Expression of a Structural Protein of the Mycovirus FgV-ch9 Negatively Affects the Transcript Level of a Novel Symptom Alleviation Factor and Causes Virus Infection-Like Symptoms in Fusarium graminearum

Abstract

Infections of fungi by mycoviruses are often symptomless but sometimes also fatal as they perturb sporulation, growth, and, if applicable, virulence of the fungal host. Hypovirulence-inducing mycoviruses, therefore, represent a powerful mean to defeat fungal epidemics on crop plants. Infection with Fusarium graminearum virus China 9 (FgV-ch9), a dsRNA chrysovirus-like mycovirus, debilitates , the causal agent of Fusarium Head Blight. In search for potential symptom alleviation or aggravation factors in, we consecutively infected a custom-made mutant collection with FgV-ch9 and found a mutant with constantly elevated expression of a gene coding for a putative mRNA-binding protein that did not show any disease symptoms despite harboring high amounts of virus. Deletion of this gene, named virus response 1 (), resulted in phenotypes identical to those observed in the virus-infected wild type with respect to growth, reproduction, and virulence. Similarly, the viral structural protein coded on segment 3 (P3) caused virus-infection like symptoms when expressed in the wild-type but not in the -overexpression mutant. Gene expression analysis revealed a drastic downregulation of in the presence of virus and in mutants expressing P3. We conclude that symptom development and severity correlate with gene expression levels of This was confirmed by comparative transcriptome analysis showing a large transcriptional overlap between the virus-infected wild type, the deletion mutant and the P3-expressing mutant. Hence, represents a fundamental host factor for the expression of virus-related symptoms and helps to understand the underlying mechanism of hypovirulence. Virus infections of phytopathogenic fungi occasionally impair growth, reproduction, and virulence, a phenomenon referred to as hypovirulence. Hypovirulence-inducing mycoviruses, therefore, represent a powerful mean to defeat fungal epidemics on crop plants. However, the poor understanding of the molecular basis of hypovirulence induction limits their application. Using the devastating fungal pathogen on cereal crops, , we identified an mRNA binding protein (named virus response 1,) which is involved in symptom expression. Downregulation of in the virus-infected fungus and deletion evoke virus-infection like symptoms while constitutive expression overrules the cytopathic effects of the virus infection. Intriguingly, the presence of a specific viral structural protein is sufficient to trigger the fungal response, downregulation, and symptom development similar to virus infection. The advancements in understanding fungal infection and response may aid biological pest control approaches using mycoviruses or viral proteins to prevent future Fusarium epidemics.

Viruses of endophytic and pathogenic forest fungi

“…Moreover, significant progress has been made in identifying and characterizing the mycoviruses and domains that cause hypovirulence in the host [25,51,52]. For example, the vr1 structural domain of Fusarium graminearum virus China 9 (FgV-ch9) was found to cause [41] symptoms in Fusarium graminearum [55]. In contrast, several mycoviruses cause hypervirulence following infection [56].…”

“…Overlap extension PCR is a simple and popular technique for cloning a viral domain into a plasmid vector without the need for restrictive endonucleases [92]. Briefly, the prepared plasmid is cloned into E. coli, and after protoplast transformation the effects of the studied protein can be monitored through either growth rate, sporulation, or gene expression via qPCR [55,93]. Phylogenetic analyses can then be used to explore the evolutionary origins of structural and potential HGT-acquired domains present in mycoviruses.…”

Mycoviruses in Fusarium Species: An Update

“…Hence, vr1 represents a fundamental host factor for the expression of virus-related symptoms and helps to understand the underlying mechanism of hypovirulence. The advancements in understanding fungal infection and response may aid biological pest control approaches using mycoviruses or viral proteins to prevent future Fusarium epidemics (Bormann et al, 2018).…”

Mycoviruses infecting the forest pathogen <i>Heterobasidion annosum</i> - mutual interactions and host reactions

“…Four phylogenetically different mycoviruses infecting Fusarium graminearum (FgV1-4) generated distinct changes in host transcriptomes, however obvious gene expression changes did not always appear as phenotypic effects (Lee et al, 2014). Similarly, FgV-ch9 infecting F. graminearum did not show disease symptoms despite having the virus in large amounts and this effect was further revealed to be due to extremely low expression of the vr1 gene (Bormann et al, 2018). In this study, HetPV13-an1 caused gene expression related alterations in many basic cellular functions connected with a severely debilitated host phenotype including carbohydrate metabolism, chaperone functions, fungal self-defense and cell cycle control.…”