Characterization of a novel single-stranded RNA virus, closely related to fusariviruses, infecting the plant pathogenic fungus Alternaria brassicicola

Sensitivity of Globisporangium ultimum to the fungicide metalaxyl is enhanced by the infection with a toti-like mycovirus

In recent years, numerous oomycete mycoviruses have been discovered; however, very few studies have focused on their effects on the host oomycete phenotype. In this study, we investigated the impact of toti-like Pythium ultimum RNA virus 2 (PuRV2) infection on the phytopathogenic soil-borne oomycete Globisporangium ultimum, which serves as a model species for Globisporangium and Pythium, specifically the UOP226 isolate in Japan. We generated a PuRV2-free isogenic line through hyphal tip isolation using high-temperature culture and subsequently compared the phenotypic characteristics and gene expression profiles of UOP226 and the PuRV2-free isogenic line. Our findings revealed that the metalaxyl sensitivity of UOP226 was greater than that of the PuRV2-free isogenic line, whereas the mycelial growth rate and colony morphology remained unchanged in the absence of the fungicide. Furthermore, transcriptome analyses using RNA-seq revealed significant downregulation of ABC-type transporter genes, which are involved in fungicide sensitivity, in UOP226. Our results suggest that PuRV2 infection influences the ecology of G. ultimum in agricultural ecosystems where metalaxyl is applied.

Identification of Mycoviruses in the Pathogens of Fragrant Pear Valsa Canker from Xinjiang in China

As a common disease, canker seriously affects the yield and quality of fragrant pear due to the lack of effective control measures. Some fungi have been reported to harbor rich reservoirs of viral resources, and some mycoviruses can be used as biocontrol agents against plant diseases. In this study, 199 isolates were obtained from diseased branches of fragrant pear in the main production areas of Xinjiang. Among them, 134 belonged to Valsa spp., identified using morphological and molecular biological techniques, in which V. mali was the dominant species. The mycoviruses in Valsa spp. were further identified using metatranscriptomic sequencing and RT-PCR. The results revealed that a total of seven mycoviruses were identified, belonging to Botourmiaviridae, Endornaviridae, Fusariviridae, Hypoviridae, Mitoviridae, and Narnaviridae, among which Phomopsis longicolla hypovirus (PlHV) was dominant in all the sample collection regions. The Cryphonectria hypovirus 3-XJ1 (CHV3-XJ1), Botourmiaviridae sp.-XJ1 (BVsp-XJ1), and Fusariviridae sp.-XJ1 (Fvsp-XJ1) were new mycoviruses discovered within the Valsa spp. More importantly, compared with those in the virus-free Valsa spp. strain, the growth rate and virulence of the VN-5 strain co-infected with PlHV and CHV3-XJ1 were reduced by 59% and 75%, respectively, and the growth rate and virulence of the VN-34 strain infected with PlHV were reduced by 42% and 55%, respectively. On the other hand, the horizontal transmission efficiency of PlHV decreased when PlHV was co-infected with CHV3-XJ1, indicating that PlHV and CHV3-XJ1 were antagonistic. In summary, the mycoviruses in Valsa spp. were identified in Xinjiang for the first time, and three of them were newly discovered mycoviruses, with two strains yielding good results. These results will offer potential biocontrol resources for managing pear canker disease and provide a theoretical basis for the control of fruit tree Valsa canker disease.

Complete genome sequence of a novel fusarivirus from Rhizoctonia solani AG-3 PT strain 3P-2-2

Here, we describe a novel mycovirus, tentatively designated as "Rhizoctonia solani fusarivirus 6" (RsFV6), which was discovered in Rhizoctonia solani AG-3 PT strain 3P-2-2. The virus has a single-stranded positive-sense RNA (+ssRNA) genome of 6141 nucleotides containing two open reading frames (ORFs) and a poly(A) tail. ORF1 encodes a large polypeptide of 1,862 amino acids (aa) with conserved RNA-dependent RNA polymerase (RdRp) and helicase (Hel) domains. ORF2 encodes a putative 167-aa protein of unknown function. BLASTp searches revealed that the ORF1-encoded polypeptide showed the highest sequence similarity (70.67% identity) to that of Rhizoctonia solani fusarivirus 3 (RsFV3), which was isolated from Rhizoctonia solani AG-2-2LP. Multiple sequence alignments and phylogenetic analysis based on RdRp and Hel sequences indicated that RsFV6 could be a novel member of the genus Alphafusarivirus family Fusariviridae.

