Calcium: a regulation system emerges in plant cells

Calcium occupies a pre-eminent place in the cellular control systems of animals (Campbell, 1983). Because of the cytotoxic effects of calcium, cells pay very particular attention to keeping cytoplasmic calcium levels very much lower than the normal extracellular 10~M level; usually it is in the range 1CT-10~M. This is accomplished using a variety of calcium-pumping systems located both in the plasma membrane and organelles and together these operate a very efficient calcium-stat system. But, in addition, cells use the temporary elevation of cytoplasmic calcium to between 10~ and 10~ M that may follow plasma membrane perturbation and alteration of calcium channel activity, as signals, eliciting a variety of predetermined responses. The concentration of cytoplasmic calcium is sensed by calcium-binding proteins, most notably calmodulin, and the calcium/ calmodulin complex in turn modulates the activity of numerous enzymes and proteins. Calcium is also associated with other signalling systems such as IP3 and cyclic AMP. Calcium injected into cytoplasm has very limited mobility. This may be the consequence of efficient internal sequestration systems which may include calcium-binding proteins bound to the cytoskeleton. A localized stimulus to the plasma membrane can elicit an equally localized response in the cytoplasm. This may explain why calcium figures so strongly in the control systems involving cytoskeleton rearrangement, secretion and endocytosis, cell division, membrane electrical activity, motor and contractile activity and cell-to-cell communication. All of these processes at some stage involve a differential response between the different parts of the cytoplasm of the same cell. Calcium research in plants has only been seriously undertaken in.the last 7-8 years and this short article outlines our current understanding; greater details can be found elsewhere (Hepler & Wayne, 1985; Trewavas, 1986). Even with the limited knowledge available, however, it is evident that the control exerted in plant cells is as profound as that in animals.