Extending chestnut blight hypovirus host range within diaporthales by biolistic delivery of viral cDNA

Fungal Viruses Unveiled: A Comprehensive Review of Mycoviruses

Mycoviruses (viruses of fungi) are ubiquitous throughout the fungal kingdom and are currently classified into 23 viral families and the genus botybirnavirus by the International Committee on the Taxonomy of Viruses (ICTV). The primary focus of mycoviral research has been on mycoviruses that infect plant pathogenic fungi, due to the ability of some to reduce the virulence of their host and thus act as potential biocontrol against these fungi. However, mycoviruses lack extracellular transmission mechanisms and rely on intercellular transmission through the hyphal anastomosis, which impedes successful transmission between different fungal strains. This review provides a comprehensive overview of mycoviruses, including their origins, host range, taxonomic classification into families, effects on their fungal counterparts, and the techniques employed in their discovery. The application of mycoviruses as biocontrol agents of plant pathogenic fungi is also discussed.

Identifying Factors that Determine Effectiveness of Delivery Agents in Biolistic Delivery Using a Library of Amine-Containing Molecules

Biolistic transfection is a popular and versatile tool for plant transformation. A key step in the biolistic process is the binding of DNA to the heavy microprojectile using a delivery agent, usually a positively charged molecule containing amine groups. Currently, the choice of the commercial delivery agent is mostly limited to spermidine. In addition, the detailed delivery mechanism has not been reported. To help broaden the selection of the delivery agent and reveal the fundamental mechanisms that lead to high delivery performance, a library of amine-containing molecules was investigated. A double-barrel biolistic delivery device was utilized for testing hundreds of samples with much-improved consistency. The performance was evaluated on onion epidermis. The binding and release of DNA were measured via direct high-performance liquid chromatography analysis. This study shows that the overwhelming majority of the amine library performed at the same level as spermidine. To further interpret these results, correlations were performed with thousands of molecular descriptors generated by chemical modeling. It was discovered that the overall charge is most likely the key factor to a successful binding and delivery. Furthermore, even after increasing the amount of the DNA concentration 50-fold to stress the binding capacity of the molecules, the amines in the library continued to deliver at a near identical level while binding all the DNA. The increased DNA was also demonstrated with a Cas9 editing test that required a large amount of DNA to be delivered, and the result was consistent with the previously determined amine performance. This study greatly expands the delivery agent selection for biolistic delivery, allowing alternatives to a commercial reagent that are more shelf-stable and cheaper. The library also offers an approach to investigate more challenging delivery of protein and CRISPR-Cas via the biolistic process in the future.

A new tetra-segmented splipalmivirus with divided RdRP domains from Cryphonectria naterciae, a fungus found on chestnut and cork oak trees in Europe

Positive-sense (+), single-stranded (ss) RNA viruses with divided RNA-dependent RNA polymerase (RdRP) domains have been reported from diverse filamentous ascomycetes since 2020. These viruses are termed splipalmiviruses or polynarnaviruses and have been characterized largely at the sequence level, but ill-defined biologically. Cryphonectria naterciae, from which only one virus has been reported, is an ascomycetous fungus potentially plant-pathogenic to chestnut and oak trees. We molecularly characterized multiple viruses in a single Portuguese isolate (C0614) of C. naterciae, taking a metatranscriptomic and conventional double-stranded RNA approach. Among them are a novel splipalmivirus (Cryphonectria naterciae splipalmivirus 1, CnSpV1) and a novel fusagravirus (Cryphonectria naterciae fusagravirus 1, CnFGV1). This study focused on the former virus. CnSpV1 has a tetra-segmented, (+)ssRNA genome (RNA1 to RNA4). As observed for other splipalmiviruses reported in 2020 and 2021, the RdRP domain is separately encoded by RNA1 (motifs F, A and B) and RNA2 (motifs C and D). A hypothetical protein encoded by the 5'-proximal open reading frame of RNA3 shows similarity to a counterpart conserved in some splipalmiviruses. The other RNA3-encoded protein and RNA4-encoded protein show no similarity with known proteins in a blastp search. The tetra-segment nature was confirmed by the conserved terminal sequences of the four CnSpV1 segments (RNA1 to RNA4) and their 100% coexistence in over 100 single conidial isolates tested. The experimental introduction of CnSpV1 along with CnFGV1 into a virus free strain C0754 of C. naterciae vegetatively incompatible with C0614 resulted in no phenotypic alteration, suggesting asymptomatic infection. The protoplast fusion assay indicates a considerably narrow host range of CnSpV1, restricted to the species C. naterciae and C. carpinicola. This study contributes to better understanding of the molecular and biological properties of this unique group of viruses.

