Ca2+ and intracellular signalling in plant cells: a role in phytochrome transduction

The role of Ca2+ as a mediator and its participation in intracellular signaling in relation to phytochrome regulatory action are considered. Using the literature data, the non-random distribution of Ca2+ in the plant cell cytoplasm, the mechanisms of Ca2+ homeostasis and the function of putative messengers of Ca2+ are described. On the basis of the data obtained at the author's laboratory, a dual effect of phytochrome on the cytosolic Ca2+ concentration is demonstrated, i.e. phytochrome controls Ca2+ permeability of plasma membrane and induces transient changes in [Ca2+]cyt. Evidence is presented that the endoplasmic reticulum and vacuole are intracellular Ca2+ stores in the plant cell sensitive to phytochrome regulatory action. It is also demonstrated that phytochrome controls the metabolism of a key phosphoinositide in plant cell membranes. It stimulates the hydrolysis of phosphatidyl 4,5-bisphosphate, a precursor of inositol 1,4,5-trisphosphate, the potent mobilizer of Ca2+ in cytoplasm. Finally, the data on the close interactions between Ca2+ and other secondary messengers in the plant cell cytoplasm are presented. Using the transgenic tobacco expressing an apoprotein of Ca(2+)-sensitive photoprotein aequorin, evidence was obtained that cyclic mononucleotides (cGMP, cAMP) control the Ca2+ concentration in the cytoplasm testifying a crosstalk among Ca2+ and cyclic mononucleotides as messengers.