Plasmodiophora brassicaechitin-binding effectors guard and mask spores during infection

Abstract

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.

Looking for a Cultured Surrogate for Effectome Studies of the Clubroot Pathogen

“…A simple solution would be to explore another culturable surrogate. In a recent study, our group identified two effectors, PbChiB2 and PbChiB4, able to bind to chitin oligomers in vitro, and suppress chitin-triggered immunity (Muirhead and Pérez-López, 2020). These effectors were able to bind chitin through a CBM18 domain as previously described for the soil-borne pathogen Verticillium dahliae (Volk et al, 2020).…”

What Can We Learn from -Omics Approaches to Understand Clubroot Disease?

“…This was done to show that overexpression of PbBSMT caused more susceptible plants [ 74 ] as well as higher levels of the SA methylester (Me-SA) vs. SA [ 73 ]. Other examples used a larger-scale experimental approach with tobacco leaves and transient transformation of numerous effectors [ 84 , 85 ]. These also led to some information on their localization but were all rather artificial involving a no-host organism and also the “wrong” organs, namely leaves.…”

“…Putative effectors with chitin-binding domains were also discovered [ 84 ], of which the secretion peptide was confirmed in yeast assays. Co-precipitation in vitro showed binding of these domains to chitin and to P. brassicae resting spores.…”

“…Co-precipitation in vitro showed binding of these domains to chitin and to P. brassicae resting spores. Furthermore, the participation of these two proteins, PbCHi2 and PbChi4, in chitin-dependent activation of the immune response in B. napus was shown [ 84 ].…”

Molecular Pathotyping of Plasmodiophora brassicae—Genomes, Marker Genes, and Obstacles

“…The genomes of P. brassicae and S. subterranea show an enrichment of proteins that include the chitin binding CBM18 domain, either as part of CE4-deacetylase proteins or as functional domains in SSP [18]. For two CBM18 domain containing SSPs of P. brassicae it indeed appears that they suppress chitin-triggered in B. napus [77].…”