*2.3. Identification of Di*ff*erentially Accumulated Metabolites and Di*ff*erentially Accumulated Flavonoids*

Differentially accumulated metabolites (DAMs) were defined as those exhibiting a fold change ≥ 2 or ≤0.5 and a variable importance of projection (VIP) ≥ 1. In total, 135 DAMs were identified, among which 63 and 72 metabolites were upregulated and downregulated, respectively. To further understand the DAMs function and the related biological processes they participated in, pathway enrichment analysis of the DAMs was conducted using KEGG. The results showed that the terms "Biosynthesis of phenylpropanoids", "Flavonoid biosynthesis", "Polycyclic aromatic hydrocarbon degradation", "Oxidative phosphorylation", and "Naphthalene degradation" were significantly enriched (Figure 3B). Among them, the flavonoid biosynthesis began with the biosynthesis of phenylpropanoids. To some

extent, these data further indicated that the flavonoid metabolism in *A. roxburghii* was significantly affected by mycorrhizal fungus AR2.

Furthermore, 148 metabolites involved in the flavonoid metabolism were identified, among which nine metabolites that belonged to two flavones (i.e., nobiletin and tangeretin), two flavonols (i.e., quercetin and isorhamnetin) and five flavonol-glycosides (i.e., narcissin, rutin, isorhamnetin-3-O-beta-d-glucoside, quercetin-7-O-glucoside, and kaempferol-3-O-glucoside) were obviously different between M and NM.
