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

A Mycovirus Representing a Novel Lineage and a Mitovirus of Botrytis cinerea Co-Infect a Basidiomycetous Fungus, Schizophyllum commune

Strain IBc-114 was isolated from a gray mold lesion and was identified as the fungus Schizophyllum commune. In this strain, two mycoviruses, Schizophyllum commune RNA virus 1 (ScRV1, C_AA053475.1) and Botrytis cinerea mitovirus 9 strain IBc-114 (BcMV9/IBc-114, C_AA053476.1), were isolated and characterized. ScRV1 has flexuous filamentous particles about 20 ± 2.1 nm in diameter and 1000 ± 94.2 nm in length. The genome of ScRV1 is 7370 nt in length and contains two open reading frames (ORFs) which encode a polyprotein and a coat protein, respectively. The polyprotein has 1967 aa, including a helicase domain and an RdRp domain which has the highest identity of 28.21% with that of Entomophthora benyvirus E (EbVE). The coat protein has 241 aa which is mostly phylogenetically close to the coat proteins of Alphatetraviridae. Based on the phylogenetic analysis of ScRV1 and viruses selected, ScRV1 might represent a new family (temporarily named Mycobenyviridae) of the order Hepelivirales. The genome of BcMV9/IBc-114 that infects S. commune is 2729 nt in length and has only one ORF encoding an RdRp protein with 719 aa. BcMV9/IBc-114 has the highest identity of 98.61% with Botrytis cinerea mitovirus 9 (BcMV9) (MT089704). ScRV1, but not BcMV9/IBc-114, has certain effects on the host growth of S. commune. Furthermore, BcMV9/IBc-114 has been demonstrated to replicate in the ascomycetous fungi Botrytis cinerea and Sclerotinia sclerotiorum, and it negatively affects the growth and pathogenicity of B. cinerea, but it does not affect S. sclerotiorum. This is the first report of mycoviruses in S. commune and cross-phyla transmission of mitovirus in nature.

New insights into RNA mycoviruses of fungal pathogens causing Fusarium head blight

Fusarium head blight (FHB) continues to be a major problem in wheat production and is considered a disease complex caused by several fungal pathogens including Fusarium culmorum, F. graminearum and F. equiseti. With the objective of investigating diversity of mycoviruses in FHB-associated pathogens, we isolated Fusarium spp. from six wheat (Triticum aestivum) cultivars. In total, 56 Fusarium isolates (29 F. culmorum, 24 F. graminearum, one F. equiseti) were screened for mycoviruses by extracting and sequencing double-stranded RNA. We found that a large proportion of Fusarium isolates (46 %) were infected with mycoviruses. F. culmorum, previously described to harbor only one mycovirus, tended to host more viruses than F. graminearum, with a few isolates harboring seven mycoviruses simultaneously. Based on the RNA-dependent RNA polymerase domain analysis, ten were positive-sense single-stranded RNA viruses (related to viruses from families Mitoviridae, Botourmiaviridae, Narnaviridae, Tymoviridae, Gammaflexiviridae, as well as proposed Ambiguiviridae and ormycovirus viral group), one was double-stranded RNA virus (Partitiviridae), and five were negative-sense single-stranded RNA viruses (related to members in the families of Yueviridae, Phenuiviridae, Mymonaviridae, as well as proposed Mycoaspiviridae). Five mycoviruses were shared between F. graminearum and F. culmorum. These results increase our general understanding of mycovirology. To our knowledge, this is the first in-depth report of the mycovirome in F. culmorum and the first report on the diversity of mycoviruses from Danish isolates of FHB-causing fungi in general.

Reexamining the Mycovirome of Botrytis spp

Botrytis species cause gray mold disease in more than 200 crops worldwide. To control this disease, chemical fungicides are usually applied. However, more sustainable control alternatives should be explored, such as the use of hypovirulent mycovirus-infected fungal strains. To determine the mycovirome of two Botrytis species, B. cinerea and B. prunorum, we reanalyzed RNA-Seq and small RNA-Seq data using different assembly programs and an updated viral database, aiming to identify new mycoviruses that were previously not described in the same dataset. New mycoviruses were identified, including those previously reported to infect or be associated with B. cinerea and Plasmopara viticola, such as Botrytis cinerea alpha-like virus 1 and Plasmopara viticola lesion-associated ourmia-like virus 80. Additionally, two novel narnaviruses, not previously identified infecting Botrytis species, have been characterized, tentatively named Botrytis cinerea narnavirus 1 and Botrytis narnavirus 1. The analysis of small RNAs suggested that all identified mycoviruses were targeted by the antiviral fungal mechanism, regardless of the viral genome type. In conclusion, the enlarged list of newly found viruses and the application of different bioinformatics approaches have enabled the identification of novel mycoviruses not previously described in Botrytis species, expanding the already extensive list.

