The Bursaphelenchus xylophilus candidate effector BxLip-3 targets the class I chitinases to suppress immunity in pine

Lipase is involved in lipid hydrolysis, which is related to nematodes' energy reserves and stress resistance. However, the role of lipases in Bursaphelenchus xylophilus, a notorious plant-parasitic nematode responsible for severe damage to pine forest ecosystems, remains largely obscure. Here, we characterized a class III lipase as a candidate effector and named it BxLip-3. It was transcriptionally up-regulated in the parasitic stages of B. xylophilus and specifically expressed in the oesophageal gland cells and the intestine. In addition, BxLip-3 suppressed cell death triggered by the pathogen-associated molecular patterns PsXEG1 and BxCDP1 in Nicotiana benthamiana, and its Lipase-3 domain is essential for immunosuppression. Silencing of the BxLip-3 gene resulted in a delay in disease onset and increased the activity of antioxidant enzymes and the expression of pathogenesis-related (PR) genes. Plant chitinases are thought to be PR proteins involved in the defence system against pathogen attack. Using yeast two-hybrid and co-immunoprecipitation assays, we identified two class I chitinases in Pinus thunbergii, PtChia1-3 and PtChia1-4, as targets of BxLip-3. The expression of these two chitinases was up-regulated during B. xylophilus inoculation and inhibited by BxLip-3. Overall, this study illustrated that BxLip-3 is a crucial virulence factor that plays a critical role in the interaction between B. xylophilus and host pine.