Activating plant immunity: the hidden dance of intracellular Ca²⁺ stores

Calcium ion (Ca²⁺) serves as a versatile and conserved second messenger in orchestrating immune responses. In plants, plasma membrane-localized Ca²⁺-permeable channels can be activated to induce Ca²⁺ influx from extracellular space to cytosol upon pathogen infection. Notably, different immune elicitors can induce dynamic Ca²⁺ signatures in the cytosol. During pattern-triggered immunity, there is a rapid and transient increase in cytosolic Ca²⁺, whereas in effector-triggered immunity, the elevation of cytosolic Ca²⁺ is strong and sustained. Numerous Ca²⁺ sensors are localized in the cytosol or different intracellular organelles, which are responsible for detecting and converting Ca²⁺ signals. In fact, Ca²⁺ signaling coordinated by cytosol and subcellular compartments plays a crucial role in activating plant immune responses. However, the complete Ca²⁺ signaling network in plant cells is still largely ambiguous. This review offers a comprehensive insight into the collaborative role of intracellular Ca²⁺ stores in shaping the Ca²⁺ signaling network during plant immunity, and several intriguing questions for future research are highlighted.