39 Calcium, Protons, and Potassium as Inorganic Second Messengers in the Cytoplasm of Plant Cells

Abstract

The cyclic nucleotides and phosphoinositides are second messengers that play recognized roles in mediating the effects of hormones in animals. Changes in the concentrations of these messengers are mediated by changes in the activities of enzymes that catalyze their synthesis or breakdown (e.g., adenyl cyclase, cAMP phosphodiesterase, phosphatidylinositol kinase, phospholipase C). In higher plants, despite several decades of physiological and biochemical research, the role of these potential signal-transducing messengers remains unclear.

It is also recognized that in all eukaryotes, the cytoplasmic concentration of calcium is maintained at a very low resting level (~100 nM), which fluctuates in response to regulatory or developmental cues. Recent results demonstrate that calcium plays an important role as a second messenger in higher plants. Unlike cAMP and the phosphoinositides, however, calcium is nonmetabolizable and, thus, changes in its concentration are mediated exclusively by alterations in transport across membranes, into or out of the cytoplasm. In this chapter, we review the state of our knowledge on calcium transport in higher plants. We also review the state of our knowledge on proton and potassium transport, since the cytoplasmic concentrations of these monovalent cations may also play key growth-regulating roles. Although it is not yet clear whether cytoplasmic proton and potassium ions can be considered true plant second messengers, we propose here a model in which changes in their concentrations cause changes in cell growth, possibly mediated by an amplification scheme involving the proton motive force and turgor pressure.

Central to this model is the recognition that all...