Molecular characterization of a novel ssRNA ourmia-like virus from the rice blast fungus Magnaporthe oryzae

Molecular characterization of a single-negative-stranded RNA virus from the rice blast fungus Magnaporthe oryzae isolate NJ39

A novel negative-sense single-stranded RNA mycovirus, designated as "Magnaporthe oryzae mymonavirus 1" (MoMNV1), was identified in the rice blast fungus Magnaporthe oryzae isolate NJ39. MoMNV1 has a single genomic RNA segment consisting of 10,515 nucleotides, which contains six open reading frames. The largest open reading frame contains 5837 bases and encodes an RNA replicase. The six open reading frames have no overlap and are arranged linearly on the genome, but the spacing of the genes is small, with a maximum of 315 bases and a minimum of 80 bases. Genome comparison and phylogenetic analysis indicated that MoMNV1 is a new member of the genus Penicillimonavirus of the family Mymonaviridae.

Determination of the Mycovirome of a Necrotrophic Fungus

Next-generation sequencing (NGS) of total RNA has allowed the detection of novel viruses infecting different hosts, such as fungi, increasing our knowledge on virus horizontal transfer events among different hosts, virus diversity, and virus evolution. Here, we describe the detailed protocols for the isolation of the plant pathogenic fungus Botrytis cinerea, from grapevine plants showing symptoms of the mold gray disease, the culture and maintenance of the isolated B. cinerea strains, the extraction of total RNA from B. cinerea strains for NGS, the bioinformatics pipeline designed and followed to detect mycoviruses in the sequenced samples, and the validation of the in silico detected mycoviruses by different approaches.

Characterization of a novel magoulivirus isolated from the phytopathogenic fungus Verticillium dahlia

A new positive-sense single-stranded RNA (+ssRNA) mycovirus, Verticillium dahliae magoulivirus 1 (VdMoV1), was isolated from two strains (2-19 and XLZ70) of Verticillium dahliae. The complete genome of VdMoV1 is 2303 nucleotides (nt) in length and has a large open reading frame (nt positions from 61 to 1938) encoding an RNA-dependent RNA polymerase (RdRp). A multiple sequence alignment indicated that the central region of the RdRp encoded by VdMoV1 contains eight typical viral RdRp motifs. BLASTp analysis demonstrated that VdMoV1 has the highest sequence identity (86.88%) to Bremia lactucae associated ourmia-like virus 2 (BlaOLV2). Phylogenetic analysis revealed that VdMoV1 is a new member of the genus Magoulivirus. As far as we know, VdMoV1 is the first reported member of the family Botourmiaviridae infecting V. dahliae.

Coinfection of Two Mycoviruses Confers Hypovirulence and Reduces the Production of Mycotoxin Alternariol in Alternaria alternata f. sp. mali

Alternaria leaf blotch caused by Alternaria alternata apple pathotype (Alternaria mali) is an important fungal disease that affects the production of apples worldwide. Mycoviruses harbored in plant pathogenic fungi can confer hypovirulence in their hosts and have attracted widespread attention as potential biocontrol tools. In this study, the coinfection of two mycoviruses, named A. alternata chrysovirus 1 strain QY2 (AaCV1-QY2) and A. alternata magoulivirus 1 (AaMV1), respectively, were isolated from A. alternata f. sp. mali strain QY21. Sequence analyses revealed that AaCV1-QY2 virus belonged to the genus Betachrysovirus and AaMV1 virus belonged to the genus Magoulvirus. These two mycoviruses were found to be associated with hypovirulence in A. alternata, among which AaCV1-QY2 might play a relatively leading role. Because the elimination of AaMV1 from the strain QY21 does not affect the hypovirulence trait, which indicates that the virus AaCV1-QY2 can independently induce slow growth and reduce host virulence. Moreover, the presence of viruses decreased the accumulation of the mycotoxin alternariol (AOH) in A. alternata strains. Intriguingly, AaCV1-QY2/AaMV1 mycoviruses can be horizontally transmitted to other A. alternata strains, and this coinfection can promote the interspecific transmission efficiency of AaCV1-QY2. To our knowledge, this study reports the first description of the member of Chrysovirus is related to hypovirulence in Alternaria spp. that facilitates the development of biocontrol measures of A. mali Roberts.

