A Victorivirus from Fusarium asiaticum, the pathogen of Fusarium head blight in China

Identification and characterization of mycoviruses in transcriptomes from the fungal family ceratocystidaceae

Mycoviruses pervade the fungal kingdom, yet their diversity within various fungal families and genera remains largely unexplored. In this study, 10 publicly available fungal transcriptomes from Ceratocystidaceae were analyzed for the presence of mycoviruses. Despite mycovirus associations being known in only four members of this family, our investigation unveiled the discovery of six novel mycoviruses. The majority of these mycoviruses are composed of positive sense single stranded RNA and are putatively assigned to the viral family Mitoviridae (with tentative classification into the genera Unuamitovirus and Duamitovirus). The double stranded RNA viruses, however, were associated with the family Totiviridae (with tentative classification into the genus Victorivirus). This study also revealed the discovery of an identical unuamitovirus in the fungal species Thielaviopsis ethacetica and Thielaviopsis paradoxa. This discovery was notable as these fungal isolates originated from distinct geographical locations, highlighting potential implications for the transmission of this mitovirus. Moreover, this investigation significantly expands the known host range for mycoviruses in this family, marking the initial identification of mycoviruses within Ceratocystis platani, Thielaviopsis paradoxa, Thielaviopsis ethacetica, and Huntiella omanensis. Future research should focus on determining the effects that these mycoviruses might have on their fungal hosts.

Coat Proteins of the Novel Victoriviruses FaVV1 and FaVV2 Suppress Sexual Reproduction and Virulence in the Pathogen of Fusarium Head Blight

Fusarium head blight (FHB), a disease inflicted by Fusarium graminearum and F. asiaticum, poses a growing threat to wheat in China, particularly in the face of climate change and evolving agricultural practices. This study unveiled the discovery of the victorivirus FgVV2 from the F. asiaticum strain F16176 and comprehensively characterized the function of the two victoriviruses FaVV1 and FaVV2 in virulence. Through comparative analysis with a virus-free strain, we established that these mycoviruses markedly repress the sexual reproduction and pathogenicity of their fungal hosts. Furthermore, we synthesized the coat protein (CP) genes CP1 from FaVV1 and CP2 from FaVV2, which were fused with the green fluorescent protein (GFP) gene and successfully expressed in Fusarium strains in wild-type isolates of F. asiaticum and F. graminearum. Similar to virus-infected strains, the transformed strains expressing CPs showed a significant decrease in perithecia formation and pathogenicity. Notably, CP2 exhibited a stronger inhibitory effect than CP1, yet the suppression of sexual reproduction in F. graminearum was less pronounced than that in F. asiaticum. Additionally, the pathogenicity of the F. asiaticum and F. graminearum strains expressing CP1 or CP2 was substantially diminished against wheat heads. The GFP-tagged CP1 and CP2 revealed distinct cellular localization patterns, suggesting various mechanisms of interaction with the host. The findings of this study provide a significant research foundation for the study of the interaction mechanisms between FaVV1 and FaVV2 with their hosts, as well as for the exploration and utilization of fungal viral resources.

Metatranscriptome-based strategy reveals the existence of novel mycoviruses in the plant pathogenic fungus Fusarium oxysporum f. sp. cubense

Fusarium oxysporum f. sp. cubense (Foc) is a devastating plant pathogen that caused a great financial loss in the banana's source area. Metatranscriptomic analysis was used to determine the diversity of mycoviruses in 246 isolates of F. oxysporum f. sp. cubense. Partial or nearly complete genomes of 20 mycoviruses were obtained by BLASTp analysis of RNA sequences using the NCBI database. These 20 viruses were grouped into five distinct lineages, namely Botourmiaviridae, Endornaviridae, Mitoviridae, Mymonaviridae, Partitiviridae, and two non-classified mycoviruses lineages. To date, there is no report of the presence of mycoviruses in this pathogen. In this study, we demonstrate the presence of mycoviruses isolated from Foc. These findings enhance our overall knowledge of viral diversity and taxonomy in Foc. Further characterization of these mycoviruses is warranted, especially in terms of exploring these novel mycoviruses for innovative biocontrol of banana Fusarium wilt disease.