Infection of Two Heterologous Mycoviruses Reduces the Virulence of Valsa mali, a Fungal Agent of Apple Valsa Canker Disease

Mycovirus infection has been widely shown to attenuate the virulence of phytopathogenic fungi. Valsa mali is an agriculturally important fungus that causes Valsa canker disease in apple trees. In this study, two unrelated mycoviruses [Cryphonectria hypovirus 1 (CHV1, genus Hypovirus, and single-stranded RNA) and Mycoreovirus 1 (MyRV1, genus Mycoreovirus, double-stranded RNA)] that originated from Cryphonectria parasitica (chestnut blight fungus) were singly or doubly introduced into V. mali via protoplast fusion. CHV1 and MyRV1 stably infected V. mali and caused a reduction in fungal vegetative growth and virulence. Co-infection of both viruses further reduced the virulence of V. mali but compromised the stability of CHV1 infection and horizontal transmission through hyphal anastomosis. Infections of MyRV1 and, to a lesser extent, CHV1 up-regulated the transcript expression of RNA silencing-related genes in V. mali. The accumulation of CHV1 (but not MyRV1) was elevated by the knockdown of dcl2, a key gene of the RNA silencing pathway. Similarly, the accumulation of CHV1 and the efficiency of the horizontal transmission of CHV1 during co-infection was restored by the knockdown of dcl2. Thus, CHV1 and MyRV1 are potential biological control agents for apple Valsa canker disease, but co-infection of both viruses has a negative effect on CHV1 infection in V. mali due to the activation of antiviral RNA silencing by MyRV1 infection.

Cryphonectria nitschkei chrysovirus 1 with unique molecular features and a very narrow host range

Cryphonectria nitschkei chrysovirus 1 (CnCV1), was described earlier from an ascomycetous fungus, Cryphonectria nitschkei strain OB5/11, collected in Japan; its partial sequence was reported a decade ago. Complete sequencing of the four genomic dsRNA segments revealed molecular features similar to but distinct from previously reported members of the family Chrysoviridae. Unique features include the presence of a mini-cistron preceding the major large open reading frame in each genomic segment. Common features include the presence of CAA repeats in the 5'-untranslated regions and conserved terminal sequences. CnCV1-OB5/11 could be laterally transferred to C. nitschkei and its relatives C. radicalis and C. naterciae via coculturing, virion transfection and protoplast fusion, but not to fungal species other than the three species mentioned above, even within the genus Cryphonectria, suggesting a very narrow host range. Phenotypic comparison of a few sets of CnCV1-infected and -free isogenic strains showed symptomless infection in new hosts.

Facilitative and synergistic interactions between fungal and plant viruses

Plants and fungi are closely associated through parasitic or symbiotic relationships in which bidirectional exchanges of cellular contents occur. Recently, a plant virus was shown to be transmitted from a plant to a fungus, but it is unknown whether fungal viruses can also cross host barriers and spread to plants. In this study, we investigated the infectivity of Cryphonectria hypovirus 1 (CHV1, family Hypoviridae), a capsidless, positive-sense (+), single-stranded RNA (ssRNA) fungal virus in a model plant, Nicotiana tabacum CHV1 replicated in mechanically inoculated leaves but did not spread systemically, but coinoculation with an unrelated plant (+)ssRNA virus, tobacco mosaic virus (TMV, family Virgaviridae), or other plant RNA viruses, enabled CHV1 to systemically infect the plant. Likewise, CHV1 systemically infected transgenic plants expressing the TMV movement protein, and coinfection with TMV further enhanced CHV1 accumulation in these plants. Conversely, CHV1 infection increased TMV accumulation when TMV was introduced into a plant pathogenic fungus, Fusarium graminearum In the in planta F. graminearum inoculation experiment, we demonstrated that TMV infection of either the plant or the fungus enabled the horizontal transfer of CHV1 from the fungus to the plant, whereas CHV1 infection enhanced fungal acquisition of TMV. Our results demonstrate two-way facilitative interactions between the plant and fungal viruses that promote cross-kingdom virus infections and suggest the presence of plant-fungal-mediated routes for dissemination of fungal and plant viruses in nature.

Mycoviruses in Biological Control: From Basic Research to Field Implementation

Mycoviruses from plant pathogens can induce hypovirulence (reduced virulence) in their host fungi and have gained considerable attention as potential biocontrol tools. An increasing number of mycoviruses that induce fungal hypovirulence, from a wide variety of taxonomic groups, are currently being reported. Successful application of these viruses in disease management is greatly dependent on their ability to spread in the natural populations of the pathogen. Mycoviruses generally lack extracellular routes of transmission. Hyphal anastomosis is the main route of horizontal mycovirus transmission to other isolates, and conidia of vertical transmission to the progeny. Transmission efficiencies are influenced by both the fungal host and the infecting virus. Interestingly, artificial transfection methods have shown that potential biocontrol mycoviruses often have the ability to infect a variety of fungi. This expands their possible use to the control of pathogens others than those where they were identified. Mycovirus research is also focused on gaining insights into their complex molecular biology and the molecular bases of fungus-virus interactions. This knowledge could be exploited to manipulate the mycovirus and/or the host and generate combinations with enhanced properties in biological control. Finally, when exploring the use of mycoviruses in field conditions, the pathogen life style and the characteristics of the disease and crops affected will deeply impact the specific challenges to overcome, and the development of biocontrol formulations and delivery methods.