Mycoviromic Analysis Unveils Complex Virus Composition in a Hypovirulent Strain of Sclerotinia sclerotiorum

Mycoviruses are ubiquitous in pathogenic fungi including Sclerotinia sclerotiorum. Using RNA sequencing, more mycoviruses have been identified in individual strains, which were previously reported to be infected by a single mycovirus. A hypovirulent strain of S. sclerotiorum, HC025, was previously thought to harbor a single mitovirus, Sclerotinia sclerotiorum mitovirus 1 (SsMV1), based on the analysis of the conventional dsRNA extraction method. We found HC025 to be co-infected by five mycoviruses. In addition to SsMV1, four mycoviruses were identified: Sclerotinia sclerotiorum narnavirus 4 (SsNV4), Sclerotinia sclerotiorum negative-stranded RNA virus 1 (SsNSRV1), Sclerotinia sclerotiorum ourmia-like virus 14 (SsOLV14), and SsOLV22. Three mycoviruses including SsNV4, SsNSRV1, and SsOLV14 share high replicase identities (more than 95%) with the previously reported corresponding mycoviruses, and SsOLV22 shows lower identity to the known viruses. The complete genome of SsOLV22 is 3987 nt long and contains a single ORF-encoded RdRp, which shares 24.84% identity with the RNA-dependent RNA polymerase (RdRp) of Hubei narna-like virus 10 (query coverage: 26%; e-value: 8 × 10⁻¹⁹). The phylogenetic tree of RdRp suggests that SsOLV22 is a new member within the family Botourmiaviridae. All of the mycoviruses except for SsNSRV1 could horizontally co-transfer from HC025 to the virulent strain Ep-1PNA367 with hypovirulent phenotypes, and converted a later strain into a hypovirulent strain. In summary, we molecularly characterized the hypovirulent strain HC025 and identified five RNA mycoviruses including a new member within Botourmiaviridae.

Molecular characterization of a novel botourmiavirus with inverted complementary termini from the rice blast fungus Magnaporthe oryzae isolate HF04

A novel positive-sense single-stranded RNA mycovirus, designated as "Magnaporthe oryzae botourmiavirus 10" (MoBV10), was identified in the rice blast fungus Magnaporthe oryzae isolate HF04. MoBV10 has a single genomic RNA segment consisting of 2,448 nucleotides, which contains a single open reading frame encoding an RNA-dependent RNA polymerase. Genome comparison and phylogenetic analysis indicated that MoBV10 is a new member of the genus Betascleroulivirus in the family Botourmiaviridae. The 5'- and 3'-terminal sequences of the genomic RNA of MoBV10 have inverted complementarity and potentially form a panhandle structure, which is very rare in RNA viruses.

Resolving Deep Evolutionary Relationships within the RNA Virus Phylum Lenarviricota

The RNA virus phylum Lenarviricota is composed of the fungi-associated families Narnaviridae and Mitoviridae, the RNA bacteriophage Leviviridae, and the plant and fungi-associated Botourmiaviridae. Members of the Lenarviricota are abundant in most environments and boast remarkable phylogenetic and genomic diversity. As this phylum includes both RNA bacteriophage and fungi- and plant-associated species, the Lenarviricota likely mark a major evolutionary transition between those RNA viruses associated with prokaryotes and eukaryotes. Despite the remarkable expansion of this phylum following metagenomic studies, the phylogenetic relationships among the families within the Lenarviricota remain uncertain. Utilising a large data set of relevant viral sequences, we performed phylogenetic and genomic analyses to resolve the complex evolutionary history within this phylum and identify patterns in the evolution of virus genome organisation. Despite limitations reflecting very high levels of sequence diversity, our phylogenetic analyses suggest that the Leviviridae comprise the basal lineage within the Lenarviricota. Our phylogenetic results also support the construction of a new virus family—the Narliviridae—comprising a set of diverse and phylogenetically distinct species, including a number of uniquely encapsidated viruses. We propose a taxonomic restructuring within the Lenarviricota to better reflect the phylogenetic relationships documented here, with the Botourmiaviridae and Narliviridae combined into the order Ourlivirales, the Narnaviridae remaining in the order Wolframvirales, and these orders combined into the single class, the Amabiliviricetes. In sum, this study provides insights into the complex evolutionary relationships among the diverse families that make up the Lenarviricota.

