<i>Phytophthora</i> Avr3a‐Like Effectors Target and Inhibit Cinnamyl Alcohol Dehydrogenase <scp>CAD5</scp> to Suppress Plant Immunity

Phytophthora species are oomycetes that cause significant losses in agricultural production and damages to natural ecosystems. Phytophthora pathogens secrete numerous cytoplasmic effectors that target distinct cellular components to suppress host immunity and facilitate pathogen colonisation. The identification of their host targets is crucial for deciphering the mechanisms they employ to modulate host immunity. Here, we found that multiple Phytophthora Avr3a-like effectors interact with host plant cinnamyl alcohol dehydrogenase CAD5, as revealed by yeast two-hybrid and co-immunoprecipitation assays. Analysis of Arabidopsis thaliana T-DNA insertion mutants and overexpression lines, as well as analysis of RNA silencing Nicotiana benthamiana plants, showed that CAD5 positively regulates plant immunity to Phytophthora pathogens. Overexpression and silencing analyses showed that CAD5 plays a positive role in plant PAMP-triggered immunity (PTI) responses, including enhanced callose deposition, promoted cell death induced by INF1, and in plant effector-triggered immunity (ETI) responses mediated by R3a/PiAvr3a^KI recognition. CAD5 enzymatic activity was inhibited by Avr3a-like effectors, and mutagenesis analyses showed its crucial role in the positive regulation of plant immunity. In conclusion, our research showed that the Phytophthora Avr3a-like effectors target the conserved immune regulator CAD5 and suppress its enzymatic activity, which is required for both plant PTI and ETI responses.