Abstract
Hypovirulence-associated mycoviruses have the potential as biocontrol agents for plant fungal disease management, and exploration of the interactions between these mycoviruses and phytopathogenic fungi can provide opportunities to elucidate the underlying mechanisms of hypovirulence and antiviruses. We previously found that Sclerotinia sclerotiorum endornavirus 3 (SsEV3), belonging to the genus Betaendornavirus within the family Endornaviridae, confers hypovirulence on the phytopathogenic fungus Sclerotinia sclerotiorum, but the underlying mechanisms remains unclear. In this study, we found that the SsEV3-infected strain produced fewer sclerotia, failed to form infection cushions on plant hosts, exhibited increased cell vacuolation, and was more sensitive to abiotic stresses. SsEV3 infection evoked transcriptional rewiring in S. sclerotiorum, affecting genes related to virulence factors for pathogenicity and RNAi pathway for antiviruses. An unknown biological function of gene Sssnf1 was downregulated following SsEV3 infection. Deletion of Sssnf1 impaired infection cushion formation and decreased virulence of S. sclerotiorum. Five key RNAi-related genes were significantly upregulated, and deletion of Ssdcl2 contributed to SsEV3 accumulation. Additionally, we identified a hypothetical protein encoded by Sshp1 that directly interacts with the RNA-dependent RNA polymerase (RdRp) domain encoded by SsEV3. Although the deletion mutants of Sshp1 exhibited normal colony morphology, they showed higher SsEV3 accumulation and reduced resistance to reactive oxygen species, indicating that this gene, similar to RNAi-related genes, plays an antiviral role in response to SsEV3 infection and may represent a new antivirus factor. Therefore, examination of the interaction between endornavirus and S. sclerotiorum provides new insights into the mechanisms of antivirus and virulence in phytopathogenic fungi.IMPORTANCEHypovirulence-associated mycoviruses have emerged as promising biocontrol agents, and studying their interactions with phytopathogenic fungi helps uncover mechanisms of fungal pathogenesis and antiviral defense. This study provides critical insights into the interaction between Sclerotinia sclerotiorum and its hypovirulence-associated endornavirus, SsEV3, elucidating the molecular mechanisms underlying mycovirus-induced changes in fungal virulence and antivirus defense. SsEV3 infection not only impairs fungal virulence traits, including infection cushion formation and sclerotial production but also triggers host antiviral responses involving typical RNA interference pathways. New virulence factors, such as Sssnf1, and antiviral factors, such as Sshp1, were identified based on the established interaction system between S. sclerotiorum and endornavirus. These findings deepen our understanding of fungus-mycovirus interactions, highlighting the role of SsEV3 in reducing the virulence of S. sclerotiorum, and facilitating the development of mycovirus-based biological control strategies.