Roles of Signaling Compounds and WRKY31 in the Defense of Pinus massoniana L. Against Dendrolimus punctatus

Dendrolimus punctatus is an important pest affecting Masson pine (Pinus massoniana L.) forests and can cause serious economic and ecological losses. WRKY transcription factors play important roles in coping with various environmental stresses. In particular, recent studies have shown that WRKY transcription factors play an important role in plant responses against herbivorous insects. However, the mechanisms underlying the actions of these genes in the defense responses of P. massoniana L. are still unclear. Our previous study provided evidence that WRKY may play an important role in the insect resistance of P. massoniana L. In this study, application of semiochemicals such as exogenous hormones and Ca2+ by spraying increased the concentrations of endogenous hormones, terpenoid synthases, and volatile substances in P. massoniana L. and effectively improved its resistance to D. punctatus. After analyzing the WRKY family of P. massoniana L., the PmWRKY31 gene was selected and studied. Yeast two-hybrid assays showed that the LP8 gene interacted with PmWRKY31. Fluorescence-based quantitative polymerase chain reaction showed that after treatment with exogenous hormones and Ca2+, the expression levels of the PmWRKY31 gene, hormonal signal-related genes, and terpene biosynthetic pathway-related genes were significantly increased, whereas the expression of the LP8 gene was decreased. Therefore, the PmWRKY31 and LP8 genes affected downstream gene expression by positively and negatively regulating the hormone signaling pathways, respectively. This result provides theoretical support for the involvement of WRKY transcription factors in the insect resistance of P. massoniana L. through their regulation of hormone signaling.