Q000494:pharmacology; D014908:Triticum / Q000187:drug effects / Q000502:physiology*; D014700:Verapamil / Q000494:pharmacology
D015220:Calcium Channels; D002132:Calcium Radioisotopes; D024642:Potassium Channels, Voltage-Gated; D015222:Sodium Channels; D001464:Barium; D019789:Tetraethylammonium; D012964:Sodium; D005682:Gadolinium; D014700:Verapamil; D011188:Potassium; D002118:Calcium