The core septin gene CgSEP5 is associated with formation of infection structures and pathogenicity in Colletotrichum gloeosporioides.

Colletotrichum gloeosporioides is a model plant pathogenic fungus, and the appressoria are the main infection structures integral to the pathogenic process. Septin proteins play fundamental roles in facilitating shape alteration and organizing the F-actin cytoskeleton, thereby aiding the invasive growth of various fungi. Herein, we examined the roles of four septin-coding genes (CgSEP3, CgSEP4, CgSEP5, and CgSEP6) in C. gloeosporioides. Our findings reveal the diverse functions of septins in C. gloeosporioides, which encompass the regulation of vegetative growth, conidiation, cell wall integrity, and stress responses. Critically, septins are involved in the formation, invasion, and expansion of infection structures and they directly influence virulence on unwounded hosts. Interestingly, the deletion of CgSEP4 resulted in the formation of hooked and bent germ tubes and caused a significant decrease in appressorium turgor pressure, which has not been reported in other fungi. Our findings demonstrated that CgSEP3 and CgSEP6 were regulated by ROS signal transduction during the formation of infection structure. Moreover, the knockout of the key component, CgSEP5, significantly decreased growth rate compared to the wild type, completely blocking the penetration of infection structures and subsequently abolishing virulence on poplar leaves. By subcellular localization of GFP fusions, it was proved that CgSEP5 may regulate the formation of appressorial pegs in C. gloeosporioides through forming a ring-like structure inside the appressorium. Collectively, our research underscores the pivotal role of septins in fungal pathogenicity, by orchestrating the formation and development of infection structures. We speculate that CgSEP5 function as a promising anti-fungal target, and believe these findings provide a substantial reference for future investigations into the mechanisms underpinning the invasion of fungi appressoria on woody plants.