Possible mechanism of contribution of a secreted aspartic proteinase FpOPSB to the virulence of Fusarium proliferatum causing banana crown rot

Fusarium proliferatum is one of the pathogenic fungi causing crown rot that results in quality deterioration and commercial value loss of banana fruit during postharvest storage. Secreted proteins from pathogenic fungi can be delivered into hosts to suppress plant immunity and establish infection. We reported here a secreted aspartic proteinase OPSB (FpOPSB) that was active during F. proliferatum infection on banana peel. FpOPSB exhibited protease activity based on in vitro assay. Immunohistochemistry assay along with subcellular localization analysis demonstrated that FpOPSB can be secreted into banana peel and localized in cytoplasm. FpOPSB deletion mutant (& UDelta;opsb) showed reduced virulence on banana fruit but no obvious effect on fungal growth compared with wild-type. Subsequently, scanning electron microscopy results suggested that FpOPSB was important for conidial production and infectious growth as well as spreading of F. proliferatum in banana peel. Heterologous expression of FpOPSB in tobacco significantly enhanced plant susceptibility to F. proliferatum. Further, seven proteins were identified as potential targets of FpOPSB in banana peel using immunoprecipitation-mass spectrometry (MS)/MS, including pathogenesis-related 10, thioredoxin H1, nonsymbiotic hemoglobin 2, fructokinase-1, patellin-1, leucine aminopeptidase 2, and l-lactate dehydrogenase A. Taken together, our results confirmed that the secreted FpOPSB is a critical virulence factor of F. proliferatum when infecting banana fruit. Additionally, we postulated that FpOPSB might interfere the host immune system or neutralize the host defenses, thereby contributing to the pathogenicity of F. proliferatum. We propose aspartic proteinase as a new potential target for controlling fungal disease of postharvest fruit.