Characterization of the Papain-Like Protease p29 of the Hypovirus CHV1-CN280 in Its Natural Host Fungus Cryphonectria parasitica and Nonhost Fungus Magnaporthe oryzae

Cryphonectria hypovirus 1 strain CN280 (CHV1-CN280) was isolated from North China and exhibited typical hypovirulence-associated traits. We previously reported that CHV1-CN280 was more aggressive and had a higher horizontal transmission ability between Cryphonectria parasitica isolates belonging to different vegetative compatibility groups than two other CHV1 hypoviruses (namely, CHV1-EP713 and CHV1-Euro7), thus displaying greater potential for biological control of chestnut blight. The genome sequence of CHV1-CN280 shared approximately 70% identity with three other hypoviruses (CHV1-EP713, CHV1-Euro7, and CHV1-EP721). The coding region for p29, a papain-like protease encoded by CHV1-CN280 hypovirus, displayed an average of only approximately 60% amino acid identity among them, while the identity between the other three CHV1 isolates was higher than 89%. Protease p29 acted as a virus-encoded determinant responsible for altering fungal host phenotypes in other CHV1 isolates. In this study, the impacts of CHV1-CN280 p29 expression in virus-free C. parasitica were investigated. CHV1-CN280 p29 expression in C. parasitica resulted in significantly reduced sporulation, pigmentation, extracellular laccase activities, and pathogenicity, which is consistent with previous investigations. Subsequently, the potential of CHV1-CN280 p29 as a viral determinant responsible for suppression of host phenotypes in other phytopathogenic fungi such as Magnaporthe oryzae, the causal agent of rice blast disease, was discussed. However, heterologous expression of p29 in M. oryzae induced the opposite effect on sporulation, extracellular laccase activities, and pathogenicity; had no significant effect on pigmentation and mycelial growth; and contributed to extracellular peroxidase activities, suggesting that CHV1-CN280 p29 may disturb a unique regulatory pathway in C. parasitica, rather than a basic regulatory pathway conserved in diverse range of fungi. Alternatively, CHV1-CN280 p29-mediated modulation of fungal phenotypes may be facilitated by the specific interaction between p29 and a special fungal-host component, which exists only with C. parasitica but not M. oryzae.

Investigation of Host Range of and Host Defense against a Mitochondrially Replicating Mitovirus

Mitoviruses (genus Mitovirus, family Narnaviridae) are mitochondrially replicating viruses that have the simplest positive-sense RNA genomes of 2.2 to 4.4 kb with a single open reading frame (ORF) encoding an RNA-dependent RNA polymerase. Cryphonectria parasitica mitovirus 1 (CpMV1) from U.S. strain NB631 of the chestnut blight fungus, Cryphonectria parasitica, was the first virus identified as a mitochondrially replicating virus. Despite subsequent discovery of many other mitoviruses from diverse fungi, no great advances in understanding mitovirus biology have emerged, partly because of the lack of inoculation methods. Here we developed a protoplast fusion-based protocol for horizontal transmission of CpMV1 that entailed fusion of recipient and donor protoplasts, hyphal anastomosis, and single-conidium isolation. This method allowed expansion of the host range to many other C. parasitica strains. Species within and outside the family Cryphonectriaceae, Cryphonectria radicalis and Valsa ceratosperma, also supported the replication of CpMV1 at a level comparable to that in the natural host. No stable maintenance of CpMV1 was observed in Helminthosporium victoriae PCR-based haplotyping of virus-infected fungal strains confirmed the recipient mitochondrial genetic background. Phenotypic comparison between CpMV1-free and -infected isogenic strains revealed no overt effects of the virus. Taking advantage of the infectivity to the standard strain C. parasitica EP155, accumulation levels were compared among antiviral RNA silencing-proficient and -deficient strains in the EP155 background. Comparable accumulation levels were observed among these strains, suggesting the avoidance of antiviral RNA silencing by CpMV1, which is consistent with its mitochondrial replication. Collectively, the results of study provide a foundation to further explore the biology of mitoviruses.IMPORTANCE Capsidless mitoviruses, which are ubiquitously detected in filamentous fungi, have the simplest RNA genomes of 2.2 to 4.4 kb, encoding only RNA-dependent RNA polymerase. Despite their simple genomes, detailed biological characterization of mitoviruses has been hampered by their mitochondrial location within the cell, posing challenges to their experimental introduction and study. Here we developed a protoplast fusion-based protocol for horizontal transfer of the prototype mitovirus, Cryphonectria parasitica mitovirus 1 (CpMV1), which was isolated from strain NB631 of the chestnut blight fungus (Cryphonectria parasitica), a model filamentous fungus for studying virus-host interactions. The host range of CpMV1 has been expanded to many different strains of C. parasitica and different fungal species within and outside the Cryphonectriaceae. Comparison of CpMV1 accumulation among various RNA silencing-deficient and -competent strains showed clearly that the virus was unaffected by RNA silencing. This study provides a solid foundation for further exploration of mitovirus-host interactions.

