Regulation of the immune system of the mushroom Schizophyllum commune

Abstract

Fungi play crucial roles in natural ecosystems and industrial applications, serving as direct food sources like edible mushrooms and contributing to fermentation processes. While scientific research has significantly advanced our understanding of fungi's clinical relevance and ecological functions, relatively little is known about interactions between fungi and between fungi and other organisms, particularly for mushroom-forming species. Mushroom-forming fungi can be strongly affected by antagonistic micro-organism, yet they have an immune system to defend themselves against these antagonists. Understanding these defense mechanisms has both fundamental scientific importance and economic relevance, as pathogenic fungi cause significant losses in commercial mushroom production. This research investigated defense mechanisms in the mushroom-forming fungus Schizophyllum commune, an ideal model organism due to its 10-day life cycle, sequenced genome, and genetic manipulability. Through macroscopic observations of interactions combined with genetic analyses, several transcription factors involved in mushroom defense were identified. S. commune was co-cultivated with the competitor fungi Trichoderma harzianum, Trichoderma aggressivum, Purpureocillium lilacinum, and the bacterium Serratia quinivorans to study gene expression changes during these interactions. Macroscopic observations revealed dark pigment production during interactions with Trichoderma species and growth inhibition during interaction with P. lilacinum and S. quinivorans. Transcriptome analysis showed both common and interaction-specific gene activation patterns. Defense-related genes activated included cell wall-modifying hydrolases, thaumatins, lectins, and genes encoding secreted small proteins. Three transcription factors - Gat1, Fst8, and Ftr3 - were identified as key regulators of defense responses. Knockout strains lacking these genes showed reduced defensive capabilities, with the Δgat1Δfst8 double knockout exhibiting the most pronounced vulnerability, suggesting synergistic effects across multiple defense networks. ABC-transporters were also implicated in defense mechanisms. Four ABC-transporter genes were studied through knockout strains and exposure to the antifungal compound fluconazole. The quadruple knockout strain (Δabcg1234) showed significantly reduced tolerance to fluconazole. Additionally, a fluorescent reporter strain demonstrated that the abcg1 promoter is activated during interactions with both living and heat-killed antagonists, suggesting responsiveness to damage-associated molecular patterns (DAMPs). The abcg1 promoter was also activated when the reporter strain was exposed to spent media derived from either axenic competitor cultures or S. commune-competitor co-cultures. This observation indicates that secreted factors, independent of cell lysates, are capable of triggering abcg1 expression. This research has identified several components of the immune system in mushroom-forming fungi, including transcription factors, ABC-transporters and other defense-related genes. While the complete signaling pathways remain to be elucidated, these findings provide fundamental knowledge that may be applied to improve disease resistance in commercially cultivated mushrooms.