<i>Bursaphelenchus xylophilus</i> Venom Allergen-Like Protein BxVAP1, Triggering Plant Defense-Related Programmed Cell Death, Plays an Important Role in Regulating <i>Pinus massoniana</i> Terpene Defense Responses

<i>Bursaphelenchus xylophilus</i> (pine wood nematode, PWN), a migratory plant-parasitic nematode, acts as an etiological agent, inflicting considerable damage to pine forests worldwide. Plant immunity constitutes a crucial factor in resisting various pathogenic invasions. The primary defensive responses of host pines against PWN infection encompass terpene accumulation, defense response-related gene expression, and programmed cell death. Venom allergen-like proteins (VAPs), as potential effectors, are instrumental in facilitating the successful colonization of PWNs. In this study, we investigated the inhibition of <i>B. xylophilus</i> VAP (<i>BxVAP1</i>) expression by RNA interference in vitro. Following <i>BxVAP1</i> silencing, the reproduction rate and migration rate of the PWN population in <i>Pinus massoniana</i> decreased, the expression of the α-pinene synthase gene was induced, other terpene synthase and pathogenesis-related genes were inhibited and delayed, the peak times and levels of terpene-related substances were changed, and the degree of cavitation in <i>P. massoniana</i> was diminished. Transient expression of BxVAP1 in <i>Nicotiana benthamiana</i> revealed that BxVAP1 was expressed in both the cell membrane and nucleus, inducing programmed cell death and the expression of pathogen-associated molecular pattern-triggered immunity marker genes (<i>NbAcre31</i> and <i>NbPTI5</i>). This study is the first to demonstrate that silencing the <i>BxVAP1</i> gene affects host defense responses, including terpenoid metabolism in <i>P. massoniana</i>, and that BxVAP1 can be recognized by <i>N. benthamiana</i> as an effector to trigger its innate immunity, expanding our understanding of the parasitic mechanism of <i>B. xylophilus</i>.