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.

Transmission of Penicillium aurantiogriseum partiti-like virus 1 to a new fungal host (Cryphonectria parasitica) confers higher resistance to salinity and reveals adaptive genomic changes

We attempted to transfect six recently characterized virus species to protoplasts of Penicillium janczewskii and Chryphonectria parasitica. None of the recovered P. janczewskii colonies was positive for the transfected viruses, but Penicillium aurantiogriseum partiti-like virus 1 (PaPLV1) was detected in three distinct regenerated C. parasitica colonies. We screened the phenotype of the infected strains in up to 45 different conditions combining different media, salinity and temperatures: our results show that the infected strains grow slower than the virus- free in most of the tested conditions with the exception of halophilic stress in a specific nutrient combination media. We proceeded to characterize molecularly the population of distinct isolates of PaPLV1 infected C. parasitica through RNAseq: comparison to the viral population present in the original host - P. auratiogriseum - showed that two isolates accumulated non-synonymous mutations suggesting adaptation to the new host. RNAseq analyses identified a second genomic RNA segment and northern blot of RNA extracted from purified virus suspensions allowed establishing that PaPLV1 is at least bipartite in nature and that it forms isometric virions of circa 36-38 nm in diameter. In light of these new acquisitions, we discuss the taxonomic placement of PaPLV1 inside the Partitiviridae.

Mycoviruses of an endophytic fungus can replicate in plant cells: evolutionary implications

So far there is no record of a specific virus able to infect both fungal and plant hosts in nature. However, experimental evidence shows that some plant virus RdRPs are able to perform replication in trans of genomic or DI RNAs in the yeast Saccharomyces cerevisiae. Furthermore, tobacco mosaic virus was recently shown to replicate in a filamentous ascomycetous fungus. Thus, at least experimentally, some plant viruses can infect some fungi. Endophytic fungi have been reported from many plants and several of these fungi have been shown to contain viruses. Here we tested if mycoviruses derived from a marine plant endophyte can replicate in plant cells. For this purpose, we used partially purified viral particles from isolate MUT4330 of Penicillium aurantiogriseum var. viridicatum which harbors six virus species, some having dsRNA and some positive-strand ssRNA genomes. These were transfected into three distinct plant protoplast cell systems. Time-course analysis of absolute RNA accumulation provided for the first time evidence that viruses of two species belonging to the Partitiviridae and Totiviridae families, can replicate in plant cells without evidence of host adaptation, i.e, changes in their nucleotide sequence.

SAGA complex mediates the transcriptional up-regulation of antiviral RNA silencing

Pathogen recognition and transcriptional activation of defense-related genes are crucial steps in cellular defense responses. RNA silencing (RNAi) functions as an antiviral defense in eukaryotic organisms. Several RNAi-related genes are known to be transcriptionally up-regulated upon virus infection in some host organisms, but little is known about their induction mechanism. A phytopathogenic ascomycete, Cryphonectria parasitica (chestnut blight fungus), provides a particularly advantageous system to study RNAi activation, because its infection by certain RNA viruses induces the transcription of dicer-like 2 (dcl2) and argonaute-like 2 (agl2), two major RNAi players. To identify cellular factors governing activation of antiviral RNAi in C. parasitica, we developed a screening protocol entailing multiple transformations of the fungus with cDNA of a hypovirus mutant lacking the RNAi suppressor (CHV1-Δp69), a reporter construct with a GFP gene driven by the dcl2 promoter, and a random mutagenic construct. Screening for GFP-negative colonies allowed the identification of sgf73, a component of the SAGA (Spt-Ada-Gcn5 acetyltransferase) complex, a well-known transcriptional coactivator. Knockout of other SAGA components showed that the histone acetyltransferase module regulates transcriptional induction of dcl2 and agl2, whereas histone deubiquitinase mediates regulation of agl2 but not dcl2 Interestingly, full-scale induction of agl2 and dcl2 by CHV1-Δp69 required both DCL2 and AGL2, whereas that by another RNA virus, mycoreovirus 1, required only DCL2, uncovering additional roles for DCL2 and AGL2 in viral recognition and/or RNAi activation. Overall, these results provide insight into the mechanism of RNAi activation.