Novel Mycoviruses Discovered from a Metatranscriptomics Survey of the Phytopathogenic Alternaria Fungus

Alternaria fungus can cause notable diseases in cereals, ornamental plants, vegetables, and fruits around the world. To date, an increasing number of mycoviruses have been accurately and successfully identified in this fungus. In this study, we discovered mycoviruses from 78 strains in 6 species of the genus Alternaria, which were collected from 10 pear production areas using high-throughput sequencing technology. Using the total RNA-seq, we detected the RNA-dependent RNA polymerase of 19 potential viruses and the coat protein of two potential viruses. We successfully confirmed these viruses using reverse transcription polymerase chain reaction with RNA as the template. We identified 12 mycoviruses that were positive-sense single-stranded RNA (+ssRNA) viruses, 5 double-strand RNA (dsRNA) viruses, and 4 negative single-stranded RNA (-ssRNA) viruses. In these viruses, five +ssRNA and four -ssRNA viruses were novel mycoviruses classified into diverse the families Botourmiaviridae, Deltaflexivirus, Mymonaviridea, and Discoviridae. We identified a novel -ssRNA mycovirus isolated from an A. tenuissima strain HB-15 as Alternaria tenuissima negative-stranded RNA virus 2 (AtNSRV2). Additionally, we characterized a novel +ssRNA mycovirus isolated from an A. tenuissima strain SC-8 as Alternaria tenuissima deltaflexivirus 1 (AtDFV1). According to phylogenetic and sequence analyses, we determined that AtNSRV2 was related to the viruses of the genus Sclerotimonavirus in the family Mymonaviridae. We also found that AtDFV1 was related to the virus family Deltaflexivirus. This study is the first to use total RNA sequencing to characterize viruses in Alternaria spp. These results expand the number of Alternaria viruses and demonstrate the diversity of these mycoviruses.

Molecular characterization of a new fusarivirus infecting Botryosphaeria dothidea, the causal agent of pear ring rot disease

Here, a novel mycovirus, tentatively designated as "Botryosphaeria dothidea fusarivirus 2" (BdFV2), was discovered in Botryosphaeria dothidea strain JZ-3. The complete genome sequence is 6,271 nucleotides (nt) in length, excluding the poly(A) tail, and contains two putative open reading frames (ORFs). The larger ORF1 encodes a polypeptide of 1,552 amino acids (aa) with conserved RNA-dependent RNA polymerase (RdRp) domains and a viral helicase domain. The ORF1-encoded polypeptide shares 19.47-78.70% sequence identity with those of other fusariviruses and shares the highest sequence identity (78.70%) with the corresponding protein aa sequences of Neofusicoccum luteum fusarivirus 1 (NlFV1) isolate CBS110299. The small ORF2 encodes a hypothetical protein with 479 aa, which is predicted to contain a chromosome segregation protein SMC domain of unknown function. Sequence alignments and phylogenetic analysis indicated that BdFV2 is a distinct member of the recently established family Fusariviridae. BdFV2 appears to be a novel fusarivirus infecting a pathogenic B. dothidea strain that causes pear ring rot disease.

Infection of Nigrospora nonsegmented RNA Virus 1 Has Important Biological Impacts on a Fungal Host