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.

New lineages of RNA viruses from clinical isolates of Rhizopus microsporus revealed by fragmented and primer-ligated dsRNA sequencing (FLDS) analysis

Rhizopus microsporus is a species in the order Mucorales that is known to cause mucormycosis, but it is poorly understood as a host of viruses. Here, we examined 25 clinical strains of R. microsporus for viral infection with a conventional double-stranded RNA (dsRNA) assay using agarose gel electrophoresis (AGE) and the recently established fragmented and primer-ligated dsRNA sequencing (FLDS) protocol. By AGE, five virus-infected strains were detected. Then, full-length genomic sequences of 12 novel RNA viruses were revealed by FLDS, which were related to the families Mitoviridae, Narnaviridae, and Endornaviridae, ill-defined groups of single-stranded RNA (ssRNA) viruses with similarity to the established families Virgaviridae and Phasmaviridae, and the proposed family "Ambiguiviridae." All the characterized viruses, except a potential phasmavirid with a negative-sense RNA genome, had positive-sense RNA genomes. One virus belonged to a previously established species within the family Mitoviridae, whereas the other 11 viruses represented new species or even new genera. These results show that the fungal pathogen R. microsporus harbors diverse RNA viruses and extend our understanding of the diversity of RNA viruses in the fungal order Mucorales, division Mucoromycota. Identifying RNA viruses from clinical isolates of R. microsporus may expand the repertoire of natural therapeutic agents for mucormycosis in the future.IMPORTANCEThe diversity of mycoviruses in fungal hosts in the division Mucoromycota has been underestimated, mainly within the species Rhizopus microsporus. Only five positive-sense RNA genomes had previously been discovered in this species. Because current sequencing methods poorly complete the termini of genomes, we used fragmented and primer-ligated double-stranded RNA sequencing to acquire the full-length genomes. Eleven novel mycoviruses were detected in this study, including the first negative-sense RNA genome reported in R. microsporus. Our findings extend the understanding of the viral diversity in clinical strains of Mucoromycota, may provide insights into the pathogenesis and ecology of this fungus, and may offer therapeutic options.

Transmission of mycoviruses: new possibilities

Mycoviruses are viruses that infect fungi. In recent years, an increasing number of mycoviruses have been reported in a wide array of fungi. With the growing interest of scientists and society in reducing the use of agrochemicals, the debate about mycoviruses as an effective next-generation biocontrol has regained momentum. Mycoviruses can have profound effects on the host phenotype, although most viruses have neutral or no effect. We speculate that understanding multiple transmission modes of mycoviruses is central to unraveling the viral ecology and their function in regulating fungal populations. Unlike plant virus transmission via vegetative plant parts, seeds, pollen, or vectors, a widely held view is that mycoviruses are transmitted via vertical routes and only under special circumstances horizontally via hyphal contact depending on the vegetative compatibility groups (i.e., the ability of different fungal strains to undergo hyphal fusion). However, this view has been challenged over the past decades, as new possible transmission routes of mycoviruses are beginning to unravel. In this perspective, we discuss emerging studies with evidence suggesting that such novel routes of mycovirus transmission exist and are pertinent to understanding the full picture of mycovirus ecology and evolution.

Cross-species transmission of a novel bisegmented orfanplasmovirus in the phytopathogenic fungus Exserohilum rostratum

Mycoviruses have been found in various fungal species across different taxonomic groups, while no viruses have been reported yet in the fungus Exserohilum rostratum. In this study, a novel orfanplasmovirus, namely Exserohilum rostratum orfanplasmovirus 1 (ErOrfV1), was identified in the Exserohilum rostratum strain JZ1 from maize leaf. The complete genome of ErOrfV1 consists of two positive single-stranded RNA segments, encoding an RNA-dependent RNA polymerase and a hypothetical protein with unknown function, respectively. Phylogenetic analysis revealed that ErOrfV1 clusters with other orfanplasmoviruses, forming a distinct phyletic clade. A new family, Orfanplasmoviridae, is proposed to encompass this newly discovered ErOrfV1 and its associated orfanplasmoviruses. ErOrfV1 exhibits effective vertical transmission through conidia, as evidenced by its 100% presence in over 200 single conidium isolates. Moreover, it can be horizontally transmitted to Exserohilum turcicum. Additionally, the infection of ErOrfV1 is cryptic in E. turcicum because there were no significant differences in mycelial growth rate and colony morphology between ErOrfV1-infected and ErOrfV1-free strains. This study represents the inaugural report of a mycovirus in E. rostratum, as well as the first documentation of the biological and transmission characteristics of orfanplasmovirus. These discoveries significantly contribute to our understanding of orfanplasmovirus.

