Principles of Calcium Signal Generation and Transduction in Plant Cells

Calcium ions exhibit unique properties and a universal ability to transmit diverse signals in plant cells under the primary action of hormones, pathogens, light, gravity, and various abiotic stressors. In the last few years, considerable progress has been achieved in deciphering the mechanisms of Ca2+ involvement in the regulation of plant responses. Recent studies revealed the genes encoding Ca2+-permeable channels that conduct Ca2+ currents across the membranes during the transduction of the Ca2+ signal. These proteins comprise the ligand-gated Ca2+-permeable channels activated by cyclic nucleotides (CNGC) and amino acids (glutamate receptor-like channels, GLR), the voltage-gated tonoplast channel (two-pore channel, TPC1), mechanosensitive channels (MSL, MCA, OSCA1), and annexins. The role of Ca2+-ATPase and Ca2+/H+-exchangers in the active extrusion of excess cytoplasmic Ca2+ into the apoplast or cell organelles was examined in detail. The calmodulins (CaM), CaM-like proteins (CML), Ca2+-dependent protein kinases (CDPK), and complexes of calcineurin-B-like proteins (CBL) with CBL-interacting protein kinases (CIPK) were found to produce intricate signaling networks that decode Ca2+ signals and elicit plant responses to external stimuli. This review analyzes the data accumulated over the past decade on the principles of formation and propagation of the calcium signal in plant cells.