Nigrospora nonsegmented RNA virus 1 (NoNRV1) has been reported previously in the fungus Nigrospora oryzae, but its biological effects on its host are unknown. In this work, we isolated a strain 9-1 of N. oryzae from a chrysanthemum leaf and identified NoNRV1 infection in the isolated strain. The genome sequence of NoNRV1 identified here is highly homologous to that of the isolate HN-21 of NoNRV1 previously reported; thus, we tentatively designated the newly identified NoNRV1 as NoNRV1-ZJ. Drug treatment with Ribavirin successfully removed NoNRV1-ZJ from the strain 9-1, which provided us with an ideal control to determine the biological impacts of NoNRV1 infection on host fungi. By comparing the virus-carrying (9-1) and virus-cured (9-1C) strains, our results indicated that infection with NoNRV1 promoted the pigmentation of the host cells, while it had no discernable effects on host growth on potato dextrose agar plates when subjected to osmotic or oxidative stress. Interestingly, we observed inhibitory impacts of virus infection on the thermotolerance of N. oryzae and the pathogenicity of the host fungus in cotton leaves. Collectively, our work provides clear evidence of the biological relevance of NoNRV1 infection in N. oryzae, including pigmentation, hypovirulence, and thermotolerance.

Role of Thioredoxin-1 and its inducers in human health and diseases

Thioredoxin-1 (Trx-1) is a small redox-active protein normally found in mammalian cells that responds to the changing redox environment by contributing electrons or regulating related proteins. There is growing evidence that Trx-1 has multiple functions, including cytoprotective, anti-apoptotic, antioxidant and anti-inflammatory effects. To date, researchers have found that Trx-1 deficiency leads to severe damage in various disease models, such as atherosclerosis, cerebral ischemia, diabetes and tumors. Conversely, activation of Trx-1 has a protective effect against these diseases. Accordingly, a variety of Trx-1 inducers have been widely used in the clinic with significant therapeutic value. In this paper, we summarize the pathogenesis of Trx-1 involvement in the above-mentioned diseases and describe the protective effects of Trx-1 inducers on them.

The unique genome organization of two novel fusariviruses hosted by the true morel mushroom Morchella esculenta

Two putative mycoviruses belonging to the proposed family "Fusariviridae" were identified in Morchella esculenta by sequencing of double-stranded RNAs extracted from the morel mushroom. These viruses were tentatively named "Morchella esculenta fusarivirus 1″ (MeFV1) and "Morchella esculenta fusarivirus 2″ (MeFV2). Including the poly(A) tail the complete genomes of MeFV1 and MeFV2 are composed of 9096 and 9011 nucleotides (nt) respectively. Both genomes contain four non-overlapping open reading frames (ORFs) in which the largest and the smallest ORFs are ORF2 and ORF3 for both genomes respectively. The ORF1 of MeFV1 and MeFV2 are preceded by the 5' untranslated regions (UTRs) of 27 and 37 nt respectively and encode 341 and 339 aa long proteins that do not exhibit significant similarity to any of the protein sequences present in GenBank database. The 1502 and 1511 aa long proteins encoded by ORF2 of MeFV1 and MeFV2 share 84.42% sequence identity to each other and are 58.54% and 58.57% identical to the RNA-dependent RNA polymerase (RdRp) of Morchella importuna fusarivirus 1 (MiFV1) respectively. Interestingly, a Promethin/LDAF1 protein domain that is associated with the endoplasmic reticulum (ER) and lipid droplet (LD) membranes was identified at the N terminal regions of MeFV1 and MeFV2 RdRps, implying that the replication of these viruses is linked to the lipid membranes. The ORF3 and ORF4 of MeFV1 and MeFV2 encode proteins (268 and 333 aa long, and 645 and 647 aa long respectively) that only share significant sequence similarities with the proteins encoded by the ORF2 and ORF3 of MiFV1 respectively. The 3' UTRs of MeFV1 and MeFV2 are 162 and 159 nt long respectively and both of them have 51 nt long terminal poly(A) traits. To our knowledge, MeFV1 and MeFV2 are the first fusariviruses identified in M. esculenta and this is the first study reporting on the presence of Promethin/LDAF1 domain in viral RdRps.

