Hypovirulence: mycoviruses at the fungal-plant interface

Homeostasis of Calnexin Is Essential for the Growth, Virulence, and Hypovirus RNA Accumulation in the Chestnut Blight Fungus

Calnexin, a calcium-binding protein, promotes correct protein folding and prevents incompletely folded glycopolypeptides from premature oxidation and degradation. Cryphonectria parasitica, an ascomycete fungus responsible for chestnut blight, poses a significant threat to the chestnut forest or orchards worldwide. Although various aspects of calnexin have been investigated, little is known about the impact of fungal viruses. CpCne was identified and characterized in this study, encoding the calnexin in C. parasitica. Strains with deletion or interference of the CpCne gene had a significant reduction in biomass and pathogenicity, and strains with overexpression of the CpCne gene had retarded growth and reduced pathogenicity. Transcriptome analysis showed that the △CpCne mutant had significant changes in the expression of genes related to carbohydrate metabolism, cell wall polysaccharide synthesis and degradation, indicating that CpCne may reduce virulence by affecting the cell wall. Additionally, the △CpCne mutant was sensitive to endoplasmic reticulum (ER) stress, suggesting that CpCne plays an important role in maintaining ER homeostasis. Furthermore, CpCne was also involved in the interaction between C. parasitica and the CHV1-EP713. Deletion or overexpression of the CpCne gene reduced viral RNA accumulation, and deletion of the CpCne gene altered the lipid and carboxylic acid metabolic pathways, thereby interfering with virus replication and assembly. Together, we demonstrated that the homeostasis of calnexin in C. parasitica (CpCne) is essential for hyphal growth and virulence, and revealed its role in viral replication and virulence.

Two novel mitoviruses coinfecting the fungus Colletotrichum karstii

Here, we isolated two novel mycoviruses coinfecting the fugus Colletotrichum karstii, which were designated as "Colletotrichum karstii mitovirus 1" (CkMV1) and "Colletotrichum karstii mitovirus 2" (CkMV2). The length of the complete genome of CkMV1 is 2,441 nucleotides, and that of CkMV2 is 2,391 nucleotides. Both contain a single open reading frame (ORF) that encodes an RNA-dependent RNA polymerase (RdRp). BLAST searches showed that the amino acids (aa) sequences of CkMV1 and CkMV2 had the highest amino acid sequence identity of 35.41% and 52.37% to the RdRp of Pleurotus pulmonarius duamitovirus 1 and Fusarium oxysporum f. sp. cubense mitovirus 4, respectively. Phylogenetic analysis based on RdRp sequences revealed that CkMV1 grouped with members of the genus Duamitovirus and that CkMV2 grouped with members of the genus Unuamitovirus, both within the family Mitoviridae.

Molecular characterization of a novel gammascleroulivirus from the rice blast fungus Magnaporthe oryzae isolate ES155

A novel single-stranded (+ss) RNA mycovirus, designated as "Magnaporthe oryzae botourmiavirus 14" (MoBV14), was identified in the rice blast fungus Magnaporthe oryzae isolate ES155. The viral genome is 2,336 nucleotides in length and contains a single open reading frame (ORF) that is predicted to encode an RNA-dependent RNA polymerase (RdRp). Genome sequence comparisons and phylogenetic analysis indicated that MoBV14 is a new member of the genus Gammascleroulivirus in the family Botourmiaviridae.

Exploring the interaction between endornavirus and Sclerotinia sclerotiorum: mechanisms of phytopathogenic fungal virulence and antivirus

Hypovirulence-associated mycoviruses have the potential as biocontrol agents for plant fungal disease management, and exploration of the interactions between these mycoviruses and phytopathogenic fungi can provide opportunities to elucidate the underlying mechanisms of hypovirulence and antiviruses. We previously found that Sclerotinia sclerotiorum endornavirus 3 (SsEV3), belonging to the genus Betaendornavirus within the family Endornaviridae, confers hypovirulence on the phytopathogenic fungus Sclerotinia sclerotiorum, but the underlying mechanisms remains unclear. In this study, we found that the SsEV3-infected strain produced fewer sclerotia, failed to form infection cushions on plant hosts, exhibited increased cell vacuolation, and was more sensitive to abiotic stresses. SsEV3 infection evoked transcriptional rewiring in S. sclerotiorum, affecting genes related to virulence factors for pathogenicity and RNAi pathway for antiviruses. An unknown biological function of gene Sssnf1 was downregulated following SsEV3 infection. Deletion of Sssnf1 impaired infection cushion formation and decreased virulence of S. sclerotiorum. Five key RNAi-related genes were significantly upregulated, and deletion of Ssdcl2 contributed to SsEV3 accumulation. Additionally, we identified a hypothetical protein encoded by Sshp1 that directly interacts with the RNA-dependent RNA polymerase (RdRp) domain encoded by SsEV3. Although the deletion mutants of Sshp1 exhibited normal colony morphology, they showed higher SsEV3 accumulation and reduced resistance to reactive oxygen species, indicating that this gene, similar to RNAi-related genes, plays an antiviral role in response to SsEV3 infection and may represent a new antivirus factor. Therefore, examination of the interaction between endornavirus and S. sclerotiorum provides new insights into the mechanisms of antivirus and virulence in phytopathogenic fungi.IMPORTANCEHypovirulence-associated mycoviruses have emerged as promising biocontrol agents, and studying their interactions with phytopathogenic fungi helps uncover mechanisms of fungal pathogenesis and antiviral defense. This study provides critical insights into the interaction between Sclerotinia sclerotiorum and its hypovirulence-associated endornavirus, SsEV3, elucidating the molecular mechanisms underlying mycovirus-induced changes in fungal virulence and antivirus defense. SsEV3 infection not only impairs fungal virulence traits, including infection cushion formation and sclerotial production but also triggers host antiviral responses involving typical RNA interference pathways. New virulence factors, such as Sssnf1, and antiviral factors, such as Sshp1, were identified based on the established interaction system between S. sclerotiorum and endornavirus. These findings deepen our understanding of fungus-mycovirus interactions, highlighting the role of SsEV3 in reducing the virulence of S. sclerotiorum, and facilitating the development of mycovirus-based biological control strategies.

A mycoviral infection drives virulence and ecological fitness of the entomopathogenic fungus Beauveria bassiana

Entomopathogenic ascomycetes are important natural regulators of insect pest populations and an increasingly adopted microbial control option. Fungal virulence in entomopathogenic ascomycetes can be modified by mycoviruses, viruses that infect fungi, whereas the possible role of these viruses on the physical and biochemical properties of the virus-containing fungal strains and on their ecological fitness has remained largely unexplored. Here, utilizing a Beauveria bassiana strain naturally infected with two mycoviruses, Beauveria bassiana partitivirus 2 (BbPV-2) and Beauveria bassiana polymycovirus 1 (BbPmV-1), we found that the mycovirus-containing strain is hypervirulent towards the experimental insect Galleria mellonella and shows major physical and biochemical changes in spore size, isoelectric point, and Pr1 activity, but even more impactful, the mycoviral infection confers a significant environmental- abiotic and biotic stress tolerance to the fungus. Hence, mycovirus infection expanded the temperature range for fungal growth and germination, and improved tolerance to osmotic stress, water stress, and UV-B radiation. Similarly, the antagonistic activity of the mycovirus-containing strain against Trichoderma harzianum was increased as compared to the mycovirus-free one. Taken together, these data suggest for the first time a mycovirus related adaptation of key traits indicators of environmental competence of a beneficial fungus, rendering these mycoviruses as potent tools for entomopathogenic fungal strain selection and development as mycoinsecticides.

