While a role of calcium-dependent protein kinase (CDPK) signalling in particular during the early onset of the local plant defence response has been described before, the objective of our project C2 is the participation and function of CDPKs as calcium sensor proteins in priming and systemic long-term resistance of Arabidopsis thaliana to biotic stresses such as an attack by bacterial phytopathogens or herbivores. In line with our key hypothesis we were able to identify CPK5 as a positive regulator, which becomes biochemically activated upon a pathogen-related priming stimulus, and which is responsible for resistance of the plant to biotrophic bacterial pathogens. In a hallmark study and subsequent publications we identified the NADPH oxidase RBOHD as an in vivo phosphorylation target of CPK5: based on a CPK5- and RBOHD-dependent mutual self-activation circuit a model for ROS-dependent systemic signal propagation from the local infection site to distal areas of the plant has been proposed by us. In addition, CPK5 mediates transcriptional reprogramming and induces salicylic acid-mediated resistance to bacterial pathogens discussed as prerequisite for systemic acquired resistance (SAR). Furthermore, our data provide evidence that CPK5 participates in particular in the onset of systemic defence responses consistent with a role in priming, in contrast to other even closely related homologues such as CPK6, or in contrast to, for example, CPK28, which functions as a negative regulator of PAMP-signalling in the context of induction of early defence responses. Therefore, we currently study how the plant benefits from CPK5-mediated priming of pathogen resistance, and besides the analysis of molecular markers, respective pathogen assays based on avirulent and virulent bacterial Psm strains have been established.
Based on our key results during phase I we will in phase II on the one hand focus on the biochemical mechanism underlying CPK5-mediated priming (and forgetting) by investigating the enzyme’s reversible modifications required for in vivo activation/inactivation such as phosphorylation, calcium binding or irreversible degradation. On the other hand we will investigate CPK5-induced downstream transcriptional reprogramming with a view to identify CPK5 targeted major transcriptional regulators. From a more ecological perspective CPK5-mediated priming of SAR to bacterial pathogens will be investigated in the context of plant ontology, and in collaborative work, we aim to elucidate the potential role of CDPKs in their role of trans-priming of the plant´s anti-herbivore defence by using pathogens as priming stimulus and insect egg deposition and/or larval feeding damage as triggering stimulus.