Molecular characterization of a novel partitivirus and a fusarivirus coinfecting the fungus Nigrospora sphaerica

In this report, we describe the molecular characterization of two novel mycoviruses coinfecting the plant pathogenic fungus Nigrospora sphaerica, which were designated "Nigrospora sphaerica fusarivirus 1" (NsFV1) and "Nigrospora sphaerica partitivirus 1" (NsPV1). NsFV1 has an undivided genome measuring 6,147 nt, excluding the polyA tail, and was predicted to contain two nonoverlapping open reading frames (ORF1 and 2). The larger ORF1 encodes a polyprotein containing a conserved RNA-dependent RNA polymerase (RdRp) and a helicase domain that has functions related to RNA replication, and the smaller ORF2 encodes a putative protein with an unknown function. NsPV1 consists of two genome segments, which measure 1,796 bp and 1,455 bp in length. Each of the two dsRNAs has a single ORF, and they are predicted to encode proteins with homology to viral RdRps and coat proteins of members of the family Partitiviridae. Phylogenetic analysis indicated that NsFV1 is a member of the recently proposed family "Fusariviridae", while NsPV1 was determined to belong to the genus Gammapartitivirus in the family Partitiviridae. To the best of our knowledge, this report is the first to describe mycoviruses infecting N. sphaerica.

Molecular characterization of a novel fusarivirus infecting the plant-pathogenic fungus Alternaria solani

A novel mycovirus belonging to the proposed family "Fusariviridae" was discovered in Alternaria solani by sequencing a cDNA corresponding to double-stranded RNA extracted from this phytopathogenic fungus. The virus was tentatively named "Alternaria solani fusarivirus 1" (AsFV1). AsFV1 has a single-stranded positive-sense (+ssRNA) genome of 6845 nucleotides containing three open reading frames (ORFs) and a poly(A) tail. The largest ORF, ORF1, encodes a large polypeptide of 1,556 amino acids (aa) with conserved RNA-dependent RNA polymerase and helicase domains. The ORF2 and ORF3 have overlapping regions, encoding a putative protein of 522 amino acids (aa) and a putative protein of 105 amino acids (aa), respectively, both of unknown function. A multiple sequence alignment and phylogenetic analysis revealed that AsFV1 could be a new member of the "Fusariviridae". This is the first report of the full-length nucleotide sequence of a fusarivirus that infects Alternaria solani.

A novel previously undescribed fusarivirus from the phytopathogenic fungus Setosphaeria turcica

A putative mycovirus belonging to the proposed family "Fusariviridae" was discovered in Setosphaeria turcica by sequencing a double-stranded RNA extracted from this phytopathogenic fungus. The virus was tentatively named "Setosphaeria turcica fusarivirus 1" (StFV1). StFV1 has a genome comprising 6685 nucleotides. The genome contains three open reading frames (ORF). The largest ORF, ORF1, is preceded by an untranslated region (UTR) of 16 nucleotides and separated from ORF2 by an intergenic region of 63 nucleotides. The smallest ORF, ORF3, overlaps ORF2 by 16 nucleotides and is followed by a 3'-UTR of 82 nucleotides. The protein encoded by ORF1 is 71.8%, 67.4% and 68.1% identical to the RNA-dependent RNA polymerases (RdRps) of Pleospora typhicola fusarivirus 1 (PtFV1), Plasmopara viticola lesion-associated fusarivirus 1 (PvlaFV1), and Plasmopara viticola lesion-associated fusarivirus 3 (PvlaFV3), respectively, but has less than 47% amino acid sequence identity to the RdRps of other fusariviruses. To our knowledge, this is the first fusarivirus discovered in S. turcica and the first virus to be identified in this fungus using conventional cloning methods.

Molecular and biological characterization of a novel strain of Alternaria alternata chrysovirus 1 identified from the pathogen Alternaria tenuissima causing watermelon leaf blight

