Chestnut Blight: Biological Control by Transmissible Hypovirulence in Endothia parasitica

Antarctica's hidden mycoviral treasures in fungi isolated from mosses: A first genomic approach

This study investigates the presence of mycoviruses in Antarctic fungi and elucidates their evolutionary relationships. To achieve this, we aligned mycoviral gene sequences with genomes of previously sequenced Antarctic endophytic fungi, made available by our research group and accessible via Joint Genome Institute. Our findings reveal that the most prevalent genetic regions in all endophytic fungi are homologous to Partitiviruses, Baculoviridae, and Phycodnaviridae. These regions display evidence of positive selection pressure, suggesting genetic diversity and the accumulation of nonsynonymous mutations. This phenomenon implies a crucial role for these regions in the adaptation and survival of these fungi in the challenging Antarctic ecosystems. The presence of mycoviruses in Antarctic endophytic fungi may indicate shared survival strategies between the virus and its host, shedding light on their evolutionary dynamics. This study underscores the significance of exploring mycoviruses within endophytic fungi and their contributions to genetic diversity. Future research avenues could delve into the functional implications of these conserved mycoviral genetic regions in Antarctic endophytic fungi, providing a comprehensive understanding of this intriguing association and genomic retention of viral region in fungi.

Exploring the Mycovirus Sclerotinia sclerotiorum Hypovirulence-Associated DNA Virus 1 as a Biocontrol Agent of White Mold Caused by Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a necrotrophic fungal pathogen causing white mold on many important economic crops. Recently, some mycoviruses such as S. sclerotiorum hypovirulence-associated DNA virus 1 (SsHADV-1) converted S. sclerotiorum into a beneficial symbiont that helps plants manage pathogens and other stresses. To explore the potential use of SsHADV-1 as a biocontrol agent in the United States and to test the efficacy of SsHADV-1-infected United States isolates in managing white mold and other crop diseases, SsHADV-1 was transferred from the Chinese strain DT-8 to United States isolates of S. sclerotiorum. SsHADV-1 is readily transmitted horizontally among United States isolates of S. sclerotiorum and consistently conferred hypovirulence to its host strains. Biopriming of dry bean seeds with hypovirulent S. sclerotiorum strains enhanced resistance to white mold, gray mold, and Rhizoctonia root rot. To investigate the underlying mechanisms, endophytic growth of hypovirulent S. sclerotiorum in dry beans was confirmed using PCR, and the expression of 12 plant defense-related genes were monitored before and after infection. The results indicated that the endophytic growth of SsHADV-1-infected strains in plants stimulated the expression of plant immunity pathway genes that assisted a rapid response from the plant to fungal infection. Finally, application of the seed biopriming technology with SsHADV-1-infected hypervirulent strain has promise for the biological control of several diseases of wheat, pea, and sunflower.

Targeting wheat fusarium head blight with mycovirus-mediated VIGS

Fusarium head blight (FHB) caused by Fusarium graminearum occurs in wheat (Triticum aestivum) and threatens food production worldwide. Wheat lacks broad, durable FHB resistance. However, Zhang et al. developed a mycovirus-based virus-induced gene-silencing system in F. graminearum, providing efficient biocontrol of this devastating fungal disease.

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

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

Mycoviral gene-incorporating phytopathogenic fungi: a biocontrol agent

Fungal pathogens cause devastating agroeconomic losses. Chemical fungicides are used to control fungal diseases, although this is not an ecofriendly approach. A recent study by Liu et al. highlighted the use of mycoviral gene-incorporating phytopathogenic fungi as biocontrol agents for disease management.

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.

Hypoviral-regulated HSP90 co-chaperone p23 (CpCop23) determines the colony morphology, virulence, and viral response of chestnut blight fungus Cryphonectria parasitica

We previously identified a protein spot that showed down-regulation in the presence of Cryphonectria hypovirus 1 (CHV1) and tannic acid supplementation as a Hsp90 co-chaperone p23 gene (CpCop23). The CpCop23-null mutant strain showed retarded growth with less aerial mycelia and intense pigmentation. Conidia of the CpCop23-null mutant were significantly decreased and their viability was dramatically diminished. The CpCop23-null mutant showed hypersensitivity to Hsp90 inhibitors. However, no differences in responsiveness were observed after exposure to other stressors such as temperature, reactive oxygen species, and high osmosis, the exception being cell wall-disturbing agents. A severe reduction in virulence was observed in the CpCop23-null mutant. Interestingly, viral transfer to the CpCop23-null mutant from CHV1-infected strain via anastomosis was more inefficient than a comparable transfer with the wild type as a result of decreased hyphal branching of the CpCop23-null mutant around the peripheral region, which resulted in less fusion of the hyphae. The CHV1-infected CpCop23-null mutant exhibited recovered mycelial growth with less pigmentation and sporulation. The CHV1-transfected CpCop23-null mutant demonstrated almost no virulence, that is, even less than that of the CHV1-infected wild type (UEP1), a further indication that reduced virulence of the mutant is not attributable exclusively to the retarded growth but rather is a function of the CpCop23 gene. Thus, this study indicates that CpCop23 plays a role in ensuring appropriate mycelial growth and development, spore viability, responses to antifungal drugs, and fungal virulence. Moreover, the CpCop23 gene acts as a host factor that affects CHV1-infected fungal growth and maintains viral symptom development.

New Insights on the Integrated Management of Plant Diseases by RNA Strategies: Mycoviruses and RNA Interference

RNA-based strategies for plant disease management offer an attractive alternative to agrochemicals that negatively impact human and ecosystem health and lead to pathogen resistance. There has been recent interest in using mycoviruses for fungal disease control after it was discovered that some cause hypovirulence in fungal pathogens, which refers to a decline in the ability of a pathogen to cause disease. Cryphonectria parasitica, the causal agent of chestnut blight, has set an ideal model of management through the release of hypovirulent strains. However, mycovirus-based management of plant diseases is still restricted by limited approaches to search for viruses causing hypovirulence and the lack of protocols allowing effective and systemic virus infection in pathogens. RNA interference (RNAi), the eukaryotic cell system that recognizes RNA sequences and specifically degrades them, represents a promising. RNA-based disease management method. The natural occurrence of cross-kingdom RNAi provides a basis for host-induced gene silencing, while the ability of most pathogens to uptake exogenous small RNAs enables the use of spray-induced gene silencing techniques. This review describes the mechanisms behind and the potential of two RNA-based strategies, mycoviruses and RNAi, for plant disease management. Successful applications are discussed, as well as the research gaps and limitations that remain to be addressed.

A Satellite dsRNA Attenuates the Induction of Helper Virus-Mediated Symptoms in Aspergillus flavus

Aspergillus flavus is an important fungal pathogen of animals and plants. Previously, we reported a novel partitivirus, Aspergillus flavus partitivirus 1 (AfPV1), infecting A. flavus. In this study, we obtained a small double-stranded (ds) RNA segment (734 bp), which is a satellite RNA of the helper virus, AfPV1. The presence of AfPV1 altered the colony morphology, decreased the number of conidiophores, created significantly larger vacuoles, and caused more sensitivity to osmotic, oxidative, and UV stresses in A. flavus, but the small RNA segment could attenuate the above symptoms caused by the helper virus AfPV1 in A. flavus. Moreover, AfPV1 infection reduced the pathogenicity of A. flavus in corn (Zea mays), honeycomb moth (Galleria mellonella), mice (Mus musculus), and the adhesion of conidia to host epithelial cells, and increased conidial death by macrophages. However, the small RNA segment could also attenuate the above symptoms caused by the helper virus AfPV1 in A. flavus, perhaps by reducing the genomic accumulation of the helper virus AfPV1 in A. flavus. We used this model to investigate transcriptional genes regulated by AfPV1 and the small RNA segment in A. flavus, and their role in generating different phenotypes. We found that the pathways of the genes regulated by AfPV1 in its host were similar to those of retroviral viruses. Therefore, some pathways may be of benefit to non-retroviral viral integration or endogenization into the genomes of its host. Moreover, some potential antiviral substances were also found in A. flavus using this system.

Interaction between hypoviral-regulated fungal virulence factor laccase3 and small heat shock protein Hsp24 from the chestnut blight fungus Cryphonectria parasitica

Laccase3 is an important virulence factor of the fungus Cryphonectria parasitica. Laccase3 gene (lac3) transcription is induced by tannic acid, a group of phenolic compounds found in chestnut trees, and its induction is regulated by the hypovirus CHV1 infection. CpHsp24, a small heat shock protein gene of C. parasitica, plays a determinative role in stress adaptation and pathogen virulence. Having uncovered in our previous study that transcriptional regulation of the CpHsp24 gene in response to tannic acid supplementation and CHV1 infection was similar to that of the lac3, and that conserved phenotypic changes of reduced virulence were observed in mutants of both genes, we inferred that both genes were implicated in a common pathway. Building on this finding, in this paper we examined whether the CpHsp24 protein (CpHSP24) was a molecular chaperone for the lac3 protein (LAC3). Our pull-down experiment indicated that the protein products of the two genes directly interacted with each other. Heterologous co-expression of CpHsp24 and lac3 genes using Saccharomyces cerevisiae resulted in more laccase activity in the cotransformant than in a parental lac3-expresssing yeast strain. These findings suggest that CpHSP24 is, in fact, a molecular chaperone for the LAC3, which is critical component of fungal pathogenesis.

Agro-Industrial Wastes: A Substrate for Multi-Enzymes Production by Cryphonectria parasitica

This study aims to produce a mix of enzymes through Solid State Fermentation (SSF) of raw materials. Four different, easily available, agro-industrial wastes were evaluated as SSF substrates for enzymes production by Cryphonectria parasitica (Murr.) Barr. environmental strains named CpA, CpB2, CpC4, and CpC7. Among the tested wastes, organic wheat bran for human use and wheat bran for animal feed better supports C. parasitica growth and protease production without any supplements. SDS-PAGE analyses highlighted the presence of three bands corresponding to an extracellular laccase (77 kDa), to the endothiapepsin (37 kDa), and to a carboxylesterase (60.6 kDa). Protease, laccase, and esterase activities by C. parasitica in SSF were evaluated for 15 days, showing the maximum protease activity at day 9 (3955.6 AU/gsf,). Conversely, the best laccase and esterase production was achieved after 15 days. The C. parasitica hypovirulent CpC4 strain showed the highest laccase and esterase activity (93.8 AU/gsf and 2.5 U/gsf, respectively). These results suggest the feasibility of a large-scale production of industrially relevant enzymes by C. parasitica strains in SSF process on low value materials.

Laccase Activity in Fungus Cryphonectria parasitica Is Affected by Growth Conditions and Fungal-Viral Genotypic Interactions

Laccase activity reduction in the chestnut blight fungus Cryphonectria parasitica usually accompanies the hypovirulence caused by the infection of fungus with Cryphonectria hypovirus 1 (CHV1). However, the different methods utilized for assessing this phenomenon has produced varied and often conflicting results. Furthermore, the majority of experimental setups included only one prototypic system, further confounding the results. Considering the diversity of fungal isolates, viral strains, and variability of their effects on the phytopathogenic process observed in nature, our goal was to ascertain if laccase activity variability is affected by (1) different C. parasitica isolates infected with several CHV1 strains, and (2) growth conditions. We have demonstrated that some CHV1 strains, contrary to previous assumptions, increase the activity of C. parasitica laccases. The specific fungal isolates used in the experiments and culture conditions also affected the results. Furthermore, we showed that two commonly used laccase substrates, 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) and 2,4-dimethoxyphenol, cannot be used interchangeably in C. parasitica laccase activity measurements. Our results illustrate the importance of conducting this type of study in experimental systems and culture conditions that resemble natural conditions as much as possible to be able to infer the most relevant conclusions applicable to natural populations.

Mycoviruses and their role in fungal pathogenesis

Nowadays, the focus of mycovirology research has expanded from plant pathogenic fungi and mycovirus mediated hypovirulence to include insect and human pathogenic fungi together with a range of mycovirus mediated phenotypes, such as hypervirulence, control of endophytic traits, regulation of metabolite production and drug resistance. In fungus-mycovirus-environmental interactions, the environment and both abiotic and biotic factors play crucial roles in whether and how mycovirus mediated phenotypes are manifest. Mycovirus infections result in alterations in the host transcriptome profile, via protein-protein interactions and triggering of antiviral RNA silencing in the fungus. These alterations, in combination with the environmental factors, may result in desirable phenotypic traits for the host, for us and in some cases for both.

Functional Analysis of an Essential GSP1/Ran Ortholog Gene, CpRan1, from the Chestnut Blight Fungus Cryphonectria parasitica Using a Heterokaryon

Functional analysis of a GSP1/Ran ortholog, CpRan1, from Cryphonectria parasitica was conducted. Genotype analysis revealed that the putative CpRan1-null mutant was a heterokaryotic transformant harboring two different types of nuclei, one with the wild-type CpRan1 allele and the other with the CpRan1-null mutant allele. The mycelial growth and colony morphology of the heterokaryotic transformant was normal. Microscopic analysis of the resulting conidia (aseptate and monokaryotic asexual spores) demonstrated that although normal germinating spores were observed from conidia harboring a nucleus with the wild-type CpRan1 allele, a number of residual conidia that did not germinate existed. Complementation analysis using protoplasts from the heterokaryon with the wild-type CpRan1 allele confirmed that the CpRan1 gene is essential to C. parasitica. Complementation analysis using the various CpRan1 chimera constructs allowed us to perform a functional analysis of essential amino acids of the CpRan1. Among the four suggested essential amino acids, Lys-97 for ubiquitination was determined to not be an essential residue. Moreover, the CpRan1-null mutant allele was successfully complemented with mouse Ran gene, which suggested that the biological function of Ran gene is evolutionary conserved and that our heterokaryon rescue can be applied for the functional analysis of heterologous genes.

A Novel Virus Alters Gene Expression and Vacuolar Morphology in Malassezia Cells and Induces a TLR3-Mediated Inflammatory Immune Response

Malassezia is the most dominant fungal genus on the human skin surface and is associated with various skin diseases including dandruff and seborrheic dermatitis. Among Malassezia species, Malassezia restricta is the most widely observed species on the human skin. In the current study, we identified a novel dsRNA virus, named MrV40, in M. restricta and characterized the sequence and structure of the viral genome along with an independent satellite dsRNA viral segment. Moreover, expression of genes involved in ribosomal synthesis and programmed cell death was altered, indicating that virus infection affected the physiology of the fungal host cells. Our data also showed that the viral nucleic acid from MrV40 induces a TLR3-mediated inflammatory immune response in bone marrow-derived dendritic cells, indicating that a viral element likely contributes to the pathogenicity of Malassezia. This is the first study to identify and characterize a novel mycovirus in Malassezia.

Most fungal viruses have been identified in plant pathogens, whereas the presence of viral particles in human-pathogenic fungi is less well studied. In the present study, we observed extrachromosomal double-stranded RNA (dsRNA) segments in various clinical isolates of Malassezia species. Malassezia is the most dominant fungal genus on the human skin surface, and species in this group are considered etiological factors of various skin diseases including dandruff, seborrheic dermatitis, and atopic dermatitis. We identified novel dsRNA segments, and our sequencing results revealed that the virus, named MrV40, belongs to the Totiviridae family and contains an additional satellite dsRNA segment encoding a novel protein. The transcriptome of virus-infected Malassezia restricta cells was compared to that of virus-cured cells, and the results showed that transcripts involved in ribosomal biosynthesis were downregulated and those involved in energy production and programmed cell death were upregulated. Moreover, transmission electron microscopy revealed significantly larger vacuoles in virus-infected M. restricta cells, indicating that MrV40 infection dramatically altered M. restricta physiology. Our analysis also revealed that viral nucleic acid from MrV40 induced a TLR3 (Toll-like receptor 3)-mediated inflammatory immune response in bone marrow-derived dendritic cells, suggesting that a viral element contributes to the pathogenicity of Malassezia.

Transcriptome Analysis of Cryphonectria parasitica Infected With Cryphonectria hypovirus 1 (CHV1) Reveals Distinct Genes Related to Fungal Metabolites, Virulence, Antiviral RNA-Silencing, and Their Regulation

Comprehensive transcriptome analysis was conducted to elucidate the molecular basis of the interaction between chestnut blight fungus, Cryphonectria parasitica, and single-stranded RNA (ssRNA) mycovirus Cryphonectria hypovirus 1 (CHV1), using RNA-sequencing (RNA-seq). A total of 1,023 differentially expressed genes (DEGs) were affected by CHV1 infection, of which 753 DEGs were upregulated and 270 DEGs were downregulated. Significant correlations in qRT-PCR analysis of 20 randomly selected DEGs and agreement with previously characterized marker genes validated our RNA-seq analysis as representing global transcriptional profiling of virus-free and -infected isogenic strains of C. parasitica. Gene Ontology (GO) analysis of DEGs indicated that “cellular aromatic compound metabolic process” and “transport” were the two most enriched components in the “biological process.” In addition, “cytoplasm” was the most enriched term in the “cellular component” and “nucleotide binding” and “cation binding” were the two most enriched terms in the “molecular function” category. These results suggested that altered expression of genes encoding numerous intracellular proteins due to hypoviral infection resulted in changes in specific metabolic processes as well as transport processes. Kyoto Encyclopedia of Genes and Genomes function analysis demonstrated that pathways for “biosynthesis of other secondary metabolites,” “amino acid metabolism,” “carbohydrate metabolism,” and “translation” were enriched among the DEGs in C. parasitica. These results demonstrate that hypoviral infection resulted in massive but specific changes in primary and secondary metabolism, of which antiviral fungal metabolites were highly induced. The results of this study provide further insights into the mechanism of fungal gene regulation by CHV1 at the transcriptome level.

Genome Sequence of the Chestnut Blight Fungus Cryphonectria parasitica EP155: A Fundamental Resource for an Archetypical Invasive Plant Pathogen

Cryphonectria parasitica is the causal agent of chestnut blight, a fungal disease that almost entirely eliminated mature American chestnut from North America over a 50-year period. Here, we formally report the genome of C. parasitica EP155 using a Sanger shotgun sequencing approach. After finishing and integration with simple-sequence repeat markers, the assembly was 43.8 Mb in 26 scaffolds (L₅₀ = 5; N₅₀ = 4.0Mb). Eight chromosomes are predicted: five scaffolds have two telomeres and six scaffolds have one telomere sequence. In total, 11,609 gene models were predicted, of which 85% show similarities to other proteins. This genome resource has already increased the utility of a fundamental plant pathogen experimental system through new understanding of the fungal vegetative incompatibility system, with significant implications for enhancing mycovirus-based biological control.

Functional analysis of an essential Ran-binding protein gene, CpRbp1, from the chestnut blight fungus Cryphonectria parasitica using heterokaryon rescue

A Ran binding protein (RanBP) homolog, CpRbp1, from Cryphonectria parasitica, has been identified as a protein that is affected by hypovirus infection or tannic acid supplementation. In this study, functional analyses of CpRbp1 were performed by constructing a knockout mutant and analyzing the resulting heterokaryon. Transformation-mediated gene replacement resulted in two putative CpRbp1-null mutants and genotype analyses identified these two mutants as heterokaryotic transformants consisting of two types of nuclei, one with the wild-type CpRbp1 allele and another with the CpRbp1-null mutant allele. Although stable mycelial growth of the heterokaryotic transformant was observed on selective medium containing hygromycin B, neither germination nor growth of the resulting conidia, which were single-cell monokaryotic progeny, was observed on the medium. In trans complementation of heterokaryons using a full-length wild-type allele of the CpRbp1 gene resulted in complemented transformants. These transformants sporulated single-cell monokaryotic conidia that were able to grow on media selective for replacing and/or complementing markers. These results clearly indicate that CpRbp1 is an essential gene, and heterokaryons allowed the fungus to maintain lethal CpRbp1-null mutant nuclei. Moreover, in trans complementation of heterokaryons using chimeric structures of the CpRbp1 gene allowed for analysis of its functional domains, which was previously hampered due to the lethality of the gene. In addition, in trans complementation using heterologous RanBP genes from Aspergillus nidulans was successful, suggesting that the function of RanBP is conserved during evolution. Furthermore, in trans complementation allowed for functional analyses of lethal orthologs. This study demonstrates that our fungal heterokaryon system can be applied effectively to determine whether a gene of interest is essential, perform functional analyses of a lethal gene, and analyze corresponding heterologous genes.

Cpvma1, a Vacuolar H+-ATPase Catalytic Subunit of Cryphonectria parasitica, is Essential for Virulence and Hypovirus RNA Accumulation

The vacuolar H⁺-ATPases (V-ATPases) are conserved ATP-dependent proton pumps that acidify intracellular compartments in eukaryotic cells. The role of Cpvma1, a V-ATPase catalytic subunit A of Cryphonectria parasitica, was investigated by generating cpvma1-overexpressing and cpvma1-silenced strains. The mutant strains were evaluated for phenotypic characteristics, V-ATPase activity, response to elevated pH and Ca²⁺ in the medium, virulence on chestnut, and accumulation of hypovirus RNA in the cells. Compared with the wild-type strain, cpvma1-overexpressing strains showed no significant difference in phenotype; however, cpvma1-silenced strains exhibited a phenotype of reduced growth rate, lower level of sporulation, and a marked decrease in V-ATPase activity and virulence. In addition, silencing of cpvma1 increased sensitivity to elevated pH and Ca²⁺, implicating an important role for Cpvma1 in pH adaptation and Ca²⁺ homeostasis. Furthermore, silencing of cpvma1 resulted in significantly decreased accumulation of hypoviral RNA. Taken together, our results indicate that Cpvma1 plays an important role in the regulation of phenotypic traits and virulence and the accumulation of hypovirus RNA in C. parasitica.

Characterization of a Hypovirus-Regulated Septin Cdc11 Ortholog, CpSep1, from the Chestnut Blight Fungus Cryphonectria parasitica

We identified a protein spot showing downregulation in the presence of Cryphonectria hypovirus 1 and tannic acid supplementation as a septin subunit with the highest homology to the Aspergillus nidulans aspA gene, an ortholog of the Saccharomyces cerevisiae Cdc11 gene. To analyze the functional role of this septin component (CpSep1), we constructed its null mutant and obtained a total of eight CpSep1-null mutants from 137 transformants. All CpSep1-null mutants showed retarded growth, with fewer aerial mycelia and intense pigmentation on plates of potato dextrose agar supplemented with L-methionine and biotin. When the marginal hyphae were examined, hyperbranching was observed in contrast to the wild type. The inhibition of colonial growth was partially recovered when the CpSep1-null mutants were cultured in the presence of the osmostabilizing sorbitol. Conidia production of the CpSep1-null mutants was significantly increased by at least 10-fold more. Interestingly, the conidial morphology of the CpSep1-null mutants changed to circular in contrast to the typical rod-shaped spores of the wild type, indicating a role of septin in the spore morphology of Cryphonectria parasitica. However, no differences in the germination process were observed. Virulence assays using excised chestnut bark, stromal pustule formation on chestnut stems, and apple inoculation indicated that the CpSep1 gene is important in pathogenicity.

Changes in Cryphonectria parasitica Populations Affect Natural Biological Control of Chestnut Blight

Invasive species, especially plant pathogens, have a potential to completely eradicate native plant species and remodel landscapes. Tripartite interactions among sweet chestnut (Castanea sativa), chestnut blight-causing invasive fungus Cryphonectria parasitica, and hyperparasitic virus Cryphonectria hypovirus 1 (CHV1) were studied in two populations. The number of different vegetative compatibility (vc) types of C. parasitica more than doubled over the 10 years, while the hypovirulence incidence dropped in one population and slightly increased in the other one. Over the course of our 3-year monitoring experiment, the prevalence of hypovirulent isolates obtained from monitored cankers increased slowly (i.e., more hypovirulent isolates were being obtained from the same cankers over time). Within studied cankers, considerable changes in vc type and CHV1 presence were observed, indicating a highly dynamic system in which virulent and hypovirulent mycelia, sometimes of discordant vc types, often appeared together. The increase in hypovirulence prevalence did not have any observable curative effect on the cankers and, occasionally, reactivation of healed cankers by new, virulent C. parasitica isolates was observed. Both short- and long-term observations and revalidation of the infected plant populations are necessary to accurately estimate disease progress and formulate an adequate disease management strategy.

Evaluation of Two Decades of Cryphonectria parasitica Hypovirus Introduction in an American Chestnut Stand in Wisconsin

Hypovirus-infected Cryphonectria parasitica strains were introduced in a large stand of American chestnut (>4,000 individuals) in western Wisconsin (USA) to evaluate whether hypoviruses can serve as biological control agents. They were deployed by treating cankers from 1992 to 1997 and again from 2004 to 2014. After 17 years of hypovirus introductions within an area of the stand with the longest history of disease, isolation of hypovirus-infected strains increased from 55% in 1994 to 86% in 2014 from cankers that were treated. During the same period, isolation from cankers that arose on trees with treated cankers increased from 29 to 72% and from 15 to 84% for cankers on nearby trees that received no treatment. Tree survivorship over the 23-year study period for trees with treated cankers was 51% compared with 31% for trees that were not treated. Introduction of hypovirus has resulted in the regrowth of the crowns of many large-diameter trees. Putative recovery of American chestnut in this stand provides evidence that prolonged hypovirus treatment can act as a biological control when limited numbers of vegetative compatibility types of C. parasitica exist.

A Novel Hypovirus Species From Xylariaceae Fungi Infecting Avocado

The white rot root disease caused by Rosellinia necatrix is a major concern for avocado cultivation in Spain. Healthy escapes of avocado trees surrounded by diseased trees prompted us to hypothesize the presence of hypovirulent R. necatrix due to mycovirus infections. Recently, we reported the presence of another fungal species, Entoleuca sp., belonging to the Xylariaceae, that was also found in healthy avocado trees and frequently co-infecting the same roots than R. necatrix. We investigated the presence of mycoviruses that might explain the hypovirulence. For that, we performed deep sequencing of dsRNAs from two isolates of Entoleuca sp. that revealed the simultaneous infection of several mycoviruses, not described previously. In this work, we report a new member of the Hypoviridae, tentatively named Entoleuca hypovirus 1 (EnHV1). The complete genome sequence was obtained for two EnHV1 strains, which lengths resulted to be 14,958 and 14,984 nt, respectively, excluding the poly(A) tails. The genome shows two ORFs separated by a 32-nt inter-ORF, and both 5′- and 3′-UTRs longer than any other hypovirus reported to date. The analysis of virus-derived siRNA populations obtained from Entoleuca sp. demonstrated antiviral silencing activity in this fungus. We screened a collection of Entoleuca sp. and R. necatrix isolates and found that EnHV1 was present in both fungal species. A genetic population analysis of EnHV1 strains revealed the presence of two main clades, each of them including members from both Entoleuca sp. and R. necatrix, which suggests intra- and interspecific virus transmission in the field. Several attempts failed to cure Entoleuca sp. from EnHV1. However, all Entoleuca sp. isolates collected from avocado, whether harboring the virus or not, showed hypovirulence. Conversely, all R. necatrix isolates were pathogenic to that crop, regardless of being infected by EnHV1.

Cryphonectria hypovirus 1-Induced Epigenetic Changes in Infected Phytopathogenic Fungus Cryphonectria parasitica

Biotic stress caused by virus infections induces epigenetic changes in infected plants and animals, but this is the first report on methylation pattern changes in a fungus after mycovirus infection. As a model pathosystem for mycovirus-host interactions, we used Cryphonectria hypovirus 1 (CHV1) and its host fungus Cryphonectria parasitica, in which deregulation of methylation cycle enzymes upon virus infection was observed previously. Six CHV1 strains of different subtypes were transferred into three different C. parasitica isolates in order to assess the effect of different CHV1 strains and/or subtypes on global cytosine methylation level in infected fungus, using methylation-sensitive amplification polymorphism (MSAP). Infection with CHV1 affected the methylation pattern of the C. parasitica genome; it increased the number and diversity of methylated, hemi-methylated, and total MSAP markers found in infected fungal isolates compared to virus-free controls. The increase in methylation levels correlated well with the CHV1-induced reduction of fungal growth in vitro, indicating that C. parasitica genome methylation upon CHV1 infection, rather than being the defensive mechanism of the fungus, is more likely to be the virulence determinant of the virus. Furthermore, the severity of CHV1 effect on methylation levels of infected C. parasitica isolates depended mostly on individual CHV1 strains and on the combination of host and virus genomes, rather than on the virus subtype. These novel findings broaden our knowledge about CHV1 strains which could potentially be used in human-aided biocontrol of chestnut blight, a disease caused by C. parasitica in chestnut forest ecosystems and orchards.

Global DNA Methylation in the Chestnut Blight Fungus Cryphonectria parasitica and Genome-Wide Changes in DNA Methylation Accompanied with Sectorization

Mutation in CpBck1, an ortholog of the cell wall integrity mitogen-activated protein kinase kinase kinase (MAPKKK) of Saccharomyces cerevisiae, in the chestnut blight fungus Cryphonectria parasitica resulted in a sporadic sectorization as culture proceeded. The progeny from the sectored area maintained the characteristics of the sector, showing a massive morphogenetic change, including robust mycelial growth without differentiation. Epigenetic changes were investigated as the genetic mechanism underlying this sectorization. Quantification of DNA methylation and whole-genome bisulfite sequencing revealed genome-wide DNA methylation of the wild-type at each nucleotide level and changes in DNA methylation of the sectored progeny. Compared to the wild-type, the sectored progeny exhibited marked genome-wide DNA hypomethylation but increased methylation sites. Expression analysis of two DNA methyltransferases, including two representative types of DNA methyltransferase (DNMTase), demonstrated that both were significantly down-regulated in the sectored progeny. However, functional analysis using mutant phenotypes of corresponding DNMTases demonstrated that a mutant of CpDmt1, an ortholog of RID of Neurospora crassa, resulted in the sectored phenotype but the CpDmt2 mutant did not, suggesting that the genetic basis of fungal sectorization is more complex. The present study revealed that a mutation in a signaling pathway component resulted in sectorization accompanied with changes in genome-wide DNA methylation, which suggests that this signal transduction pathway is important for epigenetic control of sectorization via regulation of genes involved in DNA methylation.

Mutation of the Slt2 ortholog from Cryphonectria parasitica results in abnormal cell wall integrity and sectorization with impaired pathogenicity

We assessed the biological function of CpSlt2, an ortholog of the cell wall integrity (CWI) MAPK of Saccharomyces cerevisiae, in the chestnut blight fungus Cryphonectria parasitica. The CpSlt2-null mutant exhibited marked changes in colonial growth, near absence of conidiation and aerial hyphae, and abnormal pigmentation. In addition, the CpSlt2-null mutant exhibited CWI-related phenotypic defects including hypersensitivity to cell wall-disturbing agents and other stresses. Electron microscopy revealed the presence of abnormal hyphae such as intrahyphal hyphae. In addition, virulence assays indicated that the CpSlt2 gene plays an important role in fungal pathogenesis. As cultivation of the mutant strains progressed, the majority of the colonies showed sporadic sectorization and mycelia from the sectored area stably maintained the sectored phenotype. Although mycelial growth was partially recovered, the sectored progeny had dramatically impaired virulence, confirming the CpSlt2 gene has a role in pathogenicity. Compared to a previous mutant of the CpBck1 gene, a MAPKKK gene in CWI pathway, the CpSlt2-null mutant showed similar, although not identical, phenotypic changes and most phenotypic changes were less severe than those of the CpBck1-null mutant. These results suggest that the unique sectorization is CWI pathway-specific, though the components in the same CWI pathway have common and specific functions.

Cryphonectria hypovirus 1-Induced Changes of Stress Enzyme Activity in Transfected Phytopathogenic Fungus Cryphonectria parasitica

Cryphonectria parasitica is a phytopathogenic fungus introduced from Eastern Asia to North America and to Europe, where it causes chestnut blight, a devastating disease of chestnut trees. The disease can be biologically controlled utilising the mycovirus Cryphonectria hypovirus 1 (CHV1), which changes the physiology of the host, reducing its virulence towards chestnut. We measured fungal growth in vitro and activities of glutathione S-transferase, catalase and superoxide dismutase, enzymes involved in oxidative stress response, to elucidate the effects of CHV1 infection on the host. Six CHV1 strains of different subtypes and three fungal isolates were used in different combinations to better represent natural conditions, where higher genetic diversity of both fungus and virus is expected. The infection with different CHV1 strains decreased in vitro growth rate of infected fungal isolates and increased activity of their stress enzymes in most of the studied fungus/virus combinations, indicating increased oxidative stress following CHV1 infection. All our field CHV1 strains belong to the Italian subtype, but while strain M56-1 had equal or even stronger effect on its fungal host than prototypic strain EP713 of French subtype F1, strain B11 had no effect. Thus, the severity of the observed effects depended on a particular virus strain, fungal isolate, and the combination of the two, rather than solely on the virus subtype. Since previous research showed discordance between accumulation of mRNA and stress-related proteins in CHV1 infected C. parasitica, our results emphasise the importance of enzymes' activity measurements as an invaluable extension of transcriptomic and proteomic analyses.

Investigation on the partial resistance of Cpkk2 knock out strain of Cryphonectria parasitica to Cryphonectria hypovirus 1 infection in presence of Geneticin and Geneticin resistance gene

We have recently characterized the central components of the three MAP kinase cascades present in Cryphonectria parasitica : the MEK genes cpkk1, cpkk2 and cpkk3. When we attempted to infect through anastomosis the three knock out strains with Cryphonectria hypovirus 1 (CHV1), only the deletion strain of Cpkk2, the yeast Ste7 homologue, involved in mating and filamentous growth, could not be infected. We then proceeded to attempt virus infection through transformation of Δcpkk2 protoplasts using an infectious cDNA clone able to establish virus infection through transformation. In this case, a very limited number of strains could be recovered as stable transformants compared to the efficiency of control transformations with plasmid carrying only the antibiotic marker. Furthermore, transformants carrying actively replicating virus could be isolated only if the selection marker Geneticin was used during the very initial selection process, and not maintained throughout the growth of the colonies. Moreover, Δcpkk2 isolates that maintained the virus lost Geneticin resistance. We therefore unveiled a specific negative interaction among virus infection, presence of Geneticin in the growth media, and lack of Cpkk2 MEK in the fungal host.

A Mutant of the Bck1 Homolog from Cryphonectria parasitica Resulted in Sectorization with an Impaired Pathogenicity

CpBck1, an ortholog of the cell-wall integrity mitogen-activated protein kinase kinase kinase of Saccharomyces cerevisiae, was cloned and characterized from the chestnut blight fungus Cryphonectria parasitica. The CpBck1-null mutant displayed cell wall integrity-related phenotypic changes such as abnormal cell morphology and wall formation and hypersensitivity to cell wall-disrupting agents. In addition, the mutant showed severely retarded growth without any sign of normal development, such as hyphal differentiation, conidiation, or pigmentation. As the culture proceeded, the mutant colony showed sporadic sectorization. Once sectored, the sectored phenotype of robust mycelial growth without differentiation was stably inherited. Compared with the wild type, both the parental CpBck1-null mutant and the sectored progeny exhibited marked impaired virulence. The present study revealed that a mutation in a signaling pathway component related to cell-wall integrity resulted in sporadic sectorization and these sectored phenotypes were stably inherited, suggesting that this signal transduction pathway is implicated in adaptive genetic changes for sectorization.

Multilocus PCR Assays Elucidate Vegetative Incompatibility Gene Profiles of Cryphonectria parasitica in the United States

Chestnut blight is a devastating disease of Castanea spp. Mycoviruses that reduce virulence (hypovirulence) of the causative agent, Cryphonectria parasitica, can be used to manage chestnut blight. However, vegetative incompatibility (vic) barriers that restrict anastomosis-mediated virus transmission hamper hypovirulence efficacy. In order to effectively determine the vegetative incompatibility genetic structure of C. parasitica field populations, we have designed PCR primer sets that selectively amplify and distinguish alleles for each of the six known diallelic C. parasitica vic genetic loci. PCR assay results were validated using a panel of 64 European tester strains with genetically determined vic genotypes. Analysis of 116 C. parasitica isolates collected from five locations in the eastern United States revealed 39 unique vic genotypes and generally good agreement between PCR and tester strain coculturing assays in terms of vic diversity and genotyping. However, incongruences were observed for isolates from multiple locations and suggested that the coculturing assay can overestimate diversity at the six known vic loci. The availability of molecular tools for rapid and precise vic genotyping significantly improves the ability to predict and evaluate the efficacy of hypovirulence and related management strategies.

Dynamics of Cryphonectria hypovirus infection in chestnut blight cankers

Virulent strains of the chestnut blight fungus Cryphonectria parasitica cause lethal bark cankers on chestnut trees. Infection of C. parasitica with Cryphonectria hypovirus 1 in Europe biologically controls this disease, leading to nonlethal and inactive cankers. Unexpectedly, virus-free C. parasitica strains have been isolated from inactive cankers. In this study, we compared the virulence of virus-infected and virus-free C. parasitica strains isolated from either inactive or active cankers on chestnut seedlings and sprouts. In the seedling experiment, we assessed canker growth and seedling mortality. In the sprout experiment, we also assessed canker growth and made fungal reisolations to determine virus infection and immigration of foreign vegetative compatibility (vc) types over a period of 13 years in a coppice forest. Overall, the virulence of virus-free C. parasitica strains isolated from inactive versus active cankers did not differ. Significant differences were only attributed to virus infection. Virus infection and fungal strain composition in cankers changed over time. Foreign vc types immigrated into cankers and virus-free cankers became virus-infected within a few years. Most of the cankers were callused over time and became inactive. However, we observed that the virus did not always persist in these cankers. This study demonstrates that virus spread occurs effectively in European chestnut forests and that this biocontrol system is highly dynamic.