A novel partitivirus that confers hypovirulence on plant pathogenic fungi

Members of the family Partitiviridae have bisegmented double-stranded RNA (dsRNA) genomes and are not generally known to cause obvious symptoms in their natural hosts. An unusual partitivirus, Sclerotinia sclerotiorum partitivirus 1 (SsPV1/WF-1), conferred hypovirulence on its natural plant-pathogenic fungal host, Sclerotinia sclerotiorum strain WF-1. Cellular organelles, including mitochondria, were severely damaged. Hypovirulence and associated traits of strain WF-1 and SsPV1/WF-1 were readily cotransmitted horizontally via hyphal contact to different vegetative compatibility groups of S. sclerotiorum and interspecifically to Sclerotinia nivalis and Sclerotinia minor. S. sclerotiorum strain 1980 transfected with purified SsPV1/WF-1 virions also exhibited hypovirulence and associated traits similar to those of strain WF-1. Moreover, introduction of purified SsPV1/WF-1 virions into strain KY-1 of Botrytis cinerea also resulted in reductions in virulence and mycelial growth and, unexpectedly, enhanced conidial production. However, virus infection suppressed hyphal growth of most germinating conidia of B. cinerea and was eventually lethal to infected hyphae, since very few new colonies could develop following germ tube formation. Taken together, our results support the conclusion that SsPV1/WF-1 causes hypovirulence in Sclerotinia spp. and B. cinerea. Cryo-EM (cryo-electron microscopy) reconstruction of the SsPV1 particle shows that it has a distinct structure with similarity to the closely related partitiviruses Fusarium poae virus 1 and Penicillium stoloniferum virus F. These findings provide new insights into partitivirus biological activities and clues about molecular interactions between partitiviruses and their hosts.

IMPORTANCE

Members of the Partitiviridae are believed to occur commonly in their phytopathogenic fungal and plant hosts. However, most partitiviruses examined so far appear to be associated with latent infections. Here we report a partitivirus, SsPV1/WF-1, that was isolated from a hypovirulent strain of Sclerotinia sclerotiorum and describe its biological and molecular features. We have demonstrated that SsPV1 confers hypovirulence. Furthermore, SsPV1 can infect and cause hypovirulence in Botrytis cinerea. Our study also suggests that SsPV1 has a vigorous ability to proliferate and spread via hyphal contact. SsPV1 can overcome vegetative incompatibility barriers and can be transmitted horizontally among different vegetative compatibility groups of S. sclerotiorum, even interspecifically. Cryo-EM reconstruction of SsPV1 shows that it has a distinct structure with similarity to closely related partitiviruses. Our studies exploit a novel system, SsPV1 and its hosts, which can provide the means to explore the mechanisms by which partitiviruses interact with their hosts.

Biological properties and expression strategy of rosellinia necatrix megabirnavirus 1 analysed in an experimental host, Cryphonectria parasitica

Rosellinia necatrix megabirnavirus 1 (RnMBV1) with a bipartite dsRNA genome (dsRNA1 and dsRNA2) confers hypovirulence to its natural host, the white root rot fungus, and is thus regarded as a potential virocontrol (biocontrol) agent. Each segment has two large ORFs: ORF1 and partially overlapping ORF2 on dsRNA1 encode the major capsid protein (CP) and RNA-dependent RNA polymerase (RdRp), whilst ORF3 and ORF4 on dsRNA2 encode polypeptides with unknown functions. Here, we report the biological and molecular characterization of this virus in the chestnut blight fungus, Cryphonectria parasitica, a filamentous fungus that has been used as a model for mycovirus research. Transfection with purified RnMBV1 particles into an RNA-silencing-defective strain (Δdcl-2) of C. parasitica and subsequent anastomosis with the WT strain (EP155) resulted in stable persistent infection in both host strains. However, accumulation levels in the two strains were different, being ~20-fold higher in Δdcl-2 than in EP155. Intriguingly, whilst RnMBV1 reduced both virulence and growth rate in Δdcl-2, it attenuated virulence without affecting significantly other traits in EP155. Western blot analysis using antiserum against recombinant proteins encoded by either ORF1 or ORF2 demonstrated the presence of a 250 kDa protein in purified virion preparations, suggesting that RdRp is expressed as a CP fusion product via a −1 frameshift. Antiserum against the ORF3-encoded protein allowed the detection of 150, 30 and 23 kDa polypeptides specifically in RnMBV1-infected mycelia. Some properties of an RnMBV1 mutant with genome rearrangements, which occurred after transfection of Δdcl-2 and EP155, were also presented. This study provides an additional example of C. parasitica serving as a versatile, heterologous fungus for exploring virus–host interactions and virus gene expression strategies.

New insights into mycoviruses and exploration for the biological control of crop fungal diseases

Mycoviruses are viruses that infect fungi. A growing number of novel mycoviruses have expanded our knowledge of virology, particularly in taxonomy, ecology, and evolution. Recent progress in the study of mycoviruses has comprehensively improved our understanding of the properties of mycoviruses and has strengthened our confidence to explore hypovirulence-associated mycoviruses that control crop diseases. In this review, the advantages of using hypovirulence-associated mycoviruses to control crop diseases are discussed, and, as an example, the potential for Sclerotinia sclerotiorum hypovirulence-associated DNA virus 1 (SsHADV-1) to control the stem rot of rapeseed (Brassica napus) is also introduced. Fungal vegetative incompatibility is likely to be the key factor that limits the wide utilization of mycoviruses to control crop diseases; however, there are suggested strategies for resolving this problem.