Discovery and Characterization of Putative Glycoprotein-Encoding Mycoviruses in the Bunyavirales

Although segmented negative-sense RNA viruses (SNSRVs) have been frequently discovered in various fungi, most SNSRVs reported only the large segments. In this study, we investigated the diversity of the mycoviruses in the phytopathogenic fungus Fusarium asiaticum using the metatranscriptomic technique. We identified 17 fungal single-stranded RNA (ssRNA) viruses including nine viruses within Mitoviridae, one each in Narnaviridae, Botourmiaviridae, Hypoviridae, Fusariviridae, and Narliviridae, two in Mymonaviridae, and one trisegmented virus temporarily named Fusarium asiaticum mycobunyavirus 1 (FaMBV1). The FaMBV1 genome comprises three RNA segments, large (L), medium (M), and small (S) with 6,468, 2,639, and 1,420 nucleotides, respectively. These L, M, and S segments putatively encode the L protein, glycoprotein, and nucleocapsid, respectively. Phylogenetic analysis based on the L protein showed that FaMBV1 is phylogenetically clustered with Alternaria tenuissima negative-stranded RNA virus 2 (AtNSRV2) and Sclerotinia sclerotiorum negative-stranded RNA virus 5 (SsNSRV5) but distantly related to the members of the family Phenuiviridae. FaMBV1 could be vertically transmitted by asexual spores with lower efficiency (16.7%, 2/42). Comparison between FaMBV1-free and -infected fungal strains revealed that FaMBV1 has little effect on hyphal growth, pathogenicity, and conidium production, and its M segment is dispensable for viral replication and lost during subculture and asexual conidiation. The M and S segments of AtNSRV2 and SsNSRV5 were found using bioinformatics methods, indicating that the two fungal NSRVs harbor trisegmented genomes. Our results provide a new example of the existence and evolution of the segmented negative-sense RNA viruses in fungi. IMPORTANCE Fungal segmented negative-sense RNA viruses (SNSRVs) have been frequently found. Only the large segment encoding RNA-dependent RNA polymerase (RdRp) has been reported in most fungal SNSRVs, except for a few fungal SNSRVs reported to encode nucleocapsids, nonstructural proteins, or movement proteins. Virome analysis of the Fusarium spp. that cause Fusarium head blight discovered a novel virus, Fusarium asiaticum mycobunyavirus 1 (FaMBV1), representing a novel lineage of the family Phenuiviridae. FaMBV1 harbors a trisegmented genome that putatively encodes RdRp, glycoproteins, and nucleocapsids. The putative glycoprotein was first described in fungal SNSRVs and shared homology with glycoprotein of animal phenuivirus but was dispensable for its replication in F. asiaticum. Two other trisegmented fungal SNSRVs that also encode glycoproteins were discovered, implying that three-segment bunyavirus infections may be common in fungi. These findings provide new insights into the ecology and evolution of SNSRVs, particularly those infecting fungi.

Molecular characterization of a novel victorivirus isolated from the phytopathogenic fungus Phaeobotryon rhois

Phaeobotryon rhois is an important pathogenic fungus that causes dieback and canker disease of woody hosts. A novel mycovirus, tentatively named "Phaeobotryon rhois victorivirus 1" (PrVV1), was identified in P. rhois strain SX8-4. The PrVV1 has a double-stranded RNA (dsRNA) genome that is 5,224 base pairs long and contains two open reading frames (ORF1 and ORF2), which overlap at a AUGA sequence. ORF1 encodes a polypeptide of 786 amino acids (aa) that contains the conserved coat protein (CP) domain of victoriviruses, while ORF2, encodes a large polypeptide of 826 aa that contains the conserved RNA-dependent RNA polymerase (RdRp) domain of victoriviruses. Our analysis of genomic structure, homology, and phylogeny indicated that PrVV1 is a novel member of the genus Victorivirus in the family Totiviridae. This is the first report of the complete genome sequence of a victorivirus that infects P. rhois.

Characterization of the First Alternavirus Identified in Fusarium avenaceum, the Causal Agent of Potato Dry Rot

A novel virus with a double-stranded RNA (dsRNA) genome was isolated from Fusarium avenaceum strain GS-WW-224, the causal agent of potato dry rot. The virus has been designated as Fusarium avenaceum alternavirus 1 (FaAV1). Its genome consists of two dsRNA segments, 3538 bp (dsRNA1) and 2477 bp (dsRNA2) in length, encoding RNA-dependent RNA polymerase (RdRp) and a hypothetical protein (HP), respectively. The virions of FaAV1 are isometric spherical and approximately 30 nm in diameter. Multiple sequence alignments and phylogenetic analyses based on the amino acid sequences of RdRp and HP indicated that FaAV1 appears to be a new member of the proposed family Alternaviridae. No significant differences in colony morphology and spore production were observed between strains GS-WW-224 and GS-WW-224-VF, the latter strain being one in which FaAV1 was eliminated from strain GS-WW-224. Notably, however, the dry weight of mycelial biomass of GS-WW-224 was higher than that of mycelial biomass of GS-WW-224-VF. The depth and the width of lesions on potato tubers caused by GS-WW-224 were significantly greater, relative to GS-WW-224-VF, suggesting that FaAV1 confers hypervirulence to its host, F. avenaceum. Moreover, FaAV1 was successfully transmitted horizontally from GS-WW-224 to ten other species of Fusarium, and purified virions of FaAV1 were capable of transfecting wounded hyphae of the ten species of Fusarium. This is the first report of an alternavirus infecting F. avenaceum and conferring hypervirulence.

