Functional Characterization of PeMep Gene Reveals Its Roles in the Vegetative Growth, Stress Adaptation, and Virulence of Penicillium expansum

<i>Penicillium expansum</i>, a major postharvest pathogen, causes blue mold decay in apples, resulting in substantial economic losses and mycotoxin contamination. Despite the importance of effector proteins in fungal pathogenicity, the role of metalloproteases in <i>P. expansum</i> remains unclear. Here, we characterize an effector candidate, PeMep, through whole genome sequencing and functional analyses. Functional validation confirmed the secretory capacity of its signal peptide via yeast assays and subcellular localization. Deletion of <i>PeMep</i> significantly impaired fungal growth (23% reduction), conidiation (23.3% decrease), and germination efficiency. The Δ<i>PeMep</i> mutant exhibited hypersensitivity to osmotic, oxidative, and thermal stresses, highlighting its vital role in environmental adaptability. Importantly, pathogenicity assays revealed attenuated virulence in the Δ<i>PeMep</i> mutant, with 15-30% smaller lesion sizes on apples and delayed hyphal penetration compared to the wild-type, demonstrating that PeMep is essential for the pathogenic process of <i>P. expansum</i> 3.3703. These findings identify PeMep as a potential multifunctional effector protein crucial for fungal development, environmental adaptation, and pathogenicity in <i>P. expansum</i> 3.3703, providing a novel target for postharvest disease management.