Molecular and Biological Characterization of an Isolate of Fusarium graminearum dsRNA mycovirus 4 (FgV4) from a New Host Fusarium pseudograminearum

Wheat Fusarium crown rot (FCR), mainly caused by Fusarium pseudograminearum, is one of the most important diseases. Some mycoviruses are reported to have a hypovirulence trait and considered as a biocontrol agent for plant fungal diseases. In most cases, mycovirus biological effects have not been explored clearly. In this study, we identified and characterized a novel isolate of double-stranded RNA (dsRNA) mycovirus, Fusarium graminearum dsRNA mycovirus 4 (FgV4), from a new host, an isolate WC9-2 of F. pseudograminearum. The genome of FgV4-WC9-2 includes two dsRNA segments of 2194 bp and 1738 bp. FgV4-WC9-2 dsRNA1 contains a single open reading frame (ORF1), which encodes a protein of 675 amino acids (aa) and has a conserved RNA-dependent RNA polymerase (RdRp) domain. FgV4-WC9-2 dsRNA2 contains two discontinuous ORFs (ORF2-1 and ORF2-2) that code for hypothetical proteins with unknown function. Biological characteristics research has shown that FgV4-WC9-2 infection did not change the colony morphology, but it could significantly decrease colony growth rate. FgV4-WC9-2 could also reduce the sporulation ability, change the conidia size and reduce the pathogenicity of the host to a certain extent. This study is the first to describe a hypovirulence-associated orthocurvulavirus infecting F. pseudograminearum, which has the potential to assist with FCR disease biological management.

p18 encoded by FgGMTV1 is responsible for asymptomatic infection in Fusarium graminearum

The intricate interplay between mycoviruses and their fungal hosts frequently culminates in asymptomatic infections, but the virus-derived factors underlying these infections remain poorly understood. Our study introduces p18, a novel protein encoded by the DNA-C segment of the genomovirus FgGMTV1, which facilitates the transition from virus-induced hypovirulence to asymptomatic infection within Fusarium graminearum upon its expression. We have confirmed the expression of p18 during FgGMTV1 infection and observed its presence in both the nucleus and cytoplasm. Remarkably, strains with a p18 null mutation show a significant reduction in colony expansion, conidial production, and virulence, leading to a hypovirulent phenotype. Our results also indicate that p18 hinders the accumulation of FgGMTV1, thus determining asymptomatic infection and enabling vertical transmission through conidia. Furthermore, the p18 null mutant virus converts F. graminearum from virulent to hypovirulent strains on wheat leaves after horizontal transmission. This work not only expands our knowledge of the genomovirus proteome but also provides insights into the strategies of viral evolution and adaptation. Moreover, we propose an innovative approach for creating hypovirulent strains utilizing engineered mycoviruses for the biocontrol of plant pathogenic fungi.

IMPORTANCE

Mycovirus-fungus interplay often leads to asymptomatic infections. Our study identifies p18, a novel protein from the genomovirus FgGMTV1, as a key determinant of asymptomatic infection in Fusarium graminearum. A p18-null mutant exhibits a pronounced hypovirulent phenotype. By modulating viral accumulation, p18 promotes asymptomatic infection and facilitates vertical transmission via conidia. This insight deepens our understanding of mycovirus-fungus interactions and introduces a novel strategy for biocontrol using engineered mycoviruses.

Mycovirome of Diaporthe helianthi and D. gulyae, causal agents of Phomopsis stem canker of sunflower (Helianthus annuus L.)

Diaporthe gulyae and D. helianthi cause Phomopsis stem canker, which is a yield-limiting fungal disease of sunflower (Helianthus annuus L.) in the United States. In this study, the mycovirus population was characterized in D. gulyae and D. helianthi using 52 and 42 isolates, respectively, that were recovered from diseased sunflower plants randomly sampled from commercial sunflower fields in the U.S. states of Minnesota, Nebraska, North Dakota, and South Dakota. Total RNA extracts depleted of rRNA from each fungus were pooled to construct one library for sequencing to obtain 20 GB per library of raw reads using a metatranscriptomics approach. Only the family Mitoviridae was present in both Diaporthe species. Twelve and nine novel viral contigs were discovered infecting D. gulyae and D. helianthi, respectively. Additionally, we detected two of the same viruses infecting D. helianthi, Helianthus annuus leaf-associated partitivirus 3 and 5, that were detected in a direct sunflower metatranscriptome reported before. Interestingly, Qinvirus, which is mostly known as a group of insect viruses, was found in a contig. An ambivirus that is rarely reported in the phylum Ascomycota was also discovered in this study. Besides an understanding of virome diversity, the mycovirome survey provides the first clue of biological molecules that can be further developed for antifungal purposes.

An isolate of human blood-associated partitivirus naturally infects the phytopathogenic fungus Bipolaris maydis

Human blood-associated partitivirus (HuBPV) was first identified through metagenomic analysis of serum samples from two Peruvians, but its natural host remains unknown. Here, we report the detection of an HuBPV strain (HuBPV-Bm) in the phytopathogenic fungus Bipolaris maydis strain HN11 in Hubei Province, China. The dsRNA1 and dsRNA2 of HuBPV-Bm show more than 97.6% and 98.8% nucleotide sequence identity, respectively, to those from the metagenomically discovered HuBPV strain (HuBPV-M). Notably, HuBPV-Bm contains a third dsRNA segment that was not reported for HuBPV-M. All mycelia derived from individual asexual spores of HN11 tested positive for HuBPV-Bm, as did nine out of 293 B. maydis strains collected across Hubei.

Genomic characteristics of a novel non-segmented double-stranded RNA mycovirus from the fungus Nigrospora oryzae

In this study, a novel virus isolated from Nigrospora oryzae, tentatively named "Nigrospora oryzae mycovirus 1" (NoMyV1), was identified. NoMyV1 has a non-segmented dsRNA genome that is 2891 bp in length and contains two non-overlapping open reading frames (ORF1 and 2). ORF1 encodes a protein with sequence similarity to the putative capsid proteins or hypothetical proteins of other unclassified viruses, while ORF2 encodes an RNA-dependent RNA polymerase (RdRp). Sequence comparisons showed that NoMyV1 was most similar to Penicillium janczewskii Beauveria bassiana-like virus 1 (PjBblV1), with 76.12% amino acid sequence identity in the RdRp. In a phylogenetic analysis based on RdRp sequences, NoMyV1 was found to cluster with several other unclassified viruses for which a new genus, "Unirnavirus", which is distinct from the family Partitiviridae, has been proposed. Thus, we conclude that NoMyV1 is a novel member of the proposed genus "Unirnavirus".