Characterization of Four Novel dsRNA Viruses Isolated from Mucor hiemalis Strains

We previously screened the total nucleic acid extracts of 123 Mucor strains for the presence of dsRNA molecules without further molecular analyses. Here, we characterized five novel dsRNA genomes isolated from four different Mucor hiemalis strains with next-generation sequencing (NGS), namely Mucor hiemalis virus 1a (MhV1a) from WRL CN(M) 122; Mucor hiemalis virus 1b (MhV1b) from NRRL 3624; Mucor hiemalis virus 2 (MhV2) from NRRL 3616; and Mucor hiemalis virus 3 (MhV3) and Mucor hiemalis virus (MhV4) from NRRL 3617 strains. Genomes contain two open reading frames (ORF), which encode the coat protein (CP) and the RNA dependent RNA polymerase (RdRp), respectively. In MhV1a and MhV1b, it is predicted to be translated as a fusion protein via -1 ribosomal frameshift, while in MhV4 via a rare +1 (or-2) ribosomal frameshift. In MhV2 and MhV3, the presence of specific UAAUG pentanucleotide motif points to the fact for coupled translation termination and reinitialization. MhV1a, MhV2, and MhV3 are part of the clade representing the genus Victorivirus, while MhV4 is seated in Totivirus genus clade. The detected VLPs in Mucor strains were from 33 to 36 nm in diameter. Hybridization analysis revealed that the dsRNA molecules of MhV1a-MhV4 hybridized to the corresponding molecules.

Complete genome sequence of a novel victorivirus isolated from the sesame charcoal rot fungus Macrophomina phaseolina

Macrophomina phaseolina is an important phytopathogenic fungus with a broad host range. Here, the complete genome sequence of a novel victorivirus, tentatively named Macrophomina phaseolina victorivirus 1 (MpV1), was identified from strain 2012-019 of M. phaseolina. The MpV1 genome is 5,128 nucleotides long with a predicted GC content of 62%. Sequence analysis indicated that two open reading frames (ORF 1 and 2) overlap at a tetranucleotide AUGA sequence. Proteins encoded by ORF1 and ORF2 showed significant sequence similarity to coat proteins and the RNA-dependent RNA polymerases, respectively, of members of the family Totiviridae. Analysis of the genomic structure of MpV1, homolog searches of the deduced amino acid sequences, and phylogenetic analysis indicated that MpV1 is a new member of the genus Victorivirus. As far as we know, this is the first report of the full-length nucleotide sequence of the genome of a novel victorivirus that infects M. phaseolina.

Fusarium head blight and mycotoxins in wheat: prevention and control strategies across the food chain

With 744 million metric tons produced in 2017/2018, bread wheat (Triticum aestivum) and durum wheat (Triticum durum) are the second most widely produced cereal on a global basis. Prevention or control of wheat diseases may have an enormous impact on global food security and safety. Fusarium head blight is an economically debilitating disease of wheat that reduces the quantity and quality of grain harvested, and may lead to contamination with the mycotoxin deoxynivalenol, which affects the health of humans and domesticated animals. Current climate change scenarios predict an increase in the number of epidemics caused by this disease. Multiple strategies are available for managing the disease including cultural practices, planting less-susceptible cultivars, crop rotation, and chemical and biological controls. None of these strategies, however, is completely effective by itself, and an integrated approach incorporating multiple controls simultaneously is the only effective strategy to limit the disease and reduce deoxynivalenol contamination in human food and animal feed chains. This review identifies the available tools and strategies for mitigating the damage that can result from Fusarium head blight.

Mycoviruses in Fusarium Species: An Update

Fusarium is an important genus of plant pathogenic fungi, and is widely distributed in soil and associated with plants worldwide. The diversity of mycoviruses in Fusarium is increasing continuously due to the development and extensive use of state-of-the-art RNA deep sequencing techniques. To date, fully-sequenced mycoviruses have been reported in 13 Fusarium species: Fusarium asiaticum, F. boothii, F. circinatum, F. coeruleum, F. globosum, F. graminearum, F. incarnatum, F. langsethiae, F. oxysporum, F. poae, F. pseudograminearum, F. solani, and F. virguliforme. Most Fusarium mycoviruses establish latent infections, but some mycoviruses such as Fusarium graminearum virus 1 (FgV1), Fusarium graminearum virus-ch9 (FgV-ch9), Fusarium graminearum hypovirus 2 (FgHV2), and Fusarium oxysporum f. sp. dianthi mycovirus 1 (FodV1) cause hypovirulence. Rapid advances in various omics technologies used to elucidate genes or biological processes can facilitate an improved understanding of mycovirus-host interactions. The review aims to illuminate the recent advances in studies of mycoviruses in Fusarium, including those related to diversity, molecular mechanisms of virus-host interaction. We also discuss the induction and suppression of RNA silencing including the role of RNAi components as an antiviral defense response.