A Phenome-Wide Association Study of the Effects of Fusarium graminearum Transcription Factors on Fusarium Graminearum Virus 1 Infection

Exploring the interaction between endornavirus and Sclerotinia sclerotiorum: mechanisms of phytopathogenic fungal virulence and antivirus

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.

p18 encoded by FgGMTV1 is responsible for asymptomatic infection in Fusarium graminearum

The intricate interplay between mycoviruses and their fungal hosts frequently culminates in asymptomatic infections, but the virus-derived factors underlying these infections remain poorly understood. Our study introduces p18, a novel protein encoded by the DNA-C segment of the genomovirus FgGMTV1, which facilitates the transition from virus-induced hypovirulence to asymptomatic infection within Fusarium graminearum upon its expression. We have confirmed the expression of p18 during FgGMTV1 infection and observed its presence in both the nucleus and cytoplasm. Remarkably, strains with a p18 null mutation show a significant reduction in colony expansion, conidial production, and virulence, leading to a hypovirulent phenotype. Our results also indicate that p18 hinders the accumulation of FgGMTV1, thus determining asymptomatic infection and enabling vertical transmission through conidia. Furthermore, the p18 null mutant virus converts F. graminearum from virulent to hypovirulent strains on wheat leaves after horizontal transmission. This work not only expands our knowledge of the genomovirus proteome but also provides insights into the strategies of viral evolution and adaptation. Moreover, we propose an innovative approach for creating hypovirulent strains utilizing engineered mycoviruses for the biocontrol of plant pathogenic fungi.

IMPORTANCE

Mycovirus-fungus interplay often leads to asymptomatic infections. Our study identifies p18, a novel protein from the genomovirus FgGMTV1, as a key determinant of asymptomatic infection in Fusarium graminearum. A p18-null mutant exhibits a pronounced hypovirulent phenotype. By modulating viral accumulation, p18 promotes asymptomatic infection and facilitates vertical transmission via conidia. This insight deepens our understanding of mycovirus-fungus interactions and introduces a novel strategy for biocontrol using engineered mycoviruses.

The Interaction between Hypovirulence-Associated Chrysoviruses and Their Host Fusarium Species

Chrysoviruses are isometric virus particles (35-50 nm in diameter) with a genome composed of double-stranded RNAs (dsRNA). These viruses belonged to the Chrysoviridae family, named after the first member isolated from Penicillium chrysogenum. Phylogenetic classification has divided the chrysoviruses into Alphachrysovirus and Betachrysovirus genera. Currently, these chrysoviruses have been found to infect many fungi, including Fusarium species, and cause changes in the phenotype and decline in the pathogenicity of the host. Thus, it is a microbial resource with great biocontrol potential against Fusarium species, causing destructive plant diseases and substantial economic losses. This review provides a comprehensive overview of three chrysovirus isolates (Fusarium graminearum virus 2 (FgV2), Fusarium graminearum virus-ch9 (FgV-ch9), and Fusarium oxysporum f. sp. dianthi mycovirus 1 (FodV1)) reported to decline the pathogenicity of Fusarium hosts. It also summarizes the recent studies on host response regulation, host RNA interference, and chrysovirus transmission. The information provided in the review will be a reference for analyzing the interaction of Fusarium species with chrysovirus and proposing opportunities for research on the biocontrol of Fusarium diseases. Finally, we present reasons for conducting further studies on exploring the interaction between chrysoviruses and Fusarium and improving the accumulation and transmission efficiency of these chrysoviruses.

Use of image analysis to assess radial growth of Passalora arachidicola and Nothopassalora personata on solid media

Despite significant research on early and late leaf spot diseases of peanut, in vitro study of the respective causal agents, Passalora arachidicola and Nothopassalora personata, has been limited due to cultural challenges that make growth of these fungi difficult to quantify with traditional methods. Studies were conducted to evaluate the practicality of image analysis to assess radial growth and tissue volume by correlating these assessments to dry mass. Image analysis was also used to estimate radial growth rates for these fungi over time. Tissue area and volume were significantly correlated to dry mass for P. arachidicola in two separate experiments, and for N. personata when medium had been removed from tissues prior to dry mass assessments. Tissue area densities were the same for P. arachidicola and Pseudocercospora smilacicola, evaluated as a nonstromatal cercosporoid comparison, whereas tissue volume densities were greater for P. archidicola and N. personata than P. smilacicola. A quadratic relationship was observed between radial growth and incubation time for all isolates evaluated. Growth rates of P. arachidicola isolates were 2 to 4 times faster than N. personata during the first week of incubation and slowed over time. Growth rates of NP18R, a phenotype variant of N. personata, increased after neighboring colonies met and was nearly 2.5 times faster than the fastest rates observed for P. arachidicola. These experiments demonstrate that when fungal tissues are observable, image analysis is a useful assessment tool for P. arachidicola and N. personata. Care should be taken to monitor fungal phenotypic changes in these species because phenotype degeneration can affect growth rates.

