Antioxidant potential and metabolic profiling of Viola odorata and the endophytic Rhizobium sp. BR7

The interaction between medicinal plants and their root-associated microbes plays a key role in shaping rhizosphere chemistry and bioactivity. This study evaluates the antioxidant potential and metabolic profiles of Viola odorata roots and associated Rhizobium sp. BR7 using in vitro DPPH, ABTS, FRAC based antioxidant assays and LC-MS-based untargeted metabolomics. Both plant and microbial extracts exhibited significant free radical-scavenging capacity. The isolate BR7 showed the most potent antioxidant activity (IC₅₀ = 69 µg/mL), followed by AH3, JN9, and SGA9. Isolates BR5 and KB11 showed lower efficacy. The inverse correlation was observed between flavonoid content and ABTS based antioxidant activity, indicating the key role of flavonoids in antioxidant potential. LC-MS based analysis revealed overlapping antioxidant metabolites such as caffeic acid derivatives, kaempferol-3-O46 glucoside, apigenin, and chlorogenic acid, indicating metabolic complementarity between plant and bacteria. Besides these, BR7 also contained unique compounds including indole-3-acetic acid (IAA) fragments, rhamnolipid-like biosurfactants, and alkylresorcinols, suggesting strong rhizosphere adaptability. Overall, Rhizobium sp. BR7 emerges as a promising source of antioxidant co-metabolites with potential applications in biopharmaceutical and functional food development.