Calcium signaling in plant defense

Abstract Calcium ions (Ca 2+ ) serve as ubiquitous second messengers, orchestrating various physiological and developmental processes in plants and other eukaryotes. Upon immune activation, spatiotemporal changes in cytosolic Ca 2+ concentration, referred to as calcium signatures, play a crucial role in linking pathogen recognition to specific downstream intracellular immune responses. These signatures are generated via the coordinated activity of calcium‐permeable channels, including cyclic nucleotide‐gated channels, glutamate receptor‐like channels, hyperosmolality‐gated calcium‐permeable channels, annexins, two‐pore channels, and resistosomes derived from nucleotide‐binding leucine‐rich repeat receptors, as well as by mobilization from intracellular organelles. Meanwhile, Ca 2+ pumps and antiporters maintain cytosolic homeostasis. Calcium signals are decoded by calcium sensors, such as calmodulins and calmodulin‐like proteins, calcium‐dependent protein kinases, calcineurin B‐like proteins, and CBL‐interacting protein kinases. This decoding triggers crucial downstream immune outputs, including reactive oxygen species production, defense gene expression, hormone modulation, and programmed cell death. Pathogens deploy effectors to modulate calcium influx, sensor function, or downstream signaling, highlighting calcium signaling as a primary target in host–pathogen interactions. This review summarizes the fundamental role of calcium signaling in plant defense, focusing on recent discoveries in signal decoding and pathogen counter‐strategies, and aims to provide strategies for engineering disease‐resistant crops.