Calcium homeostasis and signaling in plant immunity

Calcium (Ca²⁺) signaling consists of three steps: (1) initiation of a change in cellular Ca²⁺ concentration in response to a stimulus, (2) recognition of the change through direct binding of Ca²⁺ by its sensors, (3) transduction of the signal to elicit downstream responses. Recent studies have uncovered a central role for Ca²⁺ signaling in both layers of immune responses initiated by plasma membrane (PM) and intracellular receptors, respectively. These advances in our understanding are attributed to several lines of research, including invention of genetically-encoded Ca²⁺ reporters for the recording of intracellular Ca²⁺ signals, identification of Ca²⁺ channels and their gating mechanisms, and functional analysis of Ca²⁺ binding proteins (Ca²⁺ sensors). This review analyzes the recent literature that illustrates the importance of Ca²⁺ homeostasis and signaling in plant innate immunity, featuring intricate Ca²⁺dependent positive and negative regulations.