Pst DC3000 induces pathogenesis-uncorrelated cytosolic Ca²⁺ rise in Arabidopsis leaves

In nature, the phytopathogen usually initiates its infection on the leaf surface before moving into the internal space through natural openings. Little is known about immediate response of the leaf to the surface-colonizing phytopathogen and its correlation with individual microbe-associated molecular patterns (MAMPs). In this study, we monitored the dynamic changes in the cytosolic Ca²⁺ concentration ([Ca²⁺]cyₜ) in the Arabidopsis leaf expressing luminescence protein aequorin as the response to the surface-inoculating Pseudomonas syringae DC3000 (Pst DC3000) with a touching-free system. The significant [Ca²⁺]cyt transient rise was evoked in the leaf right after inoculation, and its magnitude was correlated with the pathogen concentration. Pharmacological studies revealed that the rising [Ca²⁺]cyₜ occurs primarily from the cAMP-mediated Ca²⁺ mobility pathway, but not Gd³⁺-sensitive Ca²⁺ influx channel in the plasma membrane, which was distinct from those induced by individual MAMPs (lipopolysaccharide, flagellin, and elongation factor Tu). Pretreating the leaf with Pst DC3000 or MAMPs significantly attenuated its responses to subsequent treatments of any of them, which indicates that the leaf has the convergent mechanism of sensitivity to the pathogen and MAMPs. Furthermore, Pst DC3000 mutants defective in flagellum, type III secretion apparatus, and phytotoxin coronine production significantly lost their multiplication ability in the leaf apoplast, but evoked [Ca²⁺]cyₜ responses comparable with that of the wild type. Taken together, these data indicates that the [Ca²⁺]cyₜ in the leaf has the sensitive response to the surface-inoculating phytopathogen, which was distinct from those of individual MAMPs and had no correlation with the pathogen pathogenesis capacity.