Functional Characterization of Aldehyde Dehydrogenase in Fusarium graminearum

Aldehyde dehydrogenase (ALDH), a common oxidoreductase in organisms, is an aldehyde scavenger involved in various metabolic processes. However, its function in different pathogenic fungi remains unknown. Fusarium graminearum causes Fusarium head blight in cereals, which reduces grain yield and quality and is an important global food security problem. To elucidate the pathogenic mechanism of F. graminearum, seven genes encoding ALDH were knocked out and then studied for their function. Single deletions of seven ALDH genes caused a decrease in spore production and weakened the pathogenicity. Furthermore, these deletions altered susceptibility to various abiotic stresses. FGSG_04194 is associated with a number of functions, including mycelial growth and development, stress sensitivity, pathogenicity, toxin production, and energy metabolism. FGSG_00139 and FGSG_11482 are involved in sporulation, pathogenicity, and SDH activity, while the other five genes are multifunctional. Notably, we found that FGSG_04194 has an inhibitory impact on ALDH activity, whereas FGSG_00979 has a positive impact. RNA sequencing and subcellular location analysis revealed that FGSG_04194 is responsible for biological process regulation, including glucose and lipid metabolism. Our results suggest that ALDH contributes to growth, stress responses, pathogenicity, deoxynivalenol synthesis, and mitochondrial energy metabolism in F. graminearum. Finally, ALDH presents a potential target and theoretical basis for fungicide development.

A novel partitivirus orchestrates conidiation, stress response, pathogenicity, and secondary metabolism of the entomopathogenic fungus Metarhizium majus

Mycoviruses are widely present in all major groups of fungi but those in entomopathogenic Metarhizium spp. remain understudied. In this investigation, a novel double-stranded (ds) RNA virus is isolated from Metarhizium majus and named Metarhizium majus partitivirus 1 (MmPV1). The complete genome sequence of MmPV1 comprises two monocistronic dsRNA segments (dsRNA 1 and dsRNA 2), which encode an RNA-dependent RNA polymerase (RdRp) and a capsid protein (CP), respectively. MmPV1 is classified as a new member of the genus Gammapartitivirus in the family Partitiviridae based on phylogenetic analysis. As compared to an MmPV1-free strain, two isogenic MmPV1-infected single-spore isolates were compromised in terms of conidiation, and tolerance to heat shock and UV-B irradiation, while these phenotypes were accompanied by transcriptional suppression of multiple genes involved in conidiation, heat shock response and DNA damage repair. MmPV1 attenuated fungal virulence since infection resulted in reduced conidiation, hydrophobicity, adhesion, and cuticular penetration. Additionally, secondary metabolites were significantly altered by MmPV1 infection, including reduced production of triterpenoids, and metarhizins A and B, and increased production of nitrogen and phosphorus compounds. However, expression of individual MmPV1 proteins in M. majus had no impact on the host phenotype, suggesting insubstantive links between defective phenotypes and a single viral protein. These findings indicate that MmPV1 infection decreases M. majus fitness to its environment and its insect-pathogenic lifestyle and environment through the orchestration of the host conidiation, stress tolerance, pathogenicity, and secondary metabolism.

Phenotypic Recovery of a Heterobasidion Isolate Infected by a Debilitation-Associated Virus Is Related to Altered Host Gene Expression and Reduced Virus Titer

The fungal genus Heterobasidion includes forest pathogenic species hosting a diverse group of partitiviruses. They include the host debilitating Heterobasidion partitivirus 13 strain an1 (HetPV13-an1), which was originally observed in a slowly growing H. annosum strain 94233. In this study, a relatively fast-growing sector strain 94233-RC3 was isolated from a highly debilitated mycelial culture of 94233, and its gene expression and virus transcript quantities as well as the genomic sequence of HetPV13-an1 were examined. The sequence of HetPV13-an1 genome in 94233-RC3 was identical to that in the original 94233, and thus not the reason for the partial phenotypic recovery. According to RNA-seq analysis, the HetPV13-an1 infected 94233-RC3 transcribed eight genes differently from the partitivirus-free 94233-32D. Three of these genes were downregulated and five upregulated. The number of differentially expressed genes was considerably lower and the changes in their expression were small compared to those of the highly debilitated original strain 94233 with the exception of the most highly upregulated ones, and therefore viral effects on the host transcriptome correlated with the degree of the virus-caused debilitation. The amounts of RdRp and CP transcripts of HetPV13-an1 were considerably lower in 94233-RC3 and also in 94233 strain infected by a closely related mildly debilitating virus HetPV13-an2, suggesting that the virus titer would have a role in determining the effect of HetPV13 viruses on their hosts.