Chitosan oligosaccharide enhances the tolerance of Pinus koraiensis to Bursaphelenchus xylophilus through the flavonoid biosynthesis pathway.

Chitosan oligosaccharide (COS) is a widely used plant inducer. However, research on its application as an inducer for controlling plant-parasitic nematodes remains limited. In this study, Pinus koraiensis was treated with three concentrations of COS, followed by inoculation with pine wood nematode (PWN, Bursaphelenchus xylophilus). The efficacy of these COS concentrations in enhancing the resistance of P. koraiensis to PWN was evaluated using the relative tolerance index (RTI) and the relative anti-nematode index (RAI). Among the treatments, 0.5 % COS exhibited the optimal induction effect, effectively delaying symptom development in P. koraiensis. To further explore the underlying mechanism, transcriptomic and metabolomic analyses were performed. At the transcriptional level, COS pretreatment followed by PWN inoculation activated the expression of genes involved in plant defense-related pathways, with the flavonoid biosynthesis pathway being the primary target. Notably, 7 days post-inoculation (dpi) emerged as a critical time point for COS-mediated resistance induction in P. koraiensis. Integrated metabolomic results revealed that COS treatment modulated the expression of a gene network centered on chalcone synthase (chs) genes within the flavonoid biosynthesis pathway. Specifically, it upregulated the expression of 21 chs, 2 chalcone isomerase (chi), 2 trans-cinnamate 4-monooxygenase (cyp73a), 3 flavanone 3 β-hydroxylase (f3h), 5 flavonoid 3'-monooxygenase (cyp75b1), 3 dihydroflavonol 4-reductase (dfr), and 7 anthocyanidin reductase (anr) genes. This upregulation promoted the biosynthesis and accumulation of chrysin, fustin, catechin, and epicatechin, thereby enhancing the tolerance of P. koraiensis to PWN.