Calcium signaling and biotic defense responses in plants.

Calcium (Ca(2+)) acts as an important second messenger in plant cells. Cytosolic free Ca(2+) concentration in plant cells changes rapidly and dynamically in response to various endogenous or environmental cues. Elevation in calcium concentration in plant cells is an essential early event during plant defense responses. Alterations in the Ca(2+) concentration are sensed by Ca(2+)-binding proteins, including calmodulin, calcium-dependent protein kinases and calcineurin B-like proteins, which relay or decode the encoded Ca(2+) signals into specific cellular and physiological responses in order to survive challenges by pathogens. Genetic and functional studies have revealed that Ca(2+) signaling plays both positive and negative roles in regulating the establishment of defense responses. Furthermore, recent studies revealed that actions of Ca(2+)-mediated signaling could be regulated by other cell signaling systems such as the ubiquitin-proteasome system to mount precise and prompt plant defense responses.