Calcium signaling system in plants

Calcium ions have unique properties and universal ability to transmit diverse signals that exert primary action on cells, such as hormones, pathogens, light, gravity, and stress factors. The principal elements in the system of calcium signaling of plant cells are different Ca2+ channels, Ca2+-ATPases, Ca2+/H+ antiporters, Ca2+-binding and Ca2+-dependent proteins. The system of calcium signaling also includes receptors, the cascades of amplifying Ca2+ signals, and transcription factors. The process of transmitting the calcium signal within the cell consists of at least two stages. At the first stage, the cytosolic calcium concentration undergoes temporal and usually local increase due to its entry through the Ca2+ channels. The second stage is related to the signal decay and represents the active removal of calcium excess from the cytosol to the extracellular medium or organelles (vacuoles, endoplasmic reticulum, mitochondria) by means of Ca2+-ATPases and/or Ca2+/H+ antiporters. The primary intracellular targets of calcium are various calcium-binding proteins. Some of these proteins ensure Ca2+ transport, others serve as a calcium buffer, and the others (e.g., calmodulin or Ca2+-dependent protein kinases) translate the calcium signal to intracellular operational mechanisms and initiate Ca2+-dependent physiological processes. An important feature of the calcium signal transduction is that this signal originates and propagates in the pulse mode. Such way of information transmission is not only faster than the diffusion but it also ensures the spatiotemporal regulation of cell functions, because the signal encoding can be realized via amplitude- and frequency-modulated oscillations in cytosolic calcium concentration.