Novel Mycoviruses Discovered in the Mycovirome of a Necrotrophic Fungus

Botrytis cinerea is one of the most important plant-pathogenic fungus. Products based on microorganisms can be used in biocontrol strategies alternative to chemical control, and mycoviruses have been explored as putative biological agents in such approaches. Here, we have explored the mycovirome of B. cinerea isolates from grapevine of Italy and Spain to increase the knowledge about mycoviral diversity and evolution, and to search for new widely distributed mycoviruses that could be active ingredients in biological products to control this hazardous fungus. A total of 248 B. cinerea field isolates were used for our metatranscriptomic study. Ninety-two mycoviruses were identified: 62 new mycoviral species constituting putative novel viral genera and families. Of these mycoviruses, 57 had a positive-sense single-stranded RNA (ssRNA) genome, 19 contained a double-stranded RNA (dsRNA) genome, 15 had a negative-sense ssRNA genome, and 1 contained a single-stranded DNA (ssDNA) genome. In general, ssRNA mycoviruses were widely distributed in all sampled regions, the ssDNA mycovirus was more frequently found in Spain, and dsRNA mycoviruses were scattered in some pools of both countries. Some of the identified mycoviruses belong to clades that have never been found associated with Botrytis species: Botrytis-infecting narnaviruses; alpha-like, umbra-like, and tymo-like ssRNA+ mycoviruses; trisegmented ssRNA- mycovirus; bisegmented and tetrasegmented dsRNA mycoviruses; and finally, an ssDNA mycovirus. Among the results obtained in this massive mycovirus screening, the discovery of novel bisegmented viruses, phylogenetically related to narnaviruses, is remarkable.IMPORTANCE The results obtained here have expanded our knowledge of mycoviral diversity, horizontal transfers, and putative cross-kingdom events. To date, this study presents the most extensive and wide diversity collection of mycoviruses infecting the necrotrophic fungus B. cinerea The collection included all types of mycoviruses, with dsRNA, ssRNA+, ssRNA-, and ssDNA genomes, most of which were discovered here, and some of which were previously reported as infecting B. cinerea or other plant-pathogenic fungi. Some of these mycoviruses are reported for the first time here associated with B. cinerea, as a trisegmented ssRNA- mycovirus and as an ssDNA mycovirus, but even more remarkablly, we also describe here four novel bisegmented viruses (binarnaviruses) not previously described in nature. The present findings significantly contribute to general knowledge in virology and more particularly in the field of mycovirology.

Complete nucleotide sequence of a novel botourmiavirus from the rice blast fungus Magnaporthe oryzae isolate SH05

A novel mycovirus with the proposed name "Magnaporthe oryzae botourmiavirus 9" (MoBV9) was found in the rice blast fungus Magnaporthe oryzae isolate SH05. The virus has a positive single-stranded RNA genome of 2,812 nucleotides and contains a single open reading frame predicted to encode an RNA-dependent RNA polymerase that is closely related to those of some unclassified viruses of the family Botourmiaviridae, including Plasmopara viticola lesion associated ourmia-like virus 44, Plasmopara viticola lesion associated ourmia-like virus 47, and Cladosporium uredinicola ourmiavirus 1. Genome sequence comparisons and phylogenetic analysis supported the notion that MoBV9 is a new member of the family Botourmiaviridae.

