Ca2+ Channels in Higher Plant Cells.

Ca2+ channels have been suggested to play major roles in the initiation of a large number of signal transduction processes in higher plant cells, including bud formation, polar growth, gas exchange regulation, secretion, movements, and light-regulated and hormone-regulated growth and development (Hepler and Wayne, 1985; Leonard and Hepler, 1990). Activation of Ca2+ channels allows transmembrane fluxes of Ca2+. These Ca2+ fluxes result in elevation of the cytosolic Ca2+ concentration from resting levels of approximately 150 nM to "excitatory" levels greater than 300 nM to 400 nM. This rise in cytosolic Ca2+ triggers numerous cell biological processes by modulation of protein kinases, ion channels, and other cellular control proteins (Leonard and Hepler, 1990). Early indications of Ca2+ channel action during signal transduction in plants were obtained in studies of cytokinin-induced budding in funaria, using Ca2+ ionophores and pharmacological blockers of voltage-dependent mammalian Ca2+ channels (Saunders and Hepler, 1982, 1983). Until recently, however, direct evidence for plant Ca'+ channels was lacking, with the exception of data indicating voltage-dependent Ca2+ channel activation in algae (Williamson and Ashley, 1982; Tazawa et al., 1987). Recent advances have demonstrated that severa1 types of Ca2+ channels exist in higher plant cells, which are regulated by membrane-associated and intracellular control mechanisms (Alexandre et al., 1990; Schroeder and Hagiwara, 1990). Figure 1 shows that these Ca2+ channels belong to two general classes of ion channels: plasma membrane Ca2+ channels, which allow Ca2+ influx from the cell wall space into the cytosol, and Ca2+ release channels located in the membrane of intracellular organelles, which allow release of stored Ca2+. In addition, new results suggest that both Ca2+ influx and Ca'+ release occur within the same cell type.