P-013 - Calcium decoding links pathogen/effector recognition with the execution of defense responses

Ca²⁺ is a crucial secondary messenger in plant-microbe interaction. The initial recognition of a microbial pathogen through conserved microbial-derived patterns initiates PTI, which includes a transient Ca²⁺ increase in the cytosol. Pathogens can suppress PTI by secreting PTI-dampening effectors. However, resistant plants detect these effectors, activating ETI, which induces a stronger defense response that involves a longer-lasting Ca²⁺ increase with higher amplitude. What distinguishes these distinct Ca²⁺ signatures in both layers of immunity and how are these translated into specific downstream defense responses? To investigate this, we selected several Ca²⁺-dependent protein kinases (CDPKs), which function as distinct ‘decoders’ (decoding synonymous to (i) bind Ca²⁺, (ii) adopt an intramolecular conformational change, and (iii) catalyze trans-phosphorylation). To explore the specificity of CDPK-dependent decoding in PTI and ETI we have developed CPKaleon, a biosensor that visualizes CDPK conformational activation and inactivation in real-time in planta. Our data identify decoding dynamics that differ significantly between PTI and ETI with respect to Ca²⁺ dependent activation patterns and to CDPK isoforms. Our data further imply that CDPK decoding functions as key driver for phosphorylation-mediated signaling. Our unique approach presents Ca²⁺ decoding as a missing link between pathogen recognition and execution of appropriate downstream responses in plant immunity.