Hypovirus molecular biology: from Koch's postulates to host self-recognition genes that restrict virus transmission

The idea that viruses can be used to control fungal diseases has been a driving force in mycovirus research since the earliest days. Viruses in the family Hypoviridae associated with reduced virulence (hypovirulence) of the chestnut blight fungus, Cryphonectria parasitica, have held a prominent place in this research. This has been due in part to the severity of the chestnut blight epidemics in North America and Europe and early reports of hypovirulence-mediated mitigation of disease in European forests and successful application for control of chestnut blight in chestnut orchards. A more recent contributing factor has been the development of a hypovirus/C. parasitica experimental system that has overcome many of the challenges associated with mycovirus research, stemming primarily from the exclusive intracellular lifestyle shared by all mycoviruses. This chapter will focus on hypovirus molecular biology with an emphasis on the development of the hypovirus/C. parasitica experimental system and its contributions to fundamental and practical advances in mycovirology and the broader understanding of virus-host interactions and fungal pathogenesis.

Viruses of the plant pathogenic fungus Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a notorious plant fungal pathogen with a broad host range including many important crops, such as oilseed rape, soybean, and numerous vegetable crops. Hypovirulence-associated mycoviruses have attracted much attention because of their potential as biological control agents for combating plant fungal diseases and for use in fundamental studies on fungal pathogenicity and other properties. This chapter describes several mycoviruses that were isolated from hypovirulent strains except for strain Sunf-M, which has a normal phenotype. These viruses include the geminivirus-like mycovirus Sclerotinia sclerotiorum hypovirulence-associated DNA virus 1 (SsHADV-1), Sclerotinia debilitation-associated RNA virus (SsDRV), Sclerotinia sclerotiorum RNA virus L (SsRV-L), Sclerotinia sclerotiorum hypovirus 1 (SsHV-1), Sclerotinia sclerotiorum mitoviruses 1 and 2 (SsMV-1, SsMV-2), and Sclerotinia sclerotiorum partitivirus S (SsPV-S). Unlike many other fungi, incidences of mixed infections with two or more mycoviruses in S. sclerotiorum are particularly high and very common. The interaction between SsDRV and S. sclerotiorum is likely to be unique. The significance of these mycoviruses to fungal ecology and viral evolution and the potential for biological control of Sclerotinia diseases using mycoviruses are discussed.

Viruses of botrytis

Botrytis cinerea (gray mold) is one of the most widespread and destructive fungal diseases of horticultural crops. Propagation and dispersal is usually by asexual conidia but the sexual stage (Botryotinia fuckeliana (de Bary) Whetzel) also occurs in nature. DsRNAs, indicative of virus infection, are common in B. cinerea, but only four viruses (Botrytis virus F (BVF), Botrytis virus X (BVX), Botrytis cinerea mitovirus 1 (BcMV1), and Botrytis porri RNA virus) have been sequenced. BVF and BVX are unusual mycoviruses being ssRNA flexous rods and have been designated the type species of the genera Mycoflexivirus and Botrexvirus (family Betaflexivirdae), respectively. The reported effects of viruses on Botrytis range from negligible to severe, with Botrytis cinerea mitovirus 1 causing hypovirulence. Little is currently known about the effects of viruses on Botrytis metabolism but recent complete sequencing of the B. cinerea genome now provides an opportunity to investigate the host-pathogen interactions at the molecular level. There is interest in the possible use of mycoviruses as biological controls for Botrytis because of the common problem of fungicide resistance. Unfortunately, hyphal anastomosis is the only known mechanism of horizontal virus transmission and the large number of vegetative incompatibility groups in Botrytis is a potential constraint on the spread of an introduced virus. Although some Botrytis viruses, such as BVF and BVX, are known to have international distribution, there is a distinct lack of epidemiological data and the means of spread are unknown.

A novel victorivirus from a phytopathogenic fungus, Rosellinia necatrix, is infectious as particles and targeted by RNA silencing

A novel victorivirus, termed Rosellinia necatrix victorivirus 1 (RnVV1), was isolated from a plant pathogenic ascomycete, white root rot fungus Rosellinia necatrix, coinfected with a partitivirus. The virus was molecularly and biologically characterized using the natural and experimental hosts (chestnut blight fungus, Cryphonectria parasitica). RnVV1 was shown to have typical molecular victorivirus attributes, including a monopartite double-stranded RNA genome with two open reading frames (ORFs) encoding capsid protein (CP) and RNA-dependent RNA polymerase (RdRp), a UAAUG pentamer presumed to facilitate the coupled termination/reinitiation for translation of the two ORFs, a spherical particle structure ~40 nm in diameter, and moderate levels of CP and RdRp sequence identity (34 to 58%) to those of members of the genus Victorivirus within the family Totiviridae. A reproducible transfection system with purified RnVV1 virions was developed for the two distinct fungal hosts. Transfection assay with purified RnVV1 virions combined with virus elimination by hyphal tipping showed that the effects of RnVV1 on the phenotype of the natural host were negligible. Interestingly, comparison of the RNA silencing-competent (standard strain EP155) and -defective (Δdcl-2) strains of C. parasitica infected with RnVV1 showed that RNA silencing acted against the virus to repress its replication, which was restored by coinfection with hypovirus or transgenic expression of an RNA silencing suppressor, hypovirus p29. Phenotypic changes were observed in the Δdcl-2 strain but not in EP155. This is the first reported study on the host range expansion of a Totiviridae member that is targeted by RNA silencing.