Complete genome sequence of a novel mitovirus identified in the phytopathogenic fungus Fusarium oxysporum f. sp. melonis strain T-SD3

A novel mitovirus was identified in Fusarium oxysporum f. sp. melonis strain T-SD3 and designated as "Fusarium oxysporum mitovirus 3" (FoMV3). The virus was isolated from diseased muskmelon plants with the typical symptom of fusarium wilt. The complete genome of FoMV3 is 2269 nt in length with a predicted AU content of 61.40% and contains a single open reading frame (ORF) using the fungal mitochondrial genetic code. The ORF was predicted to encode a polypeptide of 679 amino acids (aa) containing a conserved RNA-dependent RNA polymerase (RdRp) domain with a molecular mass of 77.39 kDa, which contains six conserved motifs with the highly conserved GDD tripeptide in motif IV. The 5'-untranslated region (UTR) and 3'-UTR of FoMV3 were predicted to fold into stem-loop structures. BLASTp analysis revealed that the RdRp of FoMV3 shared the highest aa sequence identity (83.85%) with that of Fusarium asiaticum mitovirus 5 (FaMV5, a member of the family Mitoviridae) infecting F. asiaticum, the causal agent of wheat fusarium head blight. Phylogenetic analysis further suggested that FoMV3 is a new member of the genus Unuamitovirus within the family Mitoviridae. This is the first report of a new mitovirus associated with F. oxysporum f. sp. melonis.

Expanding diversity of bunyaviruses identified in mosquitoes

Mosquitoes interact with various organisms in the environment, and female mosquitoes in particular serve as vectors that directly transmit a number of microorganisms to humans and animals by blood-sucking. Comprehensive analysis of mosquito-borne viruses has led to the understanding of the existence of diverse viral species and to the identification of zoonotic arboviruses responsible for significant outbreaks and epidemics. In the present study on mosquito-borne bunyaviruses we employed a broad-spectrum RT-PCR approach and identified eighteen different additional species in the Phenuiviridae family and also a number of related but unclassified bunyaviruses in mosquitoes collected in Zambia. The entire RNA genome segments of the newly identified viruses were further analyzed by RNA sequencing with a ribonuclease R (RNase R) treatment to reduce host-derived RNAs and enrich viral RNAs, taking advantage of the dsRNA panhandle structure of the bunyavirus genome. All three or four genome segments were identified in eight bunyavirus species. Furthermore, L segments of three different novel viruses related to the Leishbunyaviridae were found in mosquitoes together with genes from the suspected host, the Crithidia parasite. In summary, our virus detection approach using a combination of broad-spectrum RT-PCR and RNA sequencing analysis with a simple virus enrichment method allowed the discovery of novel bunyaviruses. The diversity of bunyaviruses is still expanding and studies on this will allow a better understanding of the ecology of hematophagous mosquitoes.

Virome analysis of an ectomycorrhizal fungus Suillus luteus revealing potential evolutionary implications

Suillus luteus is a widespread edible ectomycorrhizal fungus that holds significant importance in both ecological and economic value. Mycoviruses are ubiquitous infectious agents hosted in different fungi, with some known to exert beneficial or detrimental effects on their hosts. However, mycoviruses hosted in ectomycorrhizal fungi remain poorly studied. To address this gap in knowledge, we employed next-generation sequencing (NGS) to investigate the virome of S. luteus. Using BLASTp analysis and phylogenetic tree construction, we identified 33 mycovirus species, with over half of them belonging to the phylum Lenarviricota, and 29 of these viruses were novel. These mycoviruses were further grouped into 11 lineages, with the discovery of a new negative-sense single-stranded RNA viral family in the order Bunyavirales. In addition, our findings suggest the occurrence of cross-species transmission (CST) between the fungus and ticks, shedding light on potential evolutionary events that have shaped the viral community in different hosts. This study is not only the first study to characterize mycoviruses in S. luteus but highlights the enormous diversity of mycoviruses and their implications for virus evolution.

Annual (2023) taxonomic update of RNA-directed RNA polymerase-encoding negative-sense RNA viruses (realm Riboviria: kingdom Orthornavirae: phylum Negarnaviricota)

In April 2023, following the annual International Committee on Taxonomy of Viruses (ICTV) ratification vote on newly proposed taxa, the phylum Negarnaviricota was amended and emended. The phylum was expanded by one new family, 14 new genera, and 140 new species. Two genera and 538 species were renamed. One species was moved, and four were abolished. This article presents the updated taxonomy of Negarnaviricota as now accepted by the ICTV.