Biphasic Dose–Response and Mechanism Analysis of Vanillic Acid from Larix gmelinii on Neofusicoccum laricinum

Phenolic levels in Larix gmelinii are closely correlated with its interaction with Neofusicoccum laricinum. As a vital subclass of phenolics, phenolic acids participate in both constitutive and inducible plant defense responses. In the present study, metabolomic, transcriptomic, physiological and biochemical analyses were conducted to investigate the tissue heterogeneity of phenolic acid metabolism in larch stem tissues, as well as the dose-dependent effects and underlying mechanism of vanillic acid (VA) on N. laricinum, providing valuable insights into the interaction between L. gmelinii and N. laricinum. A total of 119 differentially accumulated phenolic acid metabolites were identified in this study. Shoots exhibited markedly higher levels of most phenolic acids, including VA, than trunks and branches, accompanied by significant enrichment of the phenylpropanoid biosynthesis pathway. Notably, VA exhibited a biphasic dose–response on N. laricinum, facilitating the growth of N. laricinum at low concentrations (2–1117.12 μg/mL) and exerting inhibitory effects at high concentrations (>1117.12 μg/mL). Transcriptomic analysis further suggested that phenolic acid metabolism and the overcompensation effect collectively accounted for this unique dose–response pattern. Collectively, this study verified that phenolic acid metabolism across L. gmelinii stem tissues presented distinct tissue heterogeneity, and VA exerted a biphasic dose–response on N. laricinum.