A small cysteine-rich protein identified from the Proteome of clubroot pathogen, Plasmodiophora brassicae, induces cell death in nonhost plants and host plants

Abstract Plasmodiophora brassicae is a devastating intracellular pathogen that causes clubroot disease in Brassicaceae plants, which also secrete various effector proteins promoting pathogen colonization and reprogramming host developmental processes. During screening for candidate effector proteins from the Proteome of P. brassicae, we identified a small secreted protein, PbEL04, which can trigger cell death associated with H2O2 accumulation and electrolyte leakage in the nonhost plant Nicotiana benthamiana. This ability to induce cell death was further validated using the recombinant protein produced by Escherichia coli. Moreover, PbEL04-triggered cell death did not require a signal peptide. Structural analysis showed that PbEL04 is a cysteine-rich protein consisting of four tandem EGF-like domains, few of which were homologous to other pathogens. In host plants, PbEL04 could induce cell death in Arabidopsis thaliana protoplasts, and was highly expressed in the process of gall formation. Furthermore, the severity of clubroot disease was greatly reduced in the transgenic lines of PBRA_004448 compared with wild-type Arabidopsis Col-0 after being infected by P. brassicae. Taken together, our study identifies a new secretory protein involved in plant immune response in nonhost and host plants.