Voltage-gated Ca2+ release channels in vacuolar membranes from beet storage roots and guard cells.

The ubiquity and wide-ranging roles of Ca(2+)-mediated stimulus-response coupling in plant cells is likely to demand flexibility in the duration and amplitude of the cytosolic Ca2+ signal. The presence of both ligand- and voltage-gated Ca2+ release channels at the vacuolar membrane suggests that this flexibility might be achieved through differential regulation of these discrete channel classes. The properties and distribution of voltage-gated vacuolar Ca2+ channels is described in this chapter. In beet storage roots, the channels display a single channel conductance of 12 pS (5-20 mM Ca2+ on the lumenal side), Ca2+:K+ selectivity = 15:1 and a unitary current which saturates as a function of (physiological) negative voltages at about 0.5 pA. Channel opening is enhanced by negative voltages and by intravacuolar (lumenal) Ca2+. Variation of lumenal Ca2+ concentration has enabled the binding sites both for ionic selectivity and for Ca2+ gating to be mapped with respect to their fractional electrical distance across the membrane. Voltage-gated Ca2+ channels with similar permeation properties also reside in the vacuolar membranes of Vicia faba and Commelina communis guard cells. Although these channels exhibit somewhat higher unitary conductances and voltage-saturating currents than the beet channel, they are similar in many other respects, including sensitivity to Gd3+. We conclude that the widespread distribution of an emerging class of voltage-gated Ca2+ channel in the vacuolar membrane of plants points towards a fundamental role of these channels in plant cell biology.