The leaf blight caused by the genus Alternaria is one of the most epidemic diseases on watermelon, and A. tenuissima is the dominant pathogenic species in China. Mycoviruses are found ubiquitously in filamentous fungi, and an increasing number of novel mycoviruses infecting the genus Alternaria have been reported. In this study, a mycovirus from A. tenuissima strain SD-BZF-12 was identified and characterized, whose genome size was very similar with Alternaria alternata chrysovirus 1-N18 (AaCV1-N18). The dsRNA1- and dsRNA2-encoded proteins of the virus had 99 % identities with counterparts of AaCV1-N18; and the dsRNA3- and dsRNA4-encoded proteins of the virus showed the 80 % and 94 % sequence identities with proteins deduced from dsRNA4 and dsRNA3 of AaCV1-N18, respectively. Intriguingly, dsRNA5 of the virus encoded a truncated protein with 68 amino acids (aa) by comparing with 115 aa of AaCV1-N18 dsRNA5. Phylogenetic analysis of RNA-dependent RNA polymerase domain suggested that the virus clustered together with AaCV1-N18. Based on these characteristics, the mycovirus was identified to be a novel strain of AaCV1 and designated as AaCV1-AT1. In addition, no obvious differences were observed on colony morphology between AaCV1-AT1-infected and virus-cured strains of A. tenuissima; however, AaCV1-AT1 infection reduced colony growth rate and spore production ability on host fungus, and increased the median effective concentration of difenoconazole or tebuconazole on its host. This is the first report of AaCV1-AT1 associated with A. tenuissima.

The ORF2 protein of Fusarium graminearum virus 1 suppresses the transcription of FgDICER2 and FgAGO1 to limit host antiviral defences

The filamentous fungus Fusarium graminearum possesses an RNA-interference (RNAi) pathway that acts as a defence response against virus infections and exogenous double-stranded (ds) RNA. Fusarium graminearum virus 1 (FgV1), which infects F. graminearum, confers hypovirulence-associated traits such as reduced mycelial growth, increased pigmentation and reduced pathogenicity. In this study, we found that FgV1 can suppress RNA silencing by interfering with the induction of FgDICER2 and FgAGO1, which are involved in RNAi antiviral defence and the hairpin RNA/RNAi pathway in F. graminearum. In an FgAGO1- or FgDICER2-promoter/GFP-reporter expression assay the green fluorescent protein (GFP) transcript levels were reduced in FgV1-infected transformed mutant strains. By comparing transcription levels of FgDICER2 and FgAGO1 in fungal transformed mutants expressing each open reading frame (ORF) of FgV1 with or without a hairpin RNA construct, we determined that reduction of FgDICER2 and FgAGO1 transcript levels requires only the FgV1 ORF2-encoded protein (pORF2). Moreover, we confirmed that the pORF2 binds to the upstream region of FgDICERs and FgAGOs in vitro. These combined results indicate that the pORF2 of FgV1 counteracts the RNAi defence response of F. graminearum by interfering with the induction of FgDICER2 and FgAGO1 in a promoter-dependent manner.

Novel Victorivirus from a Pakistani Isolate of Alternaria alternata Lacking a Typical Translational Stop/Restart Sequence Signature

The family Totiviridae currently contains five genera Totivirus, Victorivirus, Leishmavirus, Trichomonasvirus, and Giardiavirus. Members in this family generally have a set of two-open reading frame (ORF) elements in their genome with the 5'-proximal ORF (ORF1) encoding a capsid protein (CP) and the 3'-proximal one (ORF2) for RNA-dependent RNA polymerase (RdRp). How the downstream open reading frames (ORFs) are expressed is genus-specific. All victoriviruses characterized thus far appear to use the stop/restart translation mechanism, allowing for the expression of two separate protein products from bicitronic genome-sized viral mRNA, while the totiviruses use a -1 ribosomal frame-shifting that leads to a fusion product of CP and RdRp. We report the biological and molecular characterization of a novel victorivirus termed Alternaria alternata victorivirus 1 (AalVV1) isolated from Alternaria alternata in Pakistan. The phylogenetic and molecular analyses showed AalVV1 to be distinct from previously reported victoriviruses. AalVV1 appears to have a sequence signature required for the -1 frame-shifting at the ORF1/2 junction region, rather than a stop/restart key mediator. By contrast, SDS-polyacrylamide gel electrophoresis and peptide mass fingerprinting analyses of purified virion preparations suggested the expression of two protein products, not a CP-RdRp fusion product. How these proteins are expressed is discussed in this study. Possible effects of infection by this virus were tested in two fungal species: A. alternata and RNA silencing proficient and deficient strains of Cryphonectria parasitica, a model filamentous fungus. AalVV1 showed symptomless infection in all of these fungal strains, even in the RNA silencing deficient C. parasitica strain.