Vegetative incompatibility loci with dedicated roles in allorecognition restrict mycovirus transmission in chestnut blight fungus

Vegetative incompatibility (vic), a form of nonself allorecognition, operates widely in filamentous fungi and restricts transmission of virulence-attenuating hypoviruses in the chestnut blight fungus Cryphonectria parasitica. We report here the use of a polymorphism-based comparative genomics approach to complete the molecular identification of the genetically defined C. parasitica vic loci with the identification of vic1 and vic3. The vic1 locus in the C. parasitica reference strain EP155 consists of a polymorphic HET-domain-containing 771-aa ORF designated vic1a-2, which shares 91% identity with the corresponding vic1a-1 allele, and a small (172 aa) idiomorphic DUF1909-domain-containing ORF designated vic1b-2 that is absent at the vic1-1 locus. Gene disruption of either vic1a-2 or vic1b-2 in strain EP155 eliminated restrictions on virus transmission when paired with a vic1 heteroallelic strain; however, only disruption of vic1a-2 abolished the incompatible programmed cell death (PCD) reaction. The vic3 locus of strain EP155 contains two polymorphic ORFs of 599 aa (vic3a-1) and 102 aa (vic3b-1) that shared 46 and 85% aa identity with the corresponding vic3a-2 and vic3b-2 alleles, respectively. Disruption of either vic3a-1 or vic3b-1 resulted in increased virus transmission. However, elimination of PCD required disruption of both vic3a and vic3b. Additional allelic heterogeneity included a sequence inversion and a 8.5-kb insertion containing a LTR retrotransposon sequence and an adjacent HET-domain gene at the vic1 locus and a 7.7-kb sequence deletion associated with a nonfunctional, pseudo vic locus. Combined gene disruption studies formally confirmed restriction of mycovirus transmission by five C. parasitica vic loci and suggested dedicated roles in allorecognition. The relevance of these results to the acquisition and maintenance of vic genes and the potential for manipulation of vic alleles for enhanced mycovirus transmission are discussed.

Functional analysis of a tannic-acid-inducible and hypoviral-regulated small heat-shock protein Hsp24 from the chestnut blight fungus Cryphonectria parasitica

A small heat-shock protein gene, CpHsp24, of Cryphonectria parasitica was selected based on its expression pattern, which showed that it was tannic acid inducible and that its induction was severely hampered by a hypovirus. The predicted protein sequence of CpHsp24 consisted of a hallmark α-crystalline domain flanked by a variable N-terminal and a short C-terminal region. Disruption of CpHsp24 resulted in a slow growth rate under standard growth conditions. The CpHsp24-null mutant showed enhanced sensitivity to heat shock, which was consistent with Northern and Western analyses displaying the heat-shock induction of the CpHsp24 gene and protein, respectively. Virulence tests on the excised bark revealed a severe decrease in the necrotic area of the CpHsp24-null mutant. When the hypovirus was transferred, virus-containing CpHsp24-null progeny displayed severely retarded growth patterns with hypovirulent characteristics of reduced pigmentation and sporulation. Because the tannic-acid-inducible and hypoviral-suppressible expression and the severely impaired virulence are also characteristics of the laccase3 gene (lac3), lac3 expression in the CpHsp24-null mutant was also examined. The resulting lac3 induction was severely affected in the CpHsp24-null mutant, suggesting that CpHsp24 is important for lac3 induction and that CpHsp24 may act as a molecular chaperone for the lac3 protein.

Invasion history and demographic pattern of Cryphonectria hypovirus 1 across European populations of the chestnut blight fungus

We reconstructed the invasion history of the fungal virus Cryphonectria hypovirus 1 (CHV-1) in Europe, which infects the chestnut blight fungus Cryphonectria parasitica. The pattern of virus evolution was inferred based on nucleotide sequence variation from isolates sampled across a wide area in Europe at different points in time. Phylogeny and time estimates suggested that CHV-1 was introduced together with its fungal host to Europe and that it rapidly colonized the central range along the south facing slopes of the Alps and the north-east facing slopes of the Dinaric Alps. These central populations were the source for two waves of simultaneous invasions toward the southern Balkans and Turkey, as indicated by migration rates. Our results showed that the evolutionary scenarios for CHV-1 and C. parasitica were spatially congruent. As infection with CHV-1 reduces the pathogenicity of C. parasitica toward the chestnut tree, CHV-1 invasions of the newly established C. parasitica populations probably prevented the development of devastating chestnut blight epidemics in Europe. We propose that in this, and supposedly in other pathosystems, geographic, vegetation-related, demographic, economic, and political factors may help explain the correlated invasion pattern of a parasite and its host.

Comparative proteomic analysis of chestnut blight fungus,Cryphonectria parasitica, under tannic-acid-inducing and hypovirus-regulating conditions

Chestnut blight fungus, Cryphonectria parasitica , and its hypovirus present a useful model system for investigating the mechanisms of hypoviral infection. To identify gene products associated with fungal pathogenicity and hypoviral regulation, we attempted a proteomic analysis of the virus-free EP155/2 strain and its isogenic virus-infected UEP1 strain in response to tannic acid (TA), which is abundant in the bark of chestnut trees. In this study, pretreatment of mycelia grown on TA-supplemented media was developed for proteomic analysis. Approximately 704 proteins from the mycelia of the EP155/2 strain were reproducibly present in 3 independent extractions. Among these, 111 and 79 spots were found to be responsive to hypovirus infection and TA supplementation, respectively. The TA-grown UEP1 strain yielded 28 spots showing an expression pattern different from that of untreated UEP1. Thirty protein spots showing considerable differences in spot density were selected for further analysis. Hybrid tandem LC-MS/MS spectrometry of the 30 selected protein spots revealed that 29 were identified while 1 was unidentified. Among the identified 29 proteins, 15 were metabolic enzymes; 5 were stress-related, of which 4 were heat-shock proteins and 1 was glutathione S-transferase; 5 were signaling and cellular process-related proteins; 2 were structural proteins; and 2 matched proteins of hypothetical genes.

Characterization of CpSte11, a MAPKKK gene of Cryphonectria parasitica, and initial evidence of its involvement in the pheromone response pathway

The gene CpSte11 of Cryphonectria parasitica, which encodes a yeast Ste11 homologue, was cloned and characterized. Gene replacement analysis revealed a high frequency of CpSte11 null mutants. When compared with the wild-type parent strain, CpSte11 null mutants showed no difference in terms of growth rate or pigmentation. However, CpSte11 null mutants showed a marked decrease in both the number and size of stromal pustules on chestnut twigs. The virulence test showed that, in comparison with those of the wild-type and virus-infected hypovirulent strains, CpSte11 null mutants produced necrotic areas of intermediate size. Disruption of the CpSte11 gene also resulted in defects in female fertility. Down-regulation of transcripts for the mating pheromone precursor gene, Mf2/2, and mating response transcription factors, such as cpst12 and pro1, was observed in CpSte11 null mutants. The down-regulation of Mf2/2, cpst12 and pro1 was also observed in the mutant phenotype of Cpmk2, a mating response Fus3-like mitogen-activated protein kinase (MAPK) gene, but not in the mutant of Cpmk1, a high-osmolarity glycerol Hog1-like MAPK gene. These results indicate that the cloned CpSte11 gene is functionally involved in the mating response pathway and acts through downstream targets, including Cpmk2, cpst12, pro1 and Mf2/2. However, the characteristics of the CpSte11 null mutant were fully phenocopied only in the cpst12 null mutant, but not in other studied null mutants of components of the putative mating response pathway.

Molecular characterization of vegetative incompatibility genes that restrict hypovirus transmission in the chestnut blight fungus Cryphonectria parasitica

Genetic nonself recognition systems such as vegetative incompatibility operate in many filamentous fungi to regulate hyphal fusion between genetically dissimilar individuals and to restrict the spread of virulence-attenuating mycoviruses that have potential for biological control of pathogenic fungi. We report here the use of a comparative genomics approach to identify seven candidate polymorphic genes associated with four vegetative incompatibility (vic) loci of the chestnut blight fungus Cryphonectria parasitica. Disruption of candidate alleles in one of two strains that were heteroallelic at vic2, vic6, or vic7 resulted in enhanced virus transmission, but did not prevent barrage formation associated with mycelial incompatibility. Detailed characterization of the vic6 locus revealed the involvement of nonallelic interactions between two tightly linked genes in barrage formation, heterokaryon formation, and asymmetric, gene-specific influences on virus transmission. The combined results establish molecular identities of genes associated with four C. parasitica vic loci and provide insights into how these recognition factors interact to trigger incompatibility and restrict virus transmission.

[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.

Temperature-dependent genotype-by-genotype interaction between a pathogenic fungus and its hyperparasitic virus

The outcome of host-parasite interactions may depend not only on the genotypes of the species involved but also on environmental factors. We used the fungus Cryphonectria parasitica, the causal agent of chestnut blight, and its hyperparasitic virus, Cryphonectria hypovirus-1 (CHV1), to test for genotype-by-genotype-by-environment interactions in a host-parasite system. In C. parasitica, infection with CHV1 induces a hypovirulent phenotype with reduced virulence toward the chestnut tree (Castanea spp.) and thus controls chestnut blight in many European regions. In contrast, uninfected virulent C. parasitica have nearly eradicated the American chestnut in North America. We applied a full factorial design and assessed the fungal growth and sporulation of four C. parasitica strains, uninfected and infected with each of the four known CHV1 subtypes, at 12°, 18°, 24°, and 30°C. We found a significant (P ≤ .00001) genotype-by-genotype-by-environment interaction, demonstrating the potential for a selection mosaic. As a consequence, different host and parasite genotypes would be selected under different climatic conditions, affecting the coevolutionary dynamics of the host-parasite interaction and the course of chestnut blight epidemics. Genotype-by-genotype-by-environment interactions are essential to take into account when designing biological control strategies.

A DnaJ-like homolog from Cryphonectria parasitica is not responsive to hypoviral infection but is important for fungal growth in both wild-type and hypovirulent strains

A DnaJ-like gene, Cpdj1, a molecular chaperone and regulator of Hsp70 in Cryphonectria parasitica, was characterized. The protein product of Cpdj1 gene consists of 379 amino acids with a predicted molecular mass of 40.6 kDa and a pI of 7.79. The deduced protein sequence revealed preservation of the conserved hall-mark J-region and exhibited high homolo y to all known DnaJ-like proteins. Disruption of the Cpdj1 gene resulted in slow growth and produced colonies characterized by retarded growth and deep orange color. Accordingly, reduced virulence of the Cpdj1-null mutant was observed. This reduced growth rate was magnified when the Cpdj1-null mutant was cultured under heat-stress conditions. Reduced conidiation was also observed in the Cpdj1-null mutant, indicating that Cpdj1 gene, although not essential for cell viability, is required for appropriate cellular processes including growth and sporulation. Northern analysis showed that Cpdj1 was constitutively expressed, and when the culture was subject to high temperature, a strong induction of the transcript was observed. No significant difference in the expression and induction pattern of Cpdj1 was observed between virus-free EP155/2 and virus-infected hypovirulent UEP1 strains. However, further severe defects in mycelia growth and conidiation were observed in the hypovirus-infected Cpdj1-null mutant suggesting that the presence of Cpdj1 is required for mycelia growth and sporulation of the hypovirus-infected strain.

Biological control of chestnut blight

After 77 years of being attacked by the chestnut blight fungus, American chestnut trees continue to sprout from gradually declining root systems. The blight fungus in Italy is now associated with virus-like agents that limit its pathogenicity, and attempts have been made to introduce these controlling agents into the blight fungus in the United States. If a way can be found to help the spread here of strains of the fungus with controlling agents, it may be possible to save the American chestnut trees in our eastern forests.

An RNA symbiont enhances heat tolerance and secondary homothallism in the oomycete Phytophthora infestans

Some strains ofPhytophthora infestans,the potato late blight pathogen, harbour a small extrachromosomal RNA called PiERE1. A previous study reported that this RNA symbiont does not noticeably affect its host. Here it is revealed that PiERE1 exerts subtle effects onP. infestans, which result in greater thermotolerance during growth and an increase in secondary homothallism, i.e. oospore formation in the absence of the opposite mating type. The interaction can be considered mutualistic since these traits may increase the fitness ofP. infestansin nature. Assays of biomarkers for cellular stress revealed that an Hsp70 chaperone was upregulated by PiERE1. A genome-wide search for more members of the Hsp70 family identified ten belonging to the DnaK subfamily, one in the Hsp110/SSE subfamily, and pseudogenes. Four DnaK subfamily genes encoding predicted cytoplasmic or endoplasmic reticulum proteins were upregulated in strains harbouring PiERE1. This may explain the greater thermotolerance conferred by the RNA element, and suggests that Hsp70 may be a useful biomarker for testing organisms for the cellular effects of symbiotic elements.