Activating plant immunity: the hidden dance of intracellular Ca 2+  stores.

Calcium ion (Ca 2+ ) serves as a versatile and conserved second messenger in orchestrating immune responses. In plants, plasma membrane-localized Ca 2+ -permeable channels can be activated to induce Ca 2+ influx from extracellular space to cytosol upon pathogen infection. Notably, different immune elicitors can induce dynamic Ca 2+ signatures in the cytosol. During pattern-triggered immunity, there is a rapid and transient increase in cytosolic Ca 2+ , whereas in effector-triggered immunity, the elevation of cytosolic Ca 2+ is strong and sustained. Numerous Ca 2+ sensors are localized in the cytosol or different intracellular organelles, which are responsible for detecting and converting Ca 2+ signals. In fact, Ca 2+ signaling coordinated by cytosol and subcellular compartments plays a crucial role in activating plant immune responses. However, the complete Ca 2+ signaling network in plant cells is still largely ambiguous. This review offers a comprehensive insight into the collaborative role of intracellular Ca 2+ stores in shaping the Ca 2+ signaling network during plant immunity, and several intriguing questions for future research are highlighted.