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

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

Identification of a novel mycovirus belonging to the "flexivirus"-related family with icosahedral virion

The order Tymovirales currently comprises five viral families with positive-sense RNA [(+)RNA] genomes that infect plants, fungi, and insects. Virion morphologies within the order Tymovirales differ between families, with icosahedral virions in the Tymoviridae and filamentous virions in the other "flexi"viridae families. Despite their different morphologies, these viruses are placed in the same order based on phylogenetic analyses of replicase-associated polyproteins. However, one of the families in the Tymovirales, Deltaflexiviridae, is considered to be capsidless because there have been no published reports of virion isolation. Here, we report that a new "flexivirus"-related (+)RNA virus, prospectively named Fusarium oxysporum icosahedral virus 1 (FoIV1), is icosahedral and that most deltaflexiviruses may have icosahedral virions. Phylogenetic analyses based on replicase-associated polyproteins indicated that FoIV1 forms a distinct group in the Tymovirales with some viruses originally assigned to the Deltaflexiviridae. Electron microscopy, protein analysis, and protein structure predictions indicate that FoIV1 open reading frame 4 encodes a single jelly-roll (SJR)-like coat protein (CP) that constitutes the icosahedral virions. Results of clustering analyses based on amino acid sequences and predicted CP structures suggested that most of the deltaflexiviruses have icosahedral virions composed of SJR-like CPs as in FoIV1, rather than having filamentous virions or capsidless. These results challenge the conventional understanding of viruses in the order Tymovirales, with important implications for revising its taxonomic framework and providing insights into the evolutionary relationships within this diverse and broad host range group of (+)RNA viruses.

Restoring functional integrity of the global production ecosystem through biological control

Human society is anchored in the global agroecosystem. For millennia, this system has provided humans with copious supplies of nutrient-rich food. Yet, through chemical intensification and simplification, vast shares of present-day farmland derive insufficient benefits from biodiversity and prove highly vulnerable to biotic stressors. Here, we argue that on-farm action centered on biological control can effectively defuse pest risk by bolstering foundational ecosystem services. By harnessing plant, animal and microbial biodiversity, biological control offers safe, efficacious and economically-sound plant health solutions and coevolved options for invasive species mitigation. In recent years, its scientific foundation has been fortified and solutions have been refined for myriad ecologically brittle systems. Yet, for biological control to be mainstreamed, it needs to be rebooted, intertwined with (on- and off-farm) agroecological tactics and refurbished - from research, policy and regulation, public-private partnerships up to modes of implementation. Misaligned incentives (for chemical pesticides) and adoption barriers further need to be removed, while its scientific underpinnings should become more interdisciplinary, policy-relevant, solution-oriented and linked with market demand. Thus, biological control could ensure human wellbeing in a nature-friendly manner and retain farmland ecological functioning under global change.

Regional Variability of Chestnut (Castanea sativa) Tolerance Toward Blight Disease

Since its introduction into Europe in the first half of the 20th century, Cryphonectria parasitica has been gradually spreading across the natural range of the sweet chestnut (Castanea sativa Mill.), infecting the trees and causing lethal bark cankers. Serendipitously, a hyperparasitic Cryphonectria hypovirus 1 (CHV1), which attenuates C. parasitica virulence in combination with more tolerant European chestnut species, was able to ward off the worst effect of the disease. In North America, unfortunately, the native Castanea dentata is now functionally extinct since it occurs only as root sprouts in eastern deciduous forests where it was once dominant. In our work, we investigated changes in C. parasitica populations over time and the regional variability in chestnut populations' tolerance toward the blight disease. While vegetative compatibility (vc) type diversity and prevalence of hypovirulence remained similar as in previous studies, in the Buje population, unlike in previous studies, we were unable to find any hypovirulent fungal strains. The most common vegetative compatibility types (vc types) were EU-1, EU-2 and EU-12. However, several rare EU-types were found, including one previously unreported: EU-46. By inoculating several C. parasitica strains on tree stems from several chestnut populations, we observed that the induced lesion size was affected by the type of inoculum (CHV1-free or CHV1-infected), genotype-related individual chestnut stem and chestnut stem population of origin-related variability. The largest lesions were induced by CHV1-free fungal isolate DOB-G: 20.13 cm2 (95% C.I. 18.10-22.15) and the smallest by CHV1-infected L14/EP713: 2.49 cm2 (95% C.I. 1.59-3.39). Surprisingly, the size of the lesions induced by other CHV1-infected strains fell somewhere in between these extremes. The size of induced lesions was dependent on the population of origin as well and ranged from 11.60 cm2 (95% C.I. 9.87-13.33) for stems from the Moslavačka gora population to 17.75 cm2 (95% C.I. 15.63-19.87) for stems from Ozalj.

Reexamining the Mycovirome of Botrytis spp

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

Molecular characterization of a novel mycotombus‑like virus isolated from the phytopathogenic fungus Nigrospora oryzae

In this study, we identified a new mycotombus-like mycovirus from the phytopathogenic fungus Nigrospora oryzae, which was tentatively designated as "Nigrospora oryzae umbra-like virus 1" (NoULV1). The complete genome of NoULV1 is 3,381 nt long, containing two open reading frames (ORF1 and ORF2). ORF1 encodes a hypothetical protein with an unknown function, while ORF2 encodes an RNA-dependent RNA polymerase (RdRp) with a conserved RdRp domain containing a metal-binding 'GDN' triplet in motif C, which is distinct from the 'GDD' motif found in most + ssRNA mycoviruses. A homology search revealed that the RdRp encoded by ORF2 was similar to the RdRp of umbra-like mycoviruses. Phylogenetic analysis based on the RdRp indicated that NoULV1 was grouped into a clade together with umbra-like mycoviruses belonging to the proposed family "Mycotombusviridae".

Mycovirus-Containing Aspergillus flavus Alters Transcription Factors in Normal and Acute Lymphoblastic Leukemia Cells

Transcription factors control genes to maintain normal hemopoiesis, and dysregulation of some factors can lead to acute lymphoblastic leukemia (ALL). Mycoviruses are known to alter the genetics of their fungal host. The present study evaluates the effects of the products of a mycovirus-containing Aspergillus flavus (MCAF), isolated from the home of a patient with ALL, on certain transcription factors of normal and ALL cell lines. Our published studies have shown that ALL patients have antibodies to MCAF, and that exposure of the mononuclear leukocytes of patients in complete remission to its products, unlike controls, results in the re-development of genetic and cell surface phenotypes characteristic of ALL. For the present study, normal, pre-B, and B-cell leukemia cell lines were exposed to the culture of MCAF. Pre- and post-exposure levels of PAX5, Ikaros, and NF-κB were assessed. Exposure to MCAF resulted in apoptosis, cell cycle changes, and complete downregulation of all transcription factors in normal cell lines. In acute leukemia cell lines, cellular apoptosis and alterations in the cell cycle were also noted; however, while there was downregulation of all tested transcription factors, residual levels were retained. The noted alterations in the transcription factors caused by MCAF are novel findings. The possible role of MCAF in leukemogenesis needs to be further investigated. Mycovirus-containing Aspergillus flavus was initially isolated from a leukemia patient's home. Our prior published studies have illuminated intriguing associations of this organism with leukemia. Unlike controls, patients diagnosed with acute lymphoblastic leukemia (ALL) harbor antibodies to this organism. Furthermore, the exposure of mononuclear cells from patients with ALL in complete remission to the products of this organism reproduced genetic and cell phenotypes characteristic of ALL. These findings underscore the potential role of environmental factors in leukemogenesis and hint at novel avenues for therapeutic intervention and preventive strategies.

