Integrated transcriptomics and metabolomics analysis reveals the dynamic defense mechanisms of Pinus tabuliformis against oxalic acid stress

Oxalic acid (OA) is a fungal organic acid closely associated with host acidification, oxidative stress, and defense modulation, yet its biochemical effects on conifer root defense metabolism remain poorly understood. Sparassis latifolia, a facultative parasitic fungus associated with pine root disease, accumulated extracellular OA up to approximately 6.08 mM in liquid culture. In this study, we used exogenous OA treatments to simulate fungus-relevant chemical stress and investigated the physiological, transcriptomic, and metabolomic responses of Pinus tabuliformis roots. Seedlings were exposed to two levels of OA stress (2 and 6 mM), representing moderate and high OA accumulation scenarios, respectively, to investigate dose-dependent defense responses. OA caused concentration and duration dependent root injury, including browning, shrinkage, ultrastructural collapse, and increased accumulation of H2O2, O2⁻, and malondialdehyde. Prolonged exposure to 6 mM OA reduced antioxidant enzyme activities and depleted ascorbate and glutathione, indicating impaired redox buffering capacity. Widely targeted metabolomics and transcriptomics revealed extensive reprogramming of phenylpropanoid biosynthesis, flavonoid biosynthesis, glutathione metabolism, and plant hormone signaling. In particular, high-concentration OA (6 mM) suppressed key phenylpropanoid and flavonoid pathway genes, including PAL, 4CL, CHS, CAD1, and LAR, and reduced defense-related metabolites such as catechin, epicatechin, and quercetin. WGCNA further identified gene modules associated with flavonoid metabolites, redox regulation, and hormone related compounds. These findings indicate that fungus-relevant OA exposure not only induces oxidative injury but also uncouples stress-responsive transcriptional reprogramming from the sustained production of phenylpropanoid-flavonoid defense metabolites, providing biochemical insight into the OA-associated chemical environment of S. latifolia-pine interactions.