Deploying iron as a defense signal: Host utilization of Metarhizium anisopliae CFEM88-ferritin interaction to trigger antifungal immunity.

The interaction between the entomopathogenic fungus Metarhizium anisopliae and its insect hosts involves complex immune countermeasures; however, the function of the effector protein MaCFEM88 in this interaction remains poorly understood. In this study, we investigated the role of the secreted effector MaCFEM88 and identified an unexpected effect of its deletion on fungal virulence. Deletion of MaCFEM88 significantly enhanced fungal virulence against multiple insect hosts, including the lepidopterans Plutella xylostella (diamondback moth) and Bombyx mori (silkworm), as well as the coleopteran Tenebrio molitor (yellow mealworm), without affecting fungal growth, sporulation, or stress tolerance. Mechanistically, MaCFEM88 specifically binds to a highly conserved host ferritin (FerL) in lepidopteran species. This interaction is associated with disruption of iron homeostasis and increased iron levels in the hemolymph, accompanied by elevated reactive oxygen species (ROS). The resulting oxidative stress induces a dynamic and coordinated upregulation of key antioxidant enzymes (POD, CAT, and SOD) and activates the expression of the antimicrobial peptide defensin. RNAi-mediated silencing of BmFerL abolished MaCFEM88-associated iron overload, ROS accumulation, and defensin induction, supporting a functional role for FerL in these responses. Collectively, our findings suggest that MaCFEM88-mediated modulation of host ferritin may concurrently influence iron availability and host immune responses. We propose a testable nutritional-defense model in which the effector disrupts ferritin to release sequestered iron-a maneuver that, while initially appearing to benefit the pathogen, simultaneously generates a host danger signal implicated in antifungal defense activation. Nevertheless, the relative contribution of this mechanism to fungal virulence, whether through immune evasion or alternative iron-acquisition routes, remains unresolved. This study offers fresh insights into the molecular interplay between M. anisopliae and its insect hosts, with potential implications for refining fungal biocontrol strategies.