Plasmodiophora brassicae chitin-binding effectors guard and mask spores during infection

Plants have a sophisticated and multilayered immune system. However, plant pathogens, helped by effector proteins, have found several strategies to evade plant immunity. For instance, the clubroot pathogen, Plasmodiophora brassicae, is able to turn the roots of the susceptible hosts into nutrient-sink galls surpassing patterns-triggered immunity (PTI) and effector-triggered immunity (ETI). Chitin, the main component of P. brassicae spores cell walls and a well-known pathogens-associated molecular pattern (PAMP), can elicit PTI but is also the target of plant chitinases and chitin deacetylases. The fact that P. brassicae does not trigger PTI during the infection of the susceptible hosts motivated a genome-wide search of genes coding for secreted chitin-related proteins. We found that P. brassicae genome encodes a large repertoire of candidate-secreted effectors containing the chitin-binding domain carbohydrate-binding module family 18 (CBM18), along with chitinases and chitin deacetylases domains. The role of such proteins in the pathogenicity of the clubroot pathogen is unknown. Here, we characterized the function of two effectors, PbChiB2 and PbChiB4, which are transcriptionally activated during the spores transition to uninucleate primary plasmodium and during the spore formation. Through co-precipitation, we found that recombinant PbChiB2 and PbChiB4 bind to the spores and to chitin oligomers in vitro. We also showed that both proteins suppress chitin-triggered activation of the immune MPK3 and MPK6 in the host Brassica napus. These findings suggest a dual role for the P. brassicae CBM18 proteins as effectors for protecting zoospores and resting spores formation and for suppressing chitin-triggered immunity during the infection.