Sense and sensitivity - decoding calcium signalling across cellular, autocrine, paracrine and endocrine pathways in plant resilience

Calcium (Ca2+) signalling plays a central role in plant immunity, as underscored by recent findings showing that many disease resistance mechanisms result in formation of Ca2+-permeable pores, and that optogenetic activation of Ca2+ influx is sufficient to trigger immune responses. This review emphasizes on Ca2+ decoding, i.e. how diverse intracellular proteins interpret Ca2+ signals to drive cellular reactions. States of "Ca2+ responsiveness" - defined by the distinct sensitivities of various decoders and additional sensitization mechanisms - contribute to the regulation of immunity, possibly including the mutual potentiation of pattern- and effector-triggered immunity pathways. Additionally, the "PRIMER-bystander" model of immune signalling is interpreted within this decoding framework. Here, infected cells are proposed to enter a primed (PRIMER) immune state through strong Ca2+ signals derived from resistosome pores, while adjacent bystander cells respond to spreading signalling molecules from their neighbours. Through this spatial arrangement, coordination is achieved between cell-autonomous (autocrine) responses and non-autonomous (paracrine or endocrine) signalling, allowing robust immune propagation across plant tissues. By framing plant immunity through this Ca2+ "sense and sensitivity" paradigm, insights are provided into immune system robustness, and potential targets may be identified for future development of disease-resistant, climate-resilient crops.