Fkh dual regulation of promoter activation and miR‐34– p38b axis enhances IMD pathway immune response in Drosophila

Innate antibacterial defense in Drosophila relies on the IMD pathway to induce antimicrobial peptides (AMPs), but the auxiliary transcriptional networks that amplify or fine-tune this response remain poorly defined. Here, we identify the Fork head (Fkh) transcription factor, a FoxA-family protein classically linked to development and metabolism, as a critical amplifier of humoral immunity. Our results show that bacterial infection rapidly induces Fkh expression, and enhances host survival, as well as promotes robust induction of AMPs including Diptericin (Dpt), Attacin-A, and Cecropin-A1. Mechanistically, Fkh directly binds the Dpt promoter and synergizes with the NF-κB factor Relish, ensuring strong promoter activation. In parallel, Fkh activates miR-34 transcription to repress the negative regulator p38b, thereby relieving inhibitory pressure on IMD signaling. Disruption of either Fkh-binding motifs or the miR-34 seed site abolishes these effects. Together, our findings uncover a dual regulatory strategy in which Fkh simultaneously acts as a promoter-bound transcription factor and as an upstream activator of an immune-enhancing miRNA. This integrated Fkh-miR-34-p38b axis establishes a feed-forward mechanism that ensures rapid and high-amplitude antibacterial defense, providing a new paradigm for transcription factor-miRNA cooperation in innate immunity.