Functional Characterization of BbroAFP Reveals Its Pleiotropic Antifungal Activity in Botrytis cinerea

Fungal pathogens pose a major threat to global agriculture and human health, necessitating alternative antifungal strategies with high efficacy and low resistance potential. Antifungal proteins (AFPs) from filamentous fungi are promising candidates due to their stability, selectivity, and diverse mechanisms of action. Here, we characterize Beauveria brongniartii antifungal protein (BbroAFP), a novel cysteine-rich protein from the entomopathogenic fungus B. brongniartii, and investigate its antifungal activity against Botrytis cinerea. Recombinant BbroAFP was expressed in Pichia pastoris, purified, and verified by liquid chromatography-tandem mass spectroscopy (LC-MS/MS) and in silico modeling. BbroAFP showed potent antifungal activity with minimum inhibitory concentrations (MICs) as low as 1 µM against several phytopathogenic fungi, while exhibiting no significant antibacterial activity. Activity was maintained across a wide range of pH and temperature conditions. Confocal microscopy revealed rapid surface binding followed by cytosolic internalization without major cell wall disruption. BbroAFP induced a rapid, transient burst of reactive oxygen species (ROS), accompanied by nuclear DNA fragmentation. Gene expression analysis revealed a transient increase in aif1, whereas mca1 expression decreased at later time points and mca2 remained largely unchanged, suggesting a metacaspase-independent response. Detached tomato leaf assays showed effective protection against B. cinerea without detectable phytotoxicity. Cytotoxicity assays confirmed a favorable safety profile, supporting further evaluation of BbroAFP for plant protection.