Viruses of the white root rot fungus, Rosellinia necatrix

Rosellinia necatrix is a filamentous ascomycete that is pathogenic to a wide range of perennial plants worldwide. An extensive search for double-stranded RNA of a large collection of field isolates led to the detection of a variety of viruses. Since the first identification of a reovirus in this fungus in 2002, several novel viruses have been molecularly characterized that include members of at least five virus families. While some cause phenotypic alterations, many others show latent infections. Viruses attenuating the virulence of a host fungus to its plant hosts attract much attention as agents for virocontrol (biological control using viruses) of the fungus, one of which is currently being tested in experimental fields. Like the Cryphonectria parasitica/viruses, the R. necatrix/viruses have emerged as an amenable system for studying virus/host and virus/virus interactions. Several techniques have recently been developed that enhance the investigation of virus etiology, replication, and symptom induction in this mycovirus/fungal host system.

[Mycoviruses and virocontrol]

Viruses are widespread in all major groups of fungi. The transmission of fungal viruses occurs intracellularly during cell division, sporogenesis, and cell fusion. They apparently lack an extracellular route for infection. Recent searches of the collections of field fungal isolates have detected an increasing number of novel viruses and lead to discoveries of novel genome organizations, expression strategies and virion structures. Those findings enhanced our understanding of virus diversity and evolution. The majority of fungal viruses have dsRNA genomes packaged in spherical particles, while ssRNA mycoviruses, possessing or lacking the ability to form particles, have increasingly been reported. This review article discusses the current status of mycovirus studies and virocontrol (biocontrol) of phytopathogenic fungi using viruses that infect them and reduce their virulence. Selected examples of virocontrol-associated systems include the chestnut/chestnut blight/hypovirus and fruit trees/white root rot fungus/mycoviruses. Natural dissemination and artificial introduction of hypovirulent fungal strains efficiently contributed to virocontrol of chestnut blight in European forests. Attempts to control white root rot with hypovirulence-conferring mycoviruses are now being made in Japan.

A novel mycovirus closely related to hypoviruses that infects the plant pathogenic fungus Sclerotinia sclerotiorum

Three dsRNA segments, two similarly sized at 9.5kbp and a third one of approximately 3.6kbp, were extracted from a hypovirulent strain SZ-150 of Sclerotinia sclerotiorum. The complete cDNA sequence of one of the two large dsRNA segment (10398bp, excluding the poly (A) tail) reveals a single ORF that encodes a polyprotein with conserved domains of putative papain-like protease, UDP glucose/sterol glycosyltransferase, RNA-dependent RNA polymerase and viral RNA Helicase. This virus is closely related to Cryphonectria hypovirus (CHV) 3/GH2 and CHV4/SR2 in the family Hypoviridae and designated as Sclerotinia sclerotiorum hypovirus 1 (SsHV1/SZ-150). The satellite-like 3.6kbp dsRNA segment (S-dsRNA) shares high sequence identity with the 5'-UTR of SsHV1/SZ-150. SsHV1/SZ-150 alone is not the primary causal agent for hypovirulence of strain SZ-150 since strains without the S-dsRNA show normal phenotype. This is the first report of a naturally occurring hypovirus that infects a fungus other than Cryphonectria parasitica.

Extending the fungal host range of a partitivirus and a mycoreovirus from Rosellinia necatrix by inoculation of protoplasts with virus particles

The potential host range of mycoviruses is poorly understood because of the lack of suitable inoculation methods. Recently, successful transfection has been reported for somatically incompatible fungal isolates with purified virus particles of two mycoviruses, the partitivirus RnPV1-W8 (RnPV1) and the mycoreovirus RnMyRV3/W370 (MyRV3), from the white root rot fungus Rosellinia necatrix (class Sordariomycetes, subclass Xylariomycetidae). These studies examined and revealed the effect of the mycoviruses on growth and pathogenicity of R. necatrix. Here, we extended the experimental host range of these two mycoviruses using a transfection approach. Protoplasts of other phytopathogenic Sordariomycetous fungi-Diaporthe sp., Cryphonectria parasitica, Valsa ceratosperma (Sordariomycetidae), and Glomerella cingulata (Hypocreomycetidae)-were inoculated with RnPV1 and MyRV3 viral particles. The presence of double-stranded RNA viral genomes in regenerated mycelia of Diaporthe sp., C. parasitica, and V. ceratosperma confirmed both types of viral infections in these three novel host species. An established RnPV1 infection was confirmed in G. cingulata but MyRV3 did not infect this host. Horizontal transmission of both viruses from newly infected strains to virus-free, wild-type strains through hyphal anastomosis was readily achieved by dual culture; however, vertical transmission through conidia was rarely observed. The virulence of Diaporthe sp., C. parasitica, and V. ceratosperma strains harboring MyRV3 was reduced compared with their virus-free counterpart. In summary, our protoplast inoculation method extended the experimental host range of RnPV1-W8 and MyRV3 within the class Sordariomycetes and revealed that MyRV3 confers hypovirulence to the new hosts, as it does to R. necatrix.