A Novel Ourmia-Like Mycovirus Confers Hypovirulence-Associated Traits on Fusarium oxysporum

Fusarium wilt caused by Fusarium oxysporum f. sp. momordicae (FoM) is an important fungal disease that affects the production of bitter gourd. Hypovirulence-associated mycoviruses have great potential and application prospects for controlling the fungal disease. In this study, a novel ourmia-like virus, named Fusarium oxysporum ourmia-like virus 1 (FoOuLV1), was isolated from FoM strain HuN8. The viral genomic RNA is 2,712 nucleotides (nt) in length and contains an open reading frame (ORF) encoding a putative RNA-dependent RNA polymerase (RdRp) using either standard or mitochondrial codes. In strain HuN8, there was also a FoOuLV1-associated RNA segment with 1,173 nt in length with no sequence homology. Phylogenetic analysis showed that FoOuLV1 is a member of the genus Magoulivirus of the family Botourmiaviridae. FoOuLV1 was found to be associated with hypovirulence in FoM. Moreover, FoOuLV1 and its hypovirulence trait can be transmitted horizontally to other FoM strains and also to other formae speciale strains of F. oxysporum. In addition, FoOuLV1 showed significant biological control effect against the bitter gourd Fusarium wilt. To our knowledge, this study reveals the first description of a hypovirulence-associated ourmia-like mycovirus, which has the potential to the biological control of Fusarium wilt.

Four Novel Botourmiaviruses Co-Infecting an Isolate of the Rice Blast Fungus Magnaporthe oryzae

Via virome sequencing, six viruses were detected from Magnaporthe oryzae strains YC81-2, including one virus in the family Tombusviridae, one virus in the family Narnaviridae and four viruses in the family Botourmiaviridae. Since the RNA-dependent RNA polymerase (RdRp) of one botourmiavirus show the highest identity (79%) with Magnaporthe oryzae ourmia-like virus 1 (MOLV1), the virus that was grouped into the genus Magoulivirus was designated as Magnaporthe oryzae botourmiavirus 2 (MOBV2). The three other novel botourmiaviruses were selected for further study. The complete nucleotide sequences of the three botourmiaviruses were determined. Sequence analysis showed that virus 1, virus 2, and virus 3 were 2598, 2385, and 2326 nts in length, respectively. The variable 3' untranslated region (3'-UTR) and 5'-UTR of each virus could be folded into a stable stem-loop secondary structure. Each virus consisted of a unique ORF encoding a putative RdRp. The putative proteins with a conserved GDD motif of RdRp showed the highest sequence similarity to RdRps of viruses in the family Botourmiaviridae. Phylogenetic analysis demonstrated that these viruses were three distinct novel botourmiaviruses, clustered into the Botourmiaviridae family but not belonging to any known genera of this family. Thus, virus 1, virus 2, and virus 3 were designated as Magnaporthe oryzae botourmiavirus 5, 6, and 7 (MOBV5, MOBV6, and MOBV7), respectively. Our results suggest that four distinct botourmiaviruses, MOBV2, MOBV5, MOBV6, and MOBV7, co-infect a single strain of Magnaporthe oryzae, and MOBV5, MOBV6, and MOBV7 are members of three unclassified genera in the family Botourmiaviridae.

Population Structure of Double-Stranded RNA Mycoviruses That Infect the Rice Blast Fungus Magnaporthe oryzae in Japan

Various viruses infect Magnaporthe oryzae (syn. Pyricularia oryzae), which is a well-studied fungus that causes rice blast disease. Most research has focused on the discovery of new viruses and the hypovirulence-associated traits conferred by them. Therefore, the diversity and prevalence of viruses in wild fungal populations have not been explored. We conducted a comprehensive screening of M. oryzae mycoviruses from various regions in Japan using double-stranded RNA (dsRNA) electrophoresis and RT-PCR assays. We detected three mycoviruses, Magnaporthe oryzae virus 2 (MoV2), Magnaporthe oryzae chrysovirus 1 (MoCV1), and Magnaporthe oryzae partitivirus 1 (MoPV1), among 127 of the 194 M. oryzae strains screened. The most prevalent virus was MoPV1 (58.8%), which often co-infected in a single fungal strain together with MoV2 or MoCV1. MoV2 and MoCV1 were found in 22.7 and 10.8% of strains, respectively, and they were usually distributed in different regions so that mixed-infection with these two mycoviruses was extremely rare. The predominance of MoPV1 in M. oryzae is supported by significant negative values from neutrality tests, which indicate that the population size of MoPV1 tends to increase. Population genetic analyses revealed high nucleotide diversity and the presence of phylogenetically diverse subpopulations among the MoV2 isolates. This was not the case for MoPV1. Furthermore, studies of a virus-cured M. oryzae strain revealed that MoV2 does not cause any abnormalities or symptoms in its host. However, a leaf sheath inoculation assay showed that its presence slightly increased the speed of mycelial growth, compared with virus-free mycelia. These results demonstrate that M. oryzae in Japan harbors diverse dsRNA mycovirus communities with wide variations in their population structures among different viruses.

