Calcium Signalling: A Key Mediator in Plant Immune Mechanism

Abstract

Plants lack the adaptive immune system found in vertebrates and rely solely on innate defenses composed of two interrelated layers: pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). The perception of pathogen-associated molecular patterns (PAMPs) by pattern recognition receptors (PRRs) activates PTI leading to early immune responses such as calcium (Ca²⁺) influx and reactive oxygen species (ROS) burst. Conversely, pathogen-secreted effector proteins target host defense components to suppress PTI but their recognition by intracellular resistance (R) proteins triggers ETI and hypersensitive cell death. Calcium functions as a pivotal secondary messenger in both PTI and ETI, mediating the activation of downstream defense signalling networks. Plasma membrane channels such as cyclic nucleotide-gated channels (CNGCs) and glutamate receptor-like channels (GLRs) regulate Ca²⁺ influx while intracellular stores such as the vacuole and endoplasmic reticulum contribute additional Ca²⁺ release. Calcium sensors including calmodulins (CaMs), calcium-dependent protein kinases (CDPKs) and calcineurin B-like proteins (CBLs) decode these Ca²⁺ signals to modulate ROS bursts and hormone-mediated defenses involving salicylic acid (SA) and jasmonic acid (JA). This review synthesizes insights into calcium signalling in plant immunity, highlighting key molecular players and signalling crosstalk.