Infection of Rosellinia necatrix with purified viral particles of a member of Partitiviridae (RnPV1-W8)

Isolate W8 of the white root rot fungus, Rosellinia necatrix, harbors three dsRNA segments, L1-, L2- and M-dsRNAs, and showed an irregular colony margin, slow growth, and moderate virulence. The M-dsRNA was previously shown to be the genome of a partitivirus, RnPV1-W8. Here a transfection protocol was developed for RnPV1-W8. Protoplasts of two virus-free isolates of R. necatrix were inoculated with purified viral particles using a polyethylene glycol-mediated method. Virus infection was confirmed by electrophoresis and Northern analysis. RnPV1-W8 introduced into the new host isolates was transmissible via hyphal anastomosis. However, the infection had no effect on the morphology and virulence of infected isolates of R. necatrix. This is the first report on the transfection of a partitivirus for R. necatrix.

Hypovirulence: mycoviruses at the fungal-plant interface

Whereas most mycoviruses lead 'secret lives', some reduce the ability of their fungal hosts to cause disease in plants. This property, known as hypovirulence, has attracted attention owing to the importance of fungal diseases in agriculture and the limited strategies that are available for the control of these diseases. Using one pathogen to control another is appealing, both intellectually and ecologically. The recent development of an infectious cDNA-based reverse genetics system for members of the Hypoviridae mycovirus family has enabled the analysis of basic aspects of this fascinating virus-fungus-plant interaction, including virus-host interactions, the mechanisms underlying fungal pathogenesis, fungal signalling pathways and the evolution of RNA silencing. Such systems also provide a means for engineering mycoviruses for enhanced biocontrol potential.

Biological control of chestnut blight with hypovirulence: a critical analysis

Most hypovirulence in the chestnut blight fungus, Cryphonectria parasitica, is associated with infection by fungal viruses in the family Hypoviridae. Hypovirulence has controlled chestnut blight well in some locations in Europe and in Michigan in the United States. In contrast, with few exceptions, biological control has failed almost completely in eastern North America. Therapeutic treatment of individual cankers is successful in most cases, but the success of hypovirulence at the population level depends on the natural spread of viruses. Characteristics of three interacting trophic levels (virus, fungus, and tree), plus the environment, determine the success or failure of hypovirulence. Vegetative incompatibility restricts virus transmission, but this factor alone is a poor predictor of biological control. Any factor reducing the rate of chestnut blight epidemics enhances hypovirus invasion. Overall, however, not enough is understood about the epidemiological dynamics of this system to determine the crucial factors regulating the establishment of hypovirulence in chestnut forests.

Hypovirulence-Associated Double-Stranded RNA from Sclerotinia homoeocarpa Is Conspecific with Ophiostoma novo-ulmi Mitovirus 3a-Ld

ABSTRACT The nucleotide sequence of the hypovirulence-associated double-stranded RNA (dsRNA) in hypovirulent isolate Sh12B of Sclerotinia homoeocarpa, the causal agent of dollar spot of turf grass, was determined. This large dsRNA (L-dsRNA) is 2,632 bp long and is A and U rich (61.0% A+U residues). One strand of this dsRNA contains an open reading frame (ORF) with the potential to encode a protein of 720 amino acids. This ORF contains 12 UGA codons, predicted to encode tryptophan in ascomycete mitochondria, and has a codon bias typical of mitochondrial genes, which is consistent with a mitochondrial localization of this dsRNA. The amino acid sequence contains conserved motifs typical of RNA-dependent RNA polymerases (RdRps). Sequence analyses of the nucleotide and RdRp-like protein revealed that the L-dsRNA is homologous with previously characterized mitochondrial viruses and dsRNAs from other phytopathogenic fungi, and shares 92.4% nucleotide and 95.1% amino acid sequence identities with the Ophiostoma novo-ulmi mitovirus 3a-Ld from Ophiostoma novo-ulmi, the causal agent of Dutch elm disease. The results indicate that these two dsRNAs are conspecific. This is the first report that a hypovirulence-associated dsRNA virus naturally occurs in two taxonomically distinct fungi, and indicates that horizontal transmission of this dsRNA virus may have occurred between these fungi.