Identification of a Novel Hypovirulence-Inducing Hypovirus From Alternaria alternata

Mycoviruses are wide spread throughout almost all groups of fungi but only a small number of mycoviruses can attenuate the growth and virulence of their fungal hosts. Alternaria alternata is an ascomycete fungus that causes leaf spot diseases on various crop plants. In this study, we identified a novel ssRNA mycovirus infecting an A. alternata f. sp. mali strain isolated from an apple orchard in China. Sequence analyses revealed that this virus is related to hypoviruses, in particular to Wuhan insect virus 14, an unclassified hypovirus identified from insect meta-transcriptomics, as well as other hypoviruses belonging to the genus Hypovirus, and therefore this virus is designed as Alternaria alternata hypovirus 1 (AaHV1). The genome of AaHV1 contains a single large open-reading frame encoding a putative polyprotein (∼479 kDa) with a cysteine proteinase-like and replication-associated domains. Curing AaHV1 from the fungal host strain indicated that the virus is responsible for the slow growth and reduced virulence of the host. AaHV1 defective RNA (D-RNA) with internal deletions emerging during fungal subcultures but the presence of D-RNA does not affect AaHV1 accumulation and pathogenicities. Moreover, AaHV1 could replicate and confer hypovirulence in Botryosphaeria dothidea, a fungal pathogen of apple white rot disease. This finding could facilitate better understanding of A. alternata pathogenicity and is relevant for development of biocontrol methods of fungal diseases.

A Novel Deltaflexivirus that Infects the Plant Fungal Pathogen, Sclerotinia sclerotiorum, Can Be Transmitted Among Host Vegetative Incompatible Strains

Various mycoviruses have been isolated from Sclerotinia sclerotiorum. Here, we identified a viral RNA sequence contig, representing a novel virus, Sclerotinia sclerotiorum deltaflexivirus 2 (SsDFV2), from an RNA_Seq database. We found that SsDFV2 was harbored in the hypovirulent strain, 228, which grew slowly on potato dextrose agar, produced a few sclerotia, and could not induce typical lesions on detached rapeseed (Brassica napus) leaves. Strain 228 was also infected by Botrytis porri RNA Virus 1 (BpRV1), a virus originally isolated from Botrytis porri. The genome of SsDFV2 comprised 6711 nucleotides, excluding the poly (A) tail, and contained a single large predicted open reading frame encoding a putative viral RNA replicase. Phylogenetic analysis demonstrated that SsDFV2 is closely related to viruses in the family Deltaflexiviridae; however, it also differs significantly from members of this family, suggesting that it may represent a new species. Further we determined that SsDFV2 could be efficiently transmitted to host vegetative incompatible individuals by dual culture. To our best knowledge, this is the first report that a (+) ssRNA mycovirus can overcome the transmission limitations of the vegetative incompatibility system, a phenomenon that may facilitate the potential use of mycoviruses for the control of crop fungal diseases.

Two Novel Hypovirulence-Associated Mycoviruses in the Phytopathogenic Fungus Botrytis cinerea: Molecular Characterization and Suppression of Infection Cushion Formation

Botrytis cinerea is a necrotrophic fungus causing disease on many important agricultural crops. Two novel mycoviruses, namely Botrytis cinerea hypovirus 1 (BcHV1) and Botrytis cinerea fusarivirus 1 (BcFV1), were fully sequenced. The genome of BcHV1 is 10,214 nt long excluding a poly-A tail and possesses one large open reading frame (ORF) encoding a polyprotein possessing several conserved domains including RNA-dependent RNA polymerase (RdRp), showing homology to hypovirus-encoded polyproteins. Phylogenetic analysis indicated that BcHV1 may belong to the proposed genus Betahypovirus in the viral family Hypoviridae. The genome of BcFV1 is 8411 nt in length excluding the poly A tail and theoretically processes two major ORFs, namely ORF1 and ORF2. The larger ORF1 encoded polypeptide contains protein domains of an RdRp and a viral helicase, whereas the function of smaller ORF2 remains unknown. The BcFV1 was phylogenetically clustered with other fusariviruses forming an independent branch, indicating BcFV1 was a member in Fusariviridae. Both BcHV1 and BcFV1 were capable of being transmitted horizontally through hyphal anastomosis. Infection by BcHV1 alone caused attenuated virulence without affecting mycelial growth, significantly inhibited infection cushion (IC) formation, and altered expression of several IC-formation-associated genes. However, wound inoculation could fully rescue the virulence phenotype of the BcHV1 infected isolate. These results indicate the BcHV1-associated hypovirulence is caused by the viral influence on IC-formation-associated pathways.

