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

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