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 Ca# + enters the root and is delivered to the xylem. The characteristics of inwardly-directed Ca# + fluxes, Ca# + currents and Ca# + -permeable channels in the plasma membrane of root cells are compared. It is concluded that all root plasma membrane Ca# + 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 La$ + , Gd$ + and verapamil. It is anticipated that further classes of Ca# + channels will be identified in the future, including stretch-activated (mechanosensitive) and second messenger-activated Ca# + channels. The roles of Ca# + channels in mineral nutrition, intracellular signalling and polarized growth are discussed.
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Philip J. White (1998) studied this question.