Calcium channels in the plasma membrane of root cells

Calcium is an essential and major plant nutrient. It is required for structural, osmotic and signalling purposes. In this review the pathway and molecular mechanisms are described by which Ca2+ enters the root and is delivered to the xylem. The characteristics of inwardly-directed Ca2+ fluxes, Ca2+ currents and Ca2+-permeable channels in the plasma membrane of root cells are compared. It is concluded that all root plasma membrane Ca2+ channels characterized to date have a similar voltage-dependence (all are activated by membrane depolarization) but at least three distinct classes can be defined based on their differential sensitivities to La3+, Gd3+ and verapamil. It is anticipated that further classes of Ca2+ channels will be identified in the future, including stretch-activated (mechanosensitive) and second messenger-activated Ca2+ channels. The roles of Ca2+ channels in mineral nutrition, intracellular signalling and polarized growth are discussed. (C) 1998 Annals of Botany Company.