Melatonin Enhances <i>Panax vietnamensis</i> Resistance to Leaf Blight Pathogen <i>Neofusicoccum ribis</i> via the <i>PvWRKY40–PvCOMT2</i> Module‐Driven Lignin Biosynthesis

Panax vietnamensis, a medicinally valuable perennial herb, is highly susceptible to leaf blight under cultivation; however, the molecular mechanisms underlying this disease remain poorly understood. In this study, we identified Neofusicoccum ribis as the causal agent of P. vietnamensis leaf blight through pathogen isolation and fulfilment of Koch's postulates. Transcriptomic analysis revealed activation of phytohormone signalling (salicylic acid, jasmonic acid, and melatonin [MT]) and phenylpropanoid metabolism during infection. Among these, MT exhibited superior efficacy in inducing lignin biosynthesis compared to other hormones, with exogenous application of MT significantly enhancing lignin accumulation and improving disease resistance by 8 days post-inoculation. Further, we identified PvWRKY40 as a negative regulator of lignin synthesis, which directly binds to the W-box motif in the PvCOMT2 promoter to suppress its expression. MT counteracted this repression by downregulating PvWRKY40. Heterologous overexpression of PvCOMT2 in Nicotiana benthamiana increased lignin content and conferred enhanced resistance to Fusarium oxysporum. This study reveals a novel MT-PvWRKY40-PvCOMT2 regulatory axis governing lignin-mediated defence in P. vietnamensis, providing critical insights for combating leaf blight in cultivated ginseng.