Strategies of host defense and viral counter defense: analysis of Fusarium graminearium responses to FgHV1 infection

Viruses that selectively infect fungi are named fungal viruses or mycoviruses. Fungal viruses exist in all major groups of fungi and are important in both fundamental research and biological control of plant pathogens. Fusarium graminearum hypovirus 1 (FgHV1) was the first virus isolated from F. graminearum belonging to the hypoviridae family. FgHV1 was supposed to encode two open reading frames, which had high sequence identities with Cryphonectria hypovirus 1 and 2 (CHV1 and CHV2). F. graminearum strain infected with FgHV1 showed lower growth rate and reduced conidia production, but with no pathogenicity change. To elucidate the host defense and viral counter defense mechanism, we analyzed the F. graminearum stress responses to FgHV1 infection from several aspects. In the first part, we preliminarily elucidated the RNA silencing mechanism of the F. graminearum/hypovirus system from a small RNA (sRNA) perspective. The length distributions of F. graminearum sRNA were altered by FgHV1 infection, especially 24-nt sRNA. Extensive FgHV1-derived sRNAs were detected, yielding the first high resolution map of sRNA from mycoviruses. In particular, the RNA silencing-related genes FgDicer1 and FgRdRp5 were predicted targets of FgHV1- and FgHV2-derived siRNAs, possibly revealing a novel anti-RNA silencing strategy employed by mycoviruses. In the second part, we performed genome-wide expression analysis to reveal stress-related genes with expression changes in response to FgHV1 infection. A total of 248 differentially expressed genes were identified. Cellular redox regulation related genes were the most affected categories in F. graminearum challenged with FgHV1. Then we verified that FgHV1 encoded protein 20 (p20) could induce the H2O2 accumulation and hypersensitive response in Nicotiana benthamiana leaves. In the last part, our experiments confirmed that p20 functioned as a RNA silencing suppressor (RSS) with the agroinfiltration-mediated technique, making p20 the third RSS identified in mycoviruses. By incorporating single strand sRNA, p20 could suppress host RNA silencing. Globally, the defense responses of F. graminearum to FgHV1 infection were studied all-around and in-depth. On the other hand, FgHV1 developed counter defense strategies to protect from invasion resistance.