Plasmodiophora brassicae Effector PbPE23 Induces Necrotic Responses in Both Host and Nonhost Plants

Plasmodiophora brassicae is an obligate biotroph that causes clubroot disease in cruciferous plants, including canola and Arabidopsis. In contrast to most known bacterial, oomycete, and fungal pathogens that colonize at the host apoplastic space, the protist P. brassicae establishes an intracellular colonization within various types of root cells and secretes a plethora of effector proteins to distinct cellular compartments favorable for the survival and growth of the pathogen during pathogenesis. Identification and functional characterization of P. brassicae effectors has been hampered by the limited understanding of this unique pathosystem. Here, we report a P. brassicae effector, PbPE23, containing a serine/threonine kinase domain, that induces necrosis after heterologous expression by leaf infiltration in both host and nonhost plants. Although PbPE23 is an active kinase, the kinase activity itself is not required for triggering necrosis in plants. PbPE23 shows a nucleocytoplasmic localization in Nicotiana benthamiana, and its N-terminal ²⁵TPDPAQKQ³² sequence, resembling the contiguous hydrophilic TPAP motif and Q-rich region in many necrosis and ethylene inducing peptide 1-like proteins from plant-associated microbes, is required for the induction of necrosis. Furthermore, transcript profiling of PbPE23 reveals its high expression at the transition stages from primary to secondary infection, suggesting its potential involvement in the development of clubroot disease.