The dsRNA mycovirus ChNRV1 causes mild hypervirulence in the fungal phytopathogen Colletotrichum higginsianum

The genus Colletotrichum comprises a large number of filamentous fungi responsible for anthracnose diseases in many tropical and subtropical fruits and vegetables. In particular, Colletotrichum higginsianum infects Brassicaceae species, including Arabidopsis. The C. higginsianum strain IMI349063A is naturally infected with a dsRNA virus, named Colletorichum higginsianum non-segmented virus (ChNRV1). Here, we investigated the biological effect of ChNRV1 in C. higginsianum by comparing strains with and without the virus. ChNRV1 does not have an effect on C. higginsianum growth under salt and cell-wall stress conditions. However, thermal stress reduced C. higginsianum growth rate, this effect being more evident in the wild-type C. higginsianum strain containing the virus. Although ChNRV1 had no effect in conidiation, conidia were narrower when the virus is present. More importantly, ChNRV1 causes a mild increase in C. higginsianum virulence (hypervirulence) when infecting Arabidopsis plants. These findings indicated that, whereas the ChNRV1 mycovirus does not impair growth and conidiation of C. higginsianum, it confers hypervirulence to the fungal host. These findings will help in future research on the effect of mycoviral infection on pathogenic fungi in plant species of agronomical relevance.

ICTV Virus Taxonomy Profile: Botourmiaviridae

The family Botourmiaviridae includes viruses infecting plants and filamentous fungi containing a positive-sense, ssRNA genome that can be mono- or multi-segmented. Genera in the family include: Ourmiavirus (plant viruses), and Botoulivirus, Magoulivirus and Scleroulivirus (fungal viruses). This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on the taxonomy of the family Botourmiaviridae, which is available at ictv.global/report/botourmiaviridae.

A Single ssRNA Segment Encoding RdRp Is Sufficient for Replication, Infection, and Transmission of Ourmia-Like Virus in Fungi

Recently, an increasing number of ourmia-like viruses have been found in fungi; however, the features of these viruses remain unknown. Here, we report a novel ourmia-like virus isolated from Sclerotinia sclerotiorum. This virus, named S. sclerotiorum ourmia-like virus 4 (SsOLV4), has a genome 2,982 nt in length with a G-pentamer (GGGGG) at the 5'-terminus and a C-pentamer (CCCCC) at the 3'-terminus. The SsOLV4 genome has only one large putative open reading frame (ORF) predicted with both standard codes and mitochondrial codes and encodes an RNA-dependent RNA polymerase (RdRp). SsOLV4 is closely phylogenetically related to Pyricularia oryzae ourmia-like virus 1, with 42% identity between the RdRp amino acid sequences. We constructed full-length cDNA of SsOLV4 and synthesized RNA in vitro using the T7 RNA polymerase. The synthesized RNA could transfect S. sclerotiorum protoplasts efficiently. We further found that viral RNA could infect mycelia when mixed with PEG buffer. Our study suggests that a novel genus in family Botourmiaviridae should be established for SsOLV4 and other related viruses and demonstrates that one single-stranded RNA segment encoding RdRp is sufficient for ourmia-like viruses in fungi.

