Fungal effector FolCP1b promotes infection by sequestering host subtilase SlSBT1 to protect apoplastic effectors from degradation

Plant pathogens use a diverse arsenal of effectors to suppress host immunity, though the precise mechanisms of their action are often not fully understood. In this study, we characterize FolCP1b, a cerato-platanin (CP) effector secreted by Fusarium oxysporum f. sp. lycopersici (Fol), as a key intracellular virulence factor that disrupts host defenses and protects other Fol effectors. FolCP1b interacts with the host apoplastic subtilase SlSBT1 within the plant cytoplasm, leading to its intracellular retention and preventing its secretion to the apoplast. As a result, SlSBT1-mediated degradation of key Fol effectors, such as FolEP1 and FolEP2, is impaired, thereby promoting Fol infection. Unlike canonical protease inhibitors, FolCP1b operates by altering host protein subcellular localization rather than inhibiting enzymatic activity. Our findings unveil a novel "effector hijacking" mechanism, through which one intracellular effector safeguards apoplastic effectors from host proteolytic degradation, thereby enhancing fungal pathogenicity.