A clubroot pathogen effector targets cruciferous cysteine proteases to suppress plant immunity

Plant pathogen effector proteins are key to pathogen virulence. In susceptible host Brassicas, the clubroot pathogen, <i>Plasmodiophora brassicae</i>, induces the production of nutrient-sink root galls, at the site of infection. Among a list of 32 <i>P. brassiae</i> effector candidates previously reported by our group, we identified SSPbP53 as a putative apoplastic cystatin-like protein highly expressed during the secondary infection. Here we found that SSPbP53 encoding gene is conserved among several <i>P. brassicae</i> pathotypes and that SSPbP53 is an apoplastic protein able to directly interact with and inhibit cruciferous papain-like cysteine proteases (PLCPs), specifically Arabidopsis XYLEM CYSTEINE PEPTIDASE 1 (<i>At</i>XCP1). The severity of clubroot disease is greatly reduced in the Arabidopsis <i>xcp1</i> null mutant (<i>At</i>Δ<i>xcp1</i>) after infection with <i>P. brassicae</i> resting spores, indicating that the interaction of <i>P. brassicae</i> SSPbP53 with XCP1 is important to clubroot susceptibility. SSPbP53 is the first cystatin-like effector identified and characterized for a plant pathogenic protist.