A novel ormycovirus isolated from the plant-pathogenic fungus Fusarium graminearum

In this study, we identified a novel mycovirus, Fusarium graminearum ormycovirus 1 (FgOV1), from the pathogenic fungus Fusarium graminearum. The virus has two RNA segments, RNA1 and RNA2, with lengths of 2,591 and 1,801 nucleotides, respectively, excluding the polyA tail. Each segment contains a single open reading frame (ORF). The ORF in RNA1 encodes an RNA-dependent RNA polymerase, while the ORF in RNA2 encodes a hypothetical protein. Phylogenetic analysis showed that FgOV1 belongs to the gammaormycovirus clade, whose members are related to betaormycoviruses. To our knowledge, this is the first report of an ormycovirus in Fusarium graminearum.

Transcriptomic analysis unveils alterations in the genetic expression profile of tree peony (Paeonia suffruticosa Andrews) infected by Alternaria alternata

BACKGROUND

Black spot disease in tree peony caused by the fungal necrotroph A. alternata, is a primary limiting factor in the production of the tree peony. The intricate molecular mechanisms underlying the tree peony resistance to A. alternata have not been thoroughly investigated.

RESULTS

The present study utilized high-throughput RNA sequencing (RNA-seq) technology to conduct global expression profiling, revealing an intricate network of genes implicated in the interaction between tree peony and A. alternata. RNA-Seq libraries were constructed from leaf samples and high-throughput sequenced using the BGISEQ-500 sequencing platform. Six distinct libraries were characterized. M1, M2 and M3 were derived from leaves that had undergone mock inoculation, while I1, I2 and I3 originated from leaves that had been inoculated with the pathogen. A range of 10.22-11.80 gigabases (Gb) of clean bases were generated, comprising 68,131,232 - 78,633,602 clean bases and 56,677 - 68,996 Unigenes. A grand total of 99,721 Unigenes were acquired, boasting a mean length of 1,266 base pairs. All these 99,721 Unigenes were annotated in various databases, including NR (Non-Redundant, 61.99%), NT (Nucleotide, 45.50%), SwissProt (46.32%), KEGG (Kyoto Encyclopedia of Genes and Genomes, 49.33%), KOG (clusters of euKaryotic Orthologous Groups, 50.18%), Pfam (Protein family, 47.16%), and GO (Gene Ontology, 34.86%). In total, 66,641 (66.83%) Unigenes had matches in at least one database. By conducting a comparative transcriptome analysis of the mock- and A. alternata-infected sample libraries, we found differentially expressed genes (DEGs) that are related to phytohormone signalling, pathogen recognition, active oxygen generation, and circadian rhythm regulation. Furthermore, multiple different kinds of transcription factors were identified. The expression levels of 10 selected genes were validated employing qRT-PCR (quantitative real-time PCR) to confirm RNA-Seq data.

CONCLUSIONS

A multitude of transcriptome sequences have been generated, thus offering a valuable genetic repository for further scholarly exploration on the immune mechanisms underlying the tree peony infected by A. alternata. While the expression of most DEGs increased, a few DEGs showed decreased expression.

A pooled mycoviral resource in a strain of Rhizoctonia solani are regulators of fungal virulence

Rhizoctonia solani is a widespread and devastating soil-borne plant fungal pathogen that causes diseases, including rice sheath blight, which are difficult to control. Some mycoviruses are potential biocontrol agents for the control of fungal diseases. In order to investigate the factors that influence the virulence of R. solani and search for mycoviruses with the potential for biocontrol of R. solani, a rice-infecting R. solani strain, ZJXD1-1, was isolated and confirmed to contain eight mycoviruses via dsRNA extraction and high-throughput sequencing. The identified mycoviruses belong to families of Endornaviridae (RsEV11 and RsEV12) and Mitoviridae (RsMV125 to RsMV129), and an unclassified Toti-like clade (RsTLV1). The C39 domain in RsEV12, which shares a close evolutionary relationship with bacteria, is observed for the first time in a mycovirus. Strains with different virus combinations were obtained through viral horizontal transfer, and pathogenicity test deduced that the Endornaviruses RsEV11 and RsEV12, and Mitovirus RsMV129 might potentially enhance the pathogenicity of R. solani, while RsMV125 might reduce the virulence or interfere with the function of other Mitoviruses. Furthermore, virus curing via protoplast regeneration and viral horizontal transfer demonstrated that RsMV129 is the causal agent of R. solani hypervirulence. Overall, our study provided the resource pool of viruses that may contribute to the discovery of new biocontrol agents against R. solani and enhance our understanding of the pathogenesis of R. solani regulated by mycoviruses.

Molecular characterization of a novel gammapartitivirus infecting the fungus Nigrospora oryzae

Here, we identified a new mycovirus infecting the phytopathogenic fungus Nigrospora oryzae, which we have designated "Nigrospora oryzae partitivirus 2" (NoPV2). The genome of NoPV2 consists of two dsRNA segments (dsRNA 1 and dsRNA 2), measuring 1771 and 1440 bp in length, respectively. dsRNA 1 and dsRNA 2 each contain a single open reading frame (ORF) that encodes the RNA-dependent RNA polymerase (RdRp) and capsid protein (CP), respectively. A BLASTp search showed that the RdRp of NoPV2 had significant sequence similarity to the RdRps of other partitiviruses, including Nigrospora sphaerica partitivirus 1 (75.61% identity) and Magnaporthe oryzae partitivirus 1 (67.53% identity). Phylogenetic analysis revealed that NoPV2 is a new member of the genus Gammapartitivirus in the family Partitiviridae. This study provides important information for understanding the diversity of mycoviruses in N. oryzae.

The Expanding Mycovirome of Aspergilli

Mycoviruses are viruses that infect fungi and are widespread across all major fungal taxa, exhibiting great biological diversity. Since their discovery in the 1960s, researchers have observed a myriad of fungal phenotypes altered due to mycoviral infection. In this review, we examine the nuanced world of mycoviruses in the context of the medically and agriculturally important fungal genus, Aspergillus. The advent of RNA sequencing has revealed a previous underestimate of viral prevalence in fungi, in particular linear single-stranded RNA viruses, and here we outline the diverse viral families known to date that contain mycoviruses infecting Aspergillus. Furthermore, we describe these novel mycoviruses, highlighting those with peculiar genome structures, such as a split RNA dependent RNA polymerase gene. Next, we delineate notable mycovirus-mediated phenotypes in Aspergillus, in particular reporting on observations of mycoviruses that affect their fungal host's virulence and explore how this may relate to virus-mediated decreased stress tolerance. Furthermore, mycovirus effects on microbial competition and antifungal resistance are discussed. The factors that influence the manifestation of these phenotypes, such as temperature, fungal life stage, and infection with multiple viruses, among others, are also evaluated. In addition, we attempt to elucidate the molecular mechanisms that underpin these phenotypes, examining how mycoviruses can be targets, triggers, and even suppressors of RNA silencing and how this can affect fungal gene expression and phenotypes. Finally, we highlight the potential therapeutic applications of mycoviruses and how, in an approach analogous to bacteriophage therapy, their ability to produce hypovirulence in Aspergillus might be used to attenuate invasive aspergillosis infections in humans.

