Local traveling waves of cytosolic Ca ²⁺ elicited by defense signals or wounding are propagated by distinct mechanisms in <i>Arabidopsis</i>

Cytosolic calcium ion (Ca²⁺) signatures with specific spatiotemporal patterns play crucial roles in plant responses to biotic and abiotic stresses. The perception of microbe- or damage-associated molecular patterns (MAMPs or DAMPs, respectively) initiates cytosolic Ca²⁺ fluxes that are essential for the induction and spread of pattern-triggered immunity, the first line of plant defense against pathogens, at the cellular, organ, and systemic levels. Here, we quantitatively assessed Ca²⁺ signatures at the single-cell level, as well as the local traveling Ca²⁺ waves induced by uniform MAMP or DAMP treatment of <i>Arabidopsis thaliana</i> cotyledons. MAMPs and DAMPs induced distinct local spatiotemporal Ca²⁺ responses in epidermal pavement cells, with traveling waves of Ca²⁺ consistently initiated from a randomly distributed subset of cells and spreading in an approximately radial pattern. These local traveling waves propagated at a slow but constant speed of ~1 micrometer per second and spread to a limited number of neighboring cells. In contrast, wound-induced traveling waves of Ca²⁺, which are propagated by the diffusion of molecules that activate Ca²⁺ channels, displayed a diffusion-like decay pattern that moved rapidly away from the wounded cell but with diminishing speed over time and distance. Mathematical modeling supported a Ca²⁺-induced Ca²⁺ release mechanism that recapitulated the constant wave speed induced by MAMPs. These findings contribute to a deeper understanding of plant defense-related Ca²⁺ signaling mechanisms, as well as how defense responses are spatially restricted in tissues.