Flavin-containing monooxygenase acts as a metabolic signaling hub linking AhR-CYP transcriptional reprogramming to insecticide resistance

Abstract Insecticide resistance is a complex adaptive trait that cannot be fully explained by individual detoxification enzymes. Flavin-containing monooxygenases (FMOs) are conserved but limited in gene number in insects, raising the question of how they contribute to broad-spectrum resistance. Whether FMOs act beyond direct xenobiotic metabolism remains unclear. We investigated FMO function in metaflumizone resistance using Spodoptera exigua and Drosophila , combined with transcriptomics, pharmacological inhibition, enzymatic assays, and molecular docking analyses. Field populations showed strong metaflumizone resistance associated with FMO upregulation, but recombinant FMOs did not directly metabolize the insecticide. In Drosophila , FMO overexpression reduced metaflumizone sensitivity, whereas knockdown increased susceptibility. Mechanistically, FMO activates the tryptophan–kynurenine metabolic pathway, thereby promoting AhR signaling and subsequently upregulating CYP detoxification genes. Pharmacological modulation of FMO or AhR consistently altered CYP expression and insecticide sensitivity in both model and field systems. These results suggest FMO contributes to insecticide resistance indirectly by functioning as a metabolic signaling regulator that links endogenous tryptophan metabolism to AhR-mediated CYP transcriptional reprogramming. These findings uncover a previously unrecognized mechanism by which an evolutionarily ancient detoxification enzyme regulates the CYP detoxification network through metabolic signaling.