Complete genome analysis of a novel hypovirus in the phytopathogenic fungus Monilinia fructicola

Monilinia fructicola is one of the most devastating fungal diseases of rosaceous fruit crops, both in the field and postharvest, causing significant yield losses. Here, we report the discovery of a novel positive single-stranded RNA virus, Monilinia fructicola hypovirus 3 (MfHV3), in a strain (hf-1) of the phytopathogenic fungus Monilinia fructicola. The complete genome of MfHV3 is 9259 nucleotides (nt) in length and contains a single large open reading frame (ORF) from nt position 462 to 8411. This ORF encodes a polyprotein with three conserved domains, namely UDP-glycosyltransferase, RNA-dependent RNA polymerase (RdRp), and DEAD-like helicase. The MfHV3 polyprotein shares the highest similarity with Colletotrichum camelliae hypovirus 1. Phylogenetic analysis indicated that MfHV3 clustered with members of the genus Betahypovirus within the family Hypoviridae. Taken together, the results of genomic organization comparisons, amino acid sequence alignments, and phylogenetic analysis convincingly show that MfHV3 is a new member of the genus Betahypovirus, family Hypoviridae.

Transmission of mycoviruses: new possibilities

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

First detection of mycoviruses in Gnomoniopsis castaneae suggests a putative horizontal gene transfer event between negative-sense and double-strand RNA viruses

Gnomoniopsis castaneae is an ascomycetous fungus mainly known as a major pathogen of chestnut causing nut rots, although it is often found as an endophyte in chestnut tissues. To date, no virus has been reported as associated with to this fungus. Here, a collection of G. castaneae isolates from several European countries was screened to detect mycoviruses infecting the fungus: for the first time we report the identification and prevalence of mitovirus Gnomoniopsis castaneae mitovirus 1 (GcMV1) and the chrysovirus Gnomoniopsis castaneae chrysovirus 1 (GcCV1). Interestingly, we provide evidence supporting a putative horizontal gene transfer between members of the phyla Negarnaviricota and Duplornaviricota: a small putative protein of unknown function encoded on the RNA3 of GcCV1 (Chrysoviridae) has homologs in the genome of viruses of the family Mymonaviridae.

Discovery of novel RNA viruses through analysis of fungi-associated next-generation sequencing data

BACKGROUND

Like all other species, fungi are susceptible to infection by viruses. The diversity of fungal viruses has been rapidly expanding in recent years due to the availability of advanced sequencing technologies. However, compared to other virome studies, the research on fungi-associated viruses remains limited.

RESULTS

In this study, we downloaded and analyzed over 200 public datasets from approximately 40 different Bioprojects to explore potential fungal-associated viral dark matter. A total of 12 novel viral sequences were identified, all of which are RNA viruses, with lengths ranging from 1,769 to 9,516 nucleotides. The amino acid sequence identity of all these viruses with any known virus is below 70%. Through phylogenetic analysis, these RNA viruses were classified into different orders or families, such as Mitoviridae, Benyviridae, Botourmiaviridae, Deltaflexiviridae, Mymonaviridae, Bunyavirales, and Partitiviridae. It is possible that these sequences represent new taxa at the level of family, genus, or species. Furthermore, a co-evolution analysis indicated that the evolutionary history of these viruses within their groups is largely driven by cross-species transmission events.

CONCLUSIONS

These findings are of significant importance for understanding the diversity, evolution, and relationships between genome structure and function of fungal viruses. However, further investigation is needed to study their interactions.

Characterization of Two Novel Single-Stranded RNA Viruses from Agroathelia rolfsii, the Causal Agent of Peanut Stem Rot

Peanut stem rot is a soil-borne disease caused by Agroathelia rolfsii. It occurs widely and seriously affects the peanut yield in most peanut-producing areas. The mycoviruses that induce the hypovirulence of some plant pathogenic fungi are potential resources for the biological control of fungal diseases in plants. Thus far, few mycoviruses have been found in A. rolfsii. In this study, two mitoviruses, namely, Agroathelia rolfsii mitovirus 1 (ArMV1) and Agroathelia rolfsii mitovirus 2 (ArMV2), were identified from the weakly virulent A. rolfsii strain GP3-1, and they were also found in other A. rolfsii isolates. High amounts of ArMV1 and ArMV2in the mycelium could reduce the virulence of A. rolfsii strains. This is the first report on the existence of mitoviruses in A. rolfsii. The results of this study may provide insights into the classification and evolution of mitoviruses in A. rolfsii and enable the exploration of the use of mycoviruses as biocontrol agents for the control of peanut stem rot.

Prevalence and species diversity of dsRNA mycoviruses from Beauveria bassiana strains in the China's Guniujiang nature

We investigated the prevalence and species diversity of dsRNA mycoviruses in Beauveria bassiana isolates from the China's Guniujiang Nature Preserve. Among the 28 isolates analyzed, electropherotyping revealed viral infections in 28.6 % (8 out of 28) of the isolates. Metatranscriptomic identification and RT-PCR confirmed the presence of six putative virus species, including two novel species: Beauveria bassiana victorivirus 2 (BbV-2) and Beauveria bassiana bipartite mycovirus 2 (BbBV-2). Four previously characterized mycoviruses were also identified: Beauveria bassiana polymycovirus 4 (BbPmV4), Beauveria bassiana partitivirus 1 (BbPV-1), Beauveria bassiana bipartite mycovirus 1 (BbBV-1), and Beauveria bassiana chrysovirus 2 (BbCV-2). BbPmV4 was found to be the prevailing mycovirus among the infected isolates, and three isolates showed co-infection with both BbPmV4 and BbBV-2. This study enhances our understanding of fungal viral taxonomy and diversity, providing insights into mycovirus infections in B. bassiana populations in China's Guniujiang Nature Preserve. Furthermore, the study on the diversity of B. bassiana viruses lays the foundation for recognizing fungal viruses as potential enhancers of biocontrol agents.

CpSmt3, an ortholog of small ubiquitin-like modifier, is essential for growth, organelle function, virulence, and antiviral defense in Cryphonectria parasitica

Introduction

SUMOylation is an important post-translational modification that regulates the expression, localization, and activity of substrate proteins, thereby participating in various important cellular processes such as the cell cycle, cell metabolism, gene transcription, and antiviral activity. However, the function of SUMOylation in phytopathogenic fungi has not yet been adequately explored.

Methods

A comprehensive analysis composed of proteomics, affinity pull-down, molecular and cellular approaches was performed to explore the roles of SUMOylation in Cryphonectria parasitica, the fungal pathogen responsible for chestnut blight.

Results and discussion

CpSmt3, the gene encoding the SUMO protein CpSmt3 in C. parasitica was identified and characterized. Deletion of the CpSmt3 gene resulted in defects in mycelial growth and hyphal morphology, suppression of sporulation, attenuation of virulence, weakening of stress tolerance, and elevated accumulation of hypovirus dsRNA. The ΔCpSmt3 deletion mutant exhibited an increase in mitochondrial ROS, swollen mitochondria, excess autophagy, and thickened cell walls. About 500 putative SUMO substrate proteins were identified by affinity pull-down, among which many were implicated in the cell cycle, ribosome, translation, and virulence. Proteomics and SUMO substrate analyses further revealed that deletion of CpSmt3 reduced the accumulation of CpRho1, an important protein that is involved in TOR signal transduction. Silencing of CpRho1 resulted in a phenotype similar to that of ΔCpSmt3, while overexpression of CpRho1 could partly rescue some of the prominent defects in ΔCpSmt3. Together, these findings demonstrate that SUMOylation by CpSmt3 is vitally important and provide new insights into the SUMOylation-related regulatory mechanisms in C. parasitica.