Molecular characterization of a novel mycovirus in Alternaria alternata manifesting two-sided effects: Down-regulation of host growth and up-regulation of host plant pathogenicity

A double-stranded RNA (dsRNA) mycovirus was detected in a strain of Alternaria alternata showing impaired growth phenotypes. The A. alternata strain is the Japanese pear pathotype, which produces a host-specific AK-toxin. Sequence analysis of the viral genome dsRNAs revealed that this mycovirus consists of five dsRNAs and is evolutionarily related to members of the family Chrysoviridae; the virus was named Alternaria alternata chrysovirus 1 (AaCV1). AaCV1-ORF2 protein accumulated in dsRNA-high-titer sub-isolates with severely impaired phenotypes; heterologous AaCV1-ORF2 overexpression in Saccharomyces cerevisiae caused growth inhibition. In contrast to this yeast growth inhibition phenomenon, the dsRNA-high-titer isolates displayed enhanced pathogenicity against Japanese pear plants, in accordance with a 13-fold increase in AK-toxin level in one such isolate. These findings indicated that AaCV1 is a novel mycovirus that exhibits two contrasting effects, impairing growth of the host fungus while rendering the host 'hypervirulent' to the plant.

Novel, diverse RNA viruses from Mediterranean isolates of the phytopathogenic fungus, Rosellinia necatrix: insights into evolutionary biology of fungal viruses

To reveal mycovirus diversity, we conducted a search of as-yet-unexplored Mediterranean isolates of the phytopathogenic ascomycete Rosellinia necatrix for virus infections. Of seventy-nine, eleven fungal isolates tested RNA virus-positive, with many showing coinfections, indicating a virus incidence of 14%, which is slightly lower than that (approximately 20%) previously reported for extensive surveys of over 1000 Japanese R. necatrix isolates. All viral sequences were fully or partially characterized by Sanger and next-generation sequencing. These sequences appear to represent isolates of various new species spanning at least 6 established or previously proposed families such as Partiti-, Hypo-, Megabirna-, Yado-kari-, Fusagra- and Fusarividae, as well as a newly proposed family, Megatotiviridae. This observation greatly expands the diversity of R. necatrix viruses, because no hypo-, fusagra- or megatotiviruses were previously reported from R. necatrix. The sequence analyses showed a rare horizontal gene transfer event of the 2A-like protease domain between a dsRNA (phlegivirus) and a positive-sense, single-stranded RNA virus (hypovirus). Moreover, many of the newly detected viruses showed the closest relation to viruses reported from fungi other than R. necatrix, such as Fusarium spp., which are sympatric to R. necatrix. These combined results imply horizontal virus transfer between these soil-inhabitant fungi.

The alkalophilic fungus Sodiomyces alkalinus hosts beta- and gammapartitiviruses together with a new fusarivirus

Mixed infection by three dsRNA viruses, a novel betapartitivirus, a gammapartitivirus, and a novel fusarivirus, has been identified in four isolates of the obligate alkalophilic fungus Sodiomyces alkalinus. The first, Sodiomyces alkalinus partitivirus 1 (SaPV1), is placed within the genus Betapartitivirus and is related to Ustilaginoidea virens partitivirus 2. The taxonomic position of the second virus is less clear as it shares high (85%) amino acid sequence identity but significantly low (77%) nucleotide sequence identity of the capsid protein with Colletotrichum truncatum partitivirus 1. The third, the novel Sodiomyces alkalinus fusarivirus 1 (SaFV1), is related to Fusarium poae fusarivirus 1. All the viruses show efficient vertical transmission through asexual and sexual spores. These novel coexisting viruses do not evoke apparent phenotypic alteration to their fungal host. This is the first description of a viral infection in an alkalophilic fungus.