A novel narnavirus from the plant-pathogenic fungus Magnaporthe oryzae

A novel mycovirus with the proposed name "Magnaporthe oryzae narnavirus virus 1" (MoNV1), was described in the rice blast fungus Magnaporthe oryzae. The virus has a single-stranded (+ss) RNA genome of 2452 nucleotides, contains a single open reading frame (ORF) predicted to encode an RNA-dependent RNA polymerase (RDRP), and is closely related to some viruses of the genus Narnavirus, family Narnaviridae, including Aspergillus fumigatus narnavirus 1 (AfNV1), Neofusicoccum parvum narnavirus 2 (NpNV2) and Alternaria tenuissima narnavirus 1 (AtNV2). Genome sequence comparisons and phylogenetic analysis suggested that MoNV1 is a new member of the genus Narnavirus. The RDRPs of MoNV1 and some closely related narnaviruses do not contain a typical metal-binding "GDD" motif and catalytic site. Further studies are needed to investigate the replication mechanism of these viruses.

Molecular characterization of a novel botoulivirus from the phytopathogenic fungus Sclerotinia minor

In this study, the complete genomic sequence of a novel botoulivirus (Sclerotinia minor botoulivirus 1, SmBV1) from the phytopathogenic fungus Sclerotinia minor strain LC45 was determined. The genome of SmBV1 is 2,882 nucleotides in length and contains a single large open reading frame (ORF) encoding a putative RNA-dependent RNA polymerase (RdRp). Phylogenetic analysis showed that SmBV1 clustered with the botoulivirus clade within the family Botourmiaviridae. This is the first report of a botoulivirus in S. minor.

A novel ourmia-like mycovirus isolated from the plant pathogenic fungus Colletotrichum gloeosporioides

In this study, a novel mycovirus designed Colletotrichum gloeosporioides ourmia-like virus 1 (CgOLV1) was isolated from a filamentous phytopathogenic fungus, Colletotrichum gloeosporioides. The virus has a genome of 2,516 nucleotides and contains a large open reading frame (ORF) encoding an RNA-dependent RNA polymerase (RdRp). Phylogenetic analysis indicated that CgOLV1 is located in the ourmia-like mycovirus clade, whose members are related to plant ourmiaviruses. To the best of our knowledge, this is the first report of an ourmia-like mycovirus in C. gloeosporioides.

Characterization of a Novel Ourmia-Like Mycovirus Infecting Magnaporthe oryzae and Implications for Viral Diversity and Evolution

Here, the molecular characterization of a novel mycovirus that was isolated from a phytopathogenic fungus Magnaporthe oryzae and designed as Magnaporthe oryzae ourmia-like virus 4 (MOLV4) is reported. MOLV4 has a genome that is 2497 bp long and possesses a single open reading frame (ORF), which encodes the product RNA-dependent RNA polymerase (RdRp). Sequence similarities were found between the MOLV4 encoded RdRp and the counterparts of a few previously reported ourmia-like mycoviruses. Virus-curing and biological comparison indicate that the virus has no or mild effects on the morphology and mycelium growth rate of the host fungus. Phylogenetic analysis using the RdRp aa sequences was performed. The results show that MOLV4 is clustered with the ourmia-like mycoviruses, forming a clade closely related to ourmiaviruses but distinct from narnaviruses. In addition, database searches revealed that several MOLV4-related sequences are present in the transcriptome shotgun assembly (TSA) library, expressed sequence tag database (ESTdb), whole-genome shotgun (WGS) library, and genomic survey sequences (GSS) libraries of a few other species of eukaryote organisms. Our results show that MOLV4, together with other similar ourmia-like mycoviruses, might represent a virus clade that links the plant ourmiaviruses and fungal narnaviruses and has a wide range of hosts.

Virulence- and signaling-associated genes display a preference for long 3'UTRs during rice infection and metabolic stress in the rice blast fungus

Generation of mRNA isoforms by alternative polyadenylation (APA) and their involvement in regulation of fungal cellular processes, including virulence, remains elusive. Here, we investigated genome-wide polyadenylation site (PAS) selection in the rice blast fungus to understand how APA regulates pathogenicity. More than half of Magnaporthe oryzae transcripts undergo APA and show novel motifs in their PAS region. Transcripts with shorter 3'UTRs are more stable and abundant in polysomal fractions, suggesting they are being translated more efficiently. Importantly, rice colonization increases the use of distal PASs of pathogenicity genes, especially those participating in signalling pathways like 14-3-3B, whose long 3'UTR is required for infection. Cleavage factor I (CFI) Rbp35 regulates expression and distal PAS selection of virulence and signalling-associated genes, tRNAs and transposable elements, pointing its potential to drive genomic rearrangements and pathogen evolution. We propose a noncanonical PAS selection mechanism for Rbp35 that recognizes UGUAH, unlike humans, without CFI25. Our results showed that APA controls turnover and translation of transcripts involved in fungal growth and environmental adaptation. Furthermore, these data provide useful information for enhancing genome annotations and for cross-species comparisons of PASs and PAS usage within the fungal kingdom and the tree of life.

Chrysoviruses in Magnaporthe oryzae

Magnaporthe oryzae, the fungus that causes rice blast, is the most destructive pathogen of rice worldwide. A number of M. oryzae mycoviruses have been identified. These include Magnaporthe oryzae. viruses 1, 2, and 3 (MoV1, MoV2, and MoV3) belonging to the genus, Victorivirus, in the family, Totiviridae; Magnaporthe oryzae. partitivirus 1 (MoPV1) in the family, Partitiviridae; Magnaporthe oryzae. chrysovirus 1 strains A and B (MoCV1-A and MoCV1-B) belonging to cluster II of the family, Chrysoviridae; a mycovirus related to plant viruses of the family, Tombusviridae (Magnaporthe oryzae. virus A); and a (+)ssRNA mycovirus closely related to the ourmia-like viruses (Magnaporthe oryzae. ourmia-like virus 1). Among these, MoCV1-A and MoCV1-B were the first reported mycoviruses that cause hypovirulence traits in their host fungus, such as impaired growth, altered colony morphology, and reduced pigmentation. Recently we reported that, although MoCV1-A infection generally confers hypovirulence to fungi, it is also a driving force behind the development of physiological diversity, including pathogenic races. Another example of modulated pathogenicity caused by mycovirus infection is that of Alternaria alternata chrysovirus 1 (AaCV1), which is closely related to MoCV1-A. AaCV1 exhibits two contrasting effects: Impaired growth of the host fungus while rendering the host hypervirulent to the plant, through increased production of the host-specific AK-toxin. It is inferred that these mycoviruses might be epigenetic factors that cause changes in the pathogenicity of phytopathogenic fungi.

Diverse, Novel Mycoviruses From the Virome of a Hypovirulent Sclerotium rolfsii Strain

Sclerotium rolfsii, which causes southern blight in a wide variety of crops, is a devastating plant pathogen worldwide. Mycoviruses that induce hypovirulence in phytopathogenic fungi are potential biological control resources against fungal plant diseases. However, in S. rolfsii, mycoviruses are rarely reported. In a previous study, we found a hypovirulent strain carrying a diverse pattern of dsRNAs. Here, we utilized the RNA_Seq technique to detect viral sequences. Deep sequencing, RT-PCR and Sanger sequencing validation analyses revealed that this strain harbors various new viral species that show affinity to the distinctly established and proposed families Benyviridae, Endornaviridae, Fusariviridae, Hypoviridae, and Fusagraviridae. Moreover, some viral sequences that could not be assigned to any of the existing families or orders were also identified and showed similarities to the Alphavirus, Ourmiavirus, phlegivirus-like and Curvularia thermal tolerance virus-like groups. In addition, we also conducted deep sequencing analysis of small RNAs in the virus-infecting fugal strain. The results indicated that the Dicer-mediated gene silencing mechanism was present in S. rolfsii. This is the first report of viral diversity in a single S. rolfsii fungal strain, and the results presented herein might provide insight into the taxonomy and evolution of mycoviruses and be useful for the exploration of mycoviruses as biocontrol agents.