Possible Biological Control of Ash Dieback Using the Mycoparasite Hymenoscyphus Fraxineus Mitovirus 2

Invasive fungal diseases represent a major threat to forest ecosystems worldwide. As the application of fungicides is often unfeasible and not a sustainable solution, only a few other control options are available, including biological control. In this context, the use of parasitic mycoviruses as biocontrol agents of fungal pathogens has recently gained particular attention. Since the 1990s, the Asian fungus Hymenoscyphus fraxineus has been causing lethal ash dieback across Europe. In the present study, we investigated the biocontrol potential of the mitovirus Hymenoscyphus fraxineus mitovirus 2 (HfMV2) previously identified in Japanese populations of the pathogen. HfMV2 could be successfully introduced via co-culturing into 16 of 105 HfMV2-free isolates. Infection with HfMV2 had contrasting effects on fungal growth in vitro, from cryptic to detrimental or beneficial. Virus-infected H. fraxineus isolates whose growth was reduced by HfMV2 showed overall a lower virulence on ash (Fraxinus excelsior) saplings as compared with their isogenic HfMV2-free lines. The results suggest that mycoviruses exist in the native populations of H. fraxineus in Asia that have the potential for biological control of ash dieback in Europe. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Fusarium sacchari hypovirus 1, a Member of Hypoviridae with Virulence Attenuation Capacity in Phytopathogenic Fusarium Species

In a survey of mycoviruses in Fusarium species that cause sugarcane Pokkah boeng disease, twelve Fusarium strains from three Fusarium species (F. sacchari, F. andiyazi, and F. solani) were found to contain Fusarium sacchari hypovirus 1 (FsHV1), which we reported previously. The genomes of these variants range from 13,966 to 13,983 nucleotides, with 98.6% to 99.9% nucleotide sequence identity and 98.70% to 99.9% protein sequence similarity. Phylogenetic analysis placed these FsHV1 variants within the Alphahypovirus cluster of Hypoviridae. Intriguingly, no clear correlation was found between the geographic origin and host specificity of these viral variants. Additionally, six out of the twelve variants displayed segmental deletions of 1.5 to 1.8 kilobases, suggesting the existence of defective viral dsRNA. The presence of defective viral dsRNA led to a two-thirds reduction in the dsRNA of the wild-type viral genome, yet a tenfold increase in the total viral dsRNA content. To standardize virulence across natural strains, all FsHV1 strains were transferred into a single, virus-free Fusarium recipient strain, FZ06-VF, via mycelial fusion. Strains of Fusarium carrying FsHV1 exhibited suppressed pigment synthesis, diminished microspore production, and a marked decrease in virulence. Inoculation tests revealed varying capacities among different FsHV1 variants to modulate fungal virulence, with the strain harboring the FsHV1-FSA1 showing the lowest virulence, with a disease severity index (DSI) of 3.33, and the FsHV1-FS1 the highest (DSI = 17.66). The identification of highly virulent FsHV1 variants holds promise for the development of biocontrol agents for Pokkah boeng management.

Characterization of Mycoviruses in Armillaria ostoyae and A. cepistipes in the Czech Republic

Members of the genus Armillaria are widespread forest pathogens against which effective protection has not yet been developed. Due to their longevity and the creation of large-scale cloning of Armillaria individuals, the use of mycoviruses as biocontrol agents (BCAs) against these pathogens could be an effective alternative. This work describes the detection and characterization of viruses in Armillaria spp. collected in the Czech Republic through the application of stranded total RNA sequencing. A total of five single-stranded RNA viruses were detected in Armillaria ostoyae and A. cepistipes, including viruses of the family Tymoviridae and four viruses belonging to the recently described "ambivirus" group with a circular ambisense genome arrangement. Both hammerhead (HHRz) and hairpin (HpRz) ribozymes were detected in all the ambiviricot sequences. Armillaria viruses were compared through phylogenetic analysis and confirmed their specific host by direct RT-PCR. One virus appears to infect both Armillaria species, suggesting the occurrence of interspecies transmission in nature.

Comparative Analysis of Viromes Identified in Multiple Macrofungi

Macrofungi play important roles in the soil elemental cycle of terrestrial ecosystems. Fungal viruses are common in filamentous fungi, and some of them can affect the growth and development of hosts. However, the composition and evolution of macrofungal viruses are understudied. In this study, ninety strains of Trametes versicolor, Coprinellus micaceus, Amanita strobiliformis, and Trametes hirsuta were collected in China. Four mixed pools were generated by combining equal quantities of total RNA from each strain, according to the fungal species, and then subjected to RNA sequencing. The sequences were assembled, annotated, and then used for phylogenetic analysis. Twenty novel viruses or viral fragments were characterized from the four species of macrofungi. Based on the phylogenetic analysis, most of the viral contigs were classified into ten viral families or orders: Barnaviridae, Benyviridae, Botourmiaviridae, Deltaflexiviridae, Fusariviridae, Hypoviridae, Totiviridae, Mitoviridae, Mymonaviridae, and Bunyavirales. Of these, ambi-like viruses with circular genomes were widely distributed among the studied species. Furthermore, the number and overall abundance of viruses in these four species of macrofungi (Basidiomycota) were found to be much lower than those in broad-host phytopathogenic fungi (Ascomycota: Sclerotinia sclerotiorum, and Botrytis cinerea). By employing metatranscriptomic analysis in this study, for the first time, we demonstrated the presence of multiple mycoviruses in Amanita strobiliformis, Coprinellus micaceus, Trametes hirsute, and Trametes versicolor, significantly contributing to research on mycoviruses in macrofungi.

Complete genome sequence of a novel dsRNA virus from the phytopathogenic fungus Fusarium oxysporum

Fusarium oxysporum is a widespread plant pathogen that causes fusarium wilt and fusarium root rot in many economically significant crops. Here, a novel dsRNA virus tentatively named "Fusarium oxysporum virus 1" (FoV1) was identified in F. oxysporum strain 3S-18. The genome of FoV1 is 2,944 nucleotides (nt) in length and contains two non-overlapping open reading frames (ORF1 and 2). The larger of these, ORF2, encodes an RNA-dependent RNA polymerase (RdRp) of 590 amino acids with a molecular mass of 67.52 kDa. ORF1 encodes a putative nucleocapsid protein consisting of 134 amino acids with a molecular mass of 34.25 kDa. The RdRp domain of FoV1 shares 60.00% to 84.24% sequence identity with non-segmented dsRNA viruses. Phylogenetic analysis further suggested that FoV1 is a new member of the proposed genus "Unirnavirus" accommodating unclassified monopartite dsRNA viruses.

Mycoviruses Increase the Attractiveness of Fusarium graminearum for Fungivores and Suppress Production of the Mycotoxin Deoxynivalenol

RNA viruses of the genera Ambivirus, Mitovirus, Sclerotimonavirus, and Partitivirus were found in a single isolate of Fusarium graminearum. The genomes of the mitovirus, sclerotimonavirus, and partitivirus were assigned to previously described viruses, whereas the ambivirus genome putatively represents a new species, named Fusarium graminearum ambivirus 1 (FgAV1). To investigate the effect of mycoviruses on the fungal phenotype, the spontaneous loss of mycoviruses during meiosis and the transmission of mycoviruses into a new strain via anastomosis were used to obtain isogenic F. graminearum strains both with and without mycoviruses. Notable effects observed in mycovirus-harboring strains were (i) the suppression of the synthesis of trichothecene mycotoxins and their precursor trichodiene, (ii) the suppression of the synthesis of the defense compound aurofusarin, (iii) the stimulation of the emission of 2-methyl-1-butanol and 3-methyl-1-butanol, and (iv) the increased attractiveness of fungal mycelia for fungivorous collembolans. The increased attractiveness of mycovirus-infected filamentous fungi to animal predators opens new perspectives on the ecological implications of the infection of fungi with viruses.

