Functional characterization of the cyclin-dependent kinase gene CsSSN8 in the pepper anthracnose fungus Colletotrichum scovillei

​ Pepper anthracnose caused by Colletotrichum scovillei is a destructive disease that results in severe losses during both field production and postharvest storage. Although components associated with the Mediator kinase module are known to regulate fungal development and pathogenicity, their roles in counteracting host immunity in plant-pathogenic fungi remain largely unknown. In this study, we functionally characterized CsSSN8 in C. scovillei, a homolog of the yeast Ume3/Ssn8 that encodes a protein containing a conserved Cyclin C-type cyclin domain. Compared with the wild type and complemented strain, the ΔCsssn8 mutant displayed reduced vegetative growth, abnormal conidial morphology, impaired conidial germination, and defects associated with cell wall biogenesis, indicating an important role for CsSSN8 in fungal growth and development. However, the mutant was still able to differentiate infection-related structures, yet completely lost pathogenicity on intact pepper fruits, while retaining pathogenicity on wounded fruits. DAB staining further showed that infection by the ΔCsssn8 mutant triggered strong accumulation of host-derived reactive oxygen species. Consistent with this, suppression of ROS by DPI treatment or inoculation on heat-killed pepper fruits restored invasive hyphal growth and virulence of the mutant. These results indicate that the loss of pathogenicity in ΔCsssn8 cannot be explained solely by defects in fungal development, but is largely attributable to a failure to overcome host defense during early infection. Collectively, our findings identify CsSSN8 as a key regulator of fungal development and uncover a previously unrecognized role for Ssn8/Cyclin C in suppressing host oxidative immunity during early fruit colonization by a plant-pathogenic fungus.