The conserved GOLD domain in the Plasmodiophora brassicae effector Pb257 is required for triggering cell death and root swelling

Plasmodiophora brassicae is one of the biggest threats to cruciferous plants and one of the most economically significant diseases worldwide. However, our current understanding of its pathogenic mechanisms remains limited. In this study, we have identified an effector, Pb257, which plays a crucial role in the virulence of P. brassicae. The expression pattern analysis revealed continuous induction of Pb257 during both primary and secondary infections. Ectopic expression of Pb257 strongly triggered cell death in Nicotiana benthamiana and several other plants, which was dependent on SOBIR1 and the salicylic acid pathway but not BAK1, a central molecular regulator, to mediate classical defense pathways. Overexpression of Pb257 increased susceptibility of Brassica rapa to P. brassicae. Silencing of the Pb257-encoding gene led to reduce root swelling. Further investigation showed that the conserved GOLD domain within Pb257 was essential for inducing root enlargement, which is similar to the mild symptoms of clubroot, indicating that it is an important effector for promoting root cell enlargement. GOLD domain-containing family proteins were widely present in the kingdoms of protozoa, fungi, and animalia, and Pb257 was clustered into a clade of protozoa, suggesting the encoding gene as a potential marker gene for classification of P. brassicae. The GOLD domain-containing proteins are known regulators involved in human cell proliferation and immune system disruption, however, no previous reports have described a pathogen-derived GOLD domain protein with elicitor activity. Our findings demonstrate that Pb257 functions as a crucial effector contributing to the virulence of P. brassicae and could be used as a potential molecular target for controlling clubroot disease.