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.

Molecular genetic variability of Cryphonectria hypovirus 1 associated with Cryphonectria parasitica in South Tyrol (northern Italy)

Cryphonectria hypovirus 1 (CHV-1) has been widely studied and used as a biocontrol agent because of its ability to infect the chestnut blight fungus, Cryphonectria parasitica, and to reduce its virulence. Knowledge about the hypovirus, its presence, and diversity is completely lacking in South Tyrol (northern Italy), which may obstruct biocontrol measures for chestnut blight based on CHV-1. This work aimed to study the occurrence of CHV-1 infecting C. parasitica in South Tyrol and to perform a genetic characterization of the hypovirus. In South Tyrol, CHV-1 was found to occur in 29.2% of the fungal isolates investigated, varying in frequency between different regions and chestnut stands. Twenty-three haplotypes based on partial cDNA (complementary DNA) sequences of open reading frame (ORF)-A and 30 haplotypes based on partial cDNA sequences of ORF-B were identified among 47 and 56 hypovirulent fungal isolates, respectively. Phylogenetic analysis showed that all the haplotypes belonged to the Italian subtype of CHV-1 and that they were closely related to the populations of Italy, Switzerland, Croatia and Slovenia. Evidence of recombination was not found in the sequences and point mutations were the main source of diversity. Overall, this study indicated that the prevalence of CHV-1 in South Tyrol is low compared to many other central and western European populations and determined a need to actively impose biocontrol measures. Using sequence analysis, we identified some variants of interest of CHV-1 that should be studied in detail for their potential use in biocontrol.

The Effects of Mycovirus BmPV36 on the Cell Structure and Transcription of Bipolaris maydis

Bipolaris maydis partitivirus 36 (BmPV36) is a mycovirus that can significantly reduce the virulence of the host Bipolaris maydis, but its hypovirulence mechanism is not clear. To investigate the response of B. maydis to BmPV36, the effects of BmPV36 on host cell structure and gene expression were studied via transmission electron microscopy and transcriptome sequencing using BmPV36-carrying and virus-free mycelium on the second and fifth culture. The results of transmission electron microscopy showed that the cell wall microfibrils of B. maydis were shortened, the cell membrane was broken, and membrane-bound vesicles and vacuoles appeared in the cells after carrying BmPV36. Transcriptome sequencing results showed that after carrying BmPV36, B. maydis membrane-related genes were significantly up-regulated, but membrane transport-related genes were significantly down-regulated. Genes related to carbohydrate macromolecule polysaccharide metabolic and catabolic processes were significantly down-regulated, as were genes related to the synthesis of toxins and cell wall degrading enzymes. Therefore, we speculated that BmPV36 reduces the virulence of B. maydis by destroying the host's cell structure, inhibiting the synthesis of toxins and cell wall degrading enzymes, and reducing cell metabolism. Gaining insights into the hypovirulence mechanism of mycoviruses will provide environmentally friendly strategies for the control of fungal diseases.

Greetings from virologists to mycologists: A review outlining viruses that live in fungi

Viruses are genetic elements that parasitize self-replicating cells. Therefore, organisms parasitized by viruses are not limited to animals and plants but also include microorganisms. Among these, viruses that parasitize fungi are known as mycoviruses. Mycoviruses with an RNA genome persistently replicate inside fungal cells and coevolve with their host cells, similar to a cellular organelle. Within host cells, mycoviruses can modulate various fungal characteristics and activities, including pathogenicity and the production of enzymes and secondary metabolites. In this review, we provide an overview of the mycovirus research field as introduction to fungal researchers. Recognition of all genetic elements in fungi aids towards better understanding and control of fungi, and makes fungi a significant model system for studying microorganisms containing multiple genetic elements.

Metagenomic Analyses of Viruses in the Orchid Mycorrhizal Interaction Using Improved Assemble Tools

In nature, mycorrhizal association with soil-borne fungi is indispensable for orchid species. Compatible mycorrhizal fungi form endo-mycorrhizal structures in orchid cells, and the fungal structures are digested in orchid cells to be supplied to orchids as nutrition. Because orchid seeds lack the reserves for germination, they keep receiving nutrition through mycorrhizal formation from seed germination until nonphotosynthetic young seedlings develop leaves and become photoautotrophic. Seeds of all orchids germinate with the help of their own fungal partners, and therefore, specific partnership has been acquired in a long evolutionary history between orchids and fungi. Assuming that horizontal transmission of viruses may occur in such a close relationship, we are focusing on viruses that infect orchids and their mycorrhizal fungi. We prepared aseptically germinated orchid plants (i.e., fungi-free plants) together with pure-cultured fungal isolates and conducted transcriptome analyses (RNA-seq) by next-generation sequencing (NGS) approach. To reconstruct virus-related sequences that would have been present in the RNA sample of interest, de novo assembly process is required using short read sequences obtained from RNA-seq. In the previous version of our protocol (see Viral Metagenomics, first edition 2018), virus searches were conducted using contig sets constructed by a single assembler, but this time we devised a method to construct more reliable contigs using multiple assemblers and again reinvestigated that viruses could be detected. Because the virus detection efficiency and number of detected virus species clearly differed depending on the assembly pipeline and the number of the input data, multiple methods should be used to identify viral infection, if possible.

Viruses of plant-pathogenic fungi: a promising biocontrol strategy for Sclerotinia sclerotiorum

Plant pathogenic fungi pose a significant and ongoing threat to agriculture and food security, causing economic losses and significantly reducing crop yields. Effectively managing these fungal diseases is crucial for sustaining agricultural productivity, and in this context, mycoviruses have emerged as a promising biocontrol option. These viruses alter the physiology of their fungal hosts and their interactions with the host plants. This review encompasses the extensive diversity of reported mycoviruses, including their taxonomic classification and range of fungal hosts. We highlight representative examples of mycoviruses that affect economically significant plant-pathogenic fungi and their distinctive characteristics, with a particular emphasis on mycoviruses impacting Sclerotinia sclerotiorum. These mycoviruses exhibit significant potential for biocontrol, supported by their specificity, efficacy, and environmental safety. This positions mycoviruses as valuable tools in crop protection against diseases caused by this pathogen, maintaining their study and application as promising research areas in agricultural biotechnology. The remarkable diversity of mycoviruses, coupled with their ability to infect a broad range of plant-pathogenic fungi, inspires optimism, and suggests that these viruses have the potential to serve as an effective management strategy against major fungi-causing plant diseases worldwide.

Spanish ecological battleground: population structure of two invasive fungi, Cryphonectria parasitica and Fusarium circinatum

Introduction

Invasive fungi distributed worldwide through globalization have caused devastating diseases in different forests, causing economic and ecologic disturbances. Two such invasive species are Cryphonectria parasitica and Fusarium circinatum, which were introduced to Europe from North America, separated temporally: C. parasitica was introduced about nine decades ago, whereas F. circinatum was introduced around two decades ago. As C. parasitica had a longer time to undergo genetic changes, we hypothesized that it has higher genetic diversity than the recently introduced F. circinatum in Spain. In addition, we studied the genetic characterization of both fungi present in similar ecological conditions in Northern Spain with the aim of providing data for biocontrol measures.

Methods

Molecular genetic markers were used to test these hypotheses, including mating type and DNA sequencing of internal transcribed spacer (ITS) regions. In addition, we used vegetative compatibility (VC) type markers in C. parasitica as the information about VC type is essential to apply biocontrol against the fungus.

Results and discussion

All the isolates of C. parasitica from the studied area belonged to only one VC type (EU-1) and one mating type (MAT-2). However, three distinct haplotypes of C. parasitica were identified through ITS sequencing, showing that multiple introductions might have happened to Cantabria. Among F. circinatum, no diversity was observed in ITS and MAT loci in the studied area but isolates from other Spanish regions showed the presence of both mating types. Overall, C. parasitica had higher genetic diversity than F. circinatum, despite both organisms appearing to reproduce clonally. This study helped understand the invasion patterns of C. parasitica and F. circinatum in northern Spain and will be useful in applying biocontrol measures against both pathogens.

Update on the state of research to manage Fusarium head blight

Fusarium head blight (FHB) is one of the most devastating diseases of cereal crops, causing severe reduction in yield and quality of grain worldwide. In the United States, the major causal agent of FHB is the mycotoxigenic fungus, Fusarium graminearum. The contamination of grain with mycotoxins, including deoxynivalenol and zearalenone, is a particularly serious concern due to its impact on the health of humans and livestock. For the past few decades, multidisciplinary studies have been conducted on management strategies designed to reduce the losses caused by FHB. However, effective management is still challenging due to the emergence of fungicide-tolerant strains of F. graminearum and the lack of highly resistant wheat and barley cultivars. This review presents multidisciplinary approaches that incorporate advances in genomics, genetic-engineering, new fungicide chemistries, applied biocontrol, and consideration of the disease cycle for management of FHB.

The Bcvic1 and Bcvic2 vegetative incompatibility genes in Botrytis cinerea encode proteins with domain architectures involved in allorecognition in other filamentous fungi

Vegetative incompatibility is a fungal allorecognition system characterised by the inability of genetically distinct conspecific fungal strains to form a viable heterokaryon and is controlled by multiple polymorphic loci termed vic (vegetative incompatibility) or het (heterokaryon incompatibility). We have genetically identified and characterised the first vic locus in the economically important, plant-pathogenic, necrotrophic fungus Botrytis cinerea. A bulked segregant approach coupled with whole genome Illumina sequencing of near-isogenic lines of B. cinerea was used to map a vic locus to a 60-kb region of the genome. Within that locus, we identified two adjacent, highly polymorphic open reading frames, Bcvic1 and Bcvic2, which encode predicted proteins that contain domain architectures implicated in vegetative incompatibility in other filamentous fungi. Bcvic1 encodes a predicted protein containing a putative serine esterase domain, a NACHT family of NTPases domain, and several Ankyrin repeats. Bcvic2 encodes a putative syntaxin protein containing a SNARE domain; such proteins typically function in vesicular transport. Deletion of Bcvic1 and Bcvic2 individually had no effect on vegetative incompatibility. However, deletion of the region containing both Bcvic1 and Bcvic2 resulted in mutant lines that were severely restricted in growth and showed loss of vegetative incompatibility. Complementation of these mutants by ectopic expression restored the growth and vegetative incompatibility phenotype, indicating that Bcvic1 and Bcvic2 are controlling vegetative incompatibility at this vic locus.

Molecular Characterization of Two Totiviruses from the Commensal Yeast Geotrichum candidum

Mycoviruses can infect many of the major taxa of fungi including yeasts. Mycoviruses in the yeast fungus Geotrichum candidum are not well studied with only three G. candidum-associated viral species characterized to date, all of which belong to the Totiviridae genus Totivirus. In this study, we report the molecular characteristics of another two totiviruses co-infecting isolate Gc6 of G. candidum. The two totiviruses were tentatively named Geotrichum candidum totivirus 2 isolate Gc6 (GcTV2-Gc6) and Geotrichum candidum totivirus 4 isolate Gc6 (GcTV4-Gc6). Both viruses have the typical genome organization of totiviruses comprising two ORFs encoding capsid protein (CP) and RNA-dependent RNA polymerase (RdRp) at the N and C termini, respectively. The genomes of GcTV2-Gc6 and GcTV4-Gc6 are 4592 and 4530 bp long, respectively. Both viruses contain the-frameshifting elements and their proteins could be expressed as a single fusion protein. GcTV2-Gc6 is closely related to a totivirus isolated from the same host whereas GcTV4-Gc6 is related to insect-associated totiviruses. The phylogenetic analysis indicated that GcTV2-Gc6 and GcTV4-Gc6 belong to two different sister clades, I-A and I-B, respectively. It is interesting that all viruses identified from G. candidum belong to the genus Totivirus; however, this might be due to the lack of research reporting the characterization of mycoviruses from this fungal host. It is possible that the RNA interference (RNAi) mechanism cannot actively suppress totivirus accumulation in G. candidum Gc6.

Horizontal and Vertical Transmission of a Mycovirus Closely Related to the Partitivirus RhsV717 That Confers Hypovirulence in Rhizoctonia solani

Rhizoctonia solani virus717 (RhsV717) was isolated from the Rhizoctonia solani (R. solani) AG-2 strain Rhs717. This study isolated a virus designated as Rhizoctonia solani partitivirus BS-5 (RsPV-BS5) from the R. solani AG-3 strain BS-5, the causal agent of tobacco target spot disease. The virus was identified as a strain of RhsV717. Transmission electron microscopy (TEM) images showed that RsPV-BS5 had virus particles with a diameter of approximately 40 nm. Importantly, it can be horizontally transmitted through hyphal anastomosis and vertically transmitted via sexual basidiospores. Furthermore, this study demonstrated that RsPV-BS5 infection significantly impedes mycelial growth and induces hypovirulence in tobacco leaves. Thus, RsPV-BS5 presents a promising avenue for biocontrolling tobacco target spot disease. Transcriptome analysis unveiled differential expression of four genes related to cell wall-degrading enzymes between two isogenic strains, 06-2-15V and 06-2-15. These findings shed light on the molecular mechanism through which RsPV-BS5 reduces host pathogenicity.

A Mycovirus VIGS Vector Confers Hypovirulence to a Plant Pathogenic Fungus to Control Wheat FHB

Mycovirus-mediated hypovirulence has the potential to control fungal diseases. However, the availability of hypovirulence-conferring mycoviruses for plant fungal disease control is limited as most fungal viruses are asymptomatic. In this study, the virus-induced gene silencing (VIGS) vector p26-D4 of Fusarium graminearum gemytripvirus 1 (FgGMTV1), a tripartite circular single-stranded DNA mycovirus, is successfully constructed to convert the causal fungus of cereal Fusarium head blight (FHB) into a hypovirulent strain. p26-D4, with an insert of a 75-150 bp fragment of the target reporter transgene transcript in both sense and antisense orientations, efficiently triggered gene silencing in Fusarium graminearum. Notably, the two hypovirulent strains, p26-D4-Tri101, and p26-D4-FgPP1, obtained by silencing the virulence-related genes Tri101 and FgPP1 with p26-D4, can be used as biocontrol agents to protect wheat from a fungal disease FHