Regulation of Cytosolic Calcium in Plants

Cytosolic Ca2+ has been likened to a kind of cellular fire (Rasmussen et al., 1990). The metaphor is apt in an obvious way because it suggests that transient or localized elevations in cytosolic Ca2+-like a fire contained in a hearth-may be useful to plant cells for performing certain tasks, whereas uncontrolled or prolonged elevations in cytosolic Caz+-like a fire loose on the roof-are destructive of normal cellular processes. The metaphor is also apt in a less obvious way because it suggests that large elevations in cytosolic Ca2+, like the combustion of roofs, are energetically favored, spontaneous events that the cell must constantly work to prevent. There is now considerable evidence to support both of these views of cytosolic Ca2+ in plants. Measurements of cellular Caz+ have shown that plant cells, like those of a11 other organisms, avoid potentially toxic effects of Ca2+ by maintaining levels in the cytosol and nucleus that are 3 to 4 orders of magnitude lower than the levels in other cellular compartments (Felle, 1988b; Bush et al., 1989; Gilroy et al., 1990). There is, therefore, a large gradient for calcium directed into the cytosol across most cellular membranes, a gradient that is potentially useful for amplifying signals that impinge on the cell by transducing them into a regulated increase in cytosolic Ca2+. At the same time that Ca2+ levels and gradients have been measured in plant cells, a number of cellular processes have been identified that depend on changes in cytosolic Ca2+ for their activation (Johannes et al., 1991). This has led plant scientists to adopt the theory that Ca2+ acts as an intracellular messenger, conveying information about the nature of a particular stimulus or stress impinging on the cell to target proteins that guide the cellular response. Establishing the validity of this theory for plant cells has been the aim of much of the work on Ca2+ regulation in recent years. The purpose of this review is to consider what is currently known about Ca2+ regulation in plant cells with the aim of understanding, metaphorically speaking, why plant cells play with fire and how they avoid getting bumed. According to the intracellular messenger theory, Ca2+ and a small group of other compounds play a pivotal role in signal transduction by communicating signal perception at a localized receptor to other parts of the cell, where the effectors of the cellular response are located (Fig. 1). In animal cells, Ca2+mediated signal transduction pathways have been shown to be enormously varied and complex in the temporal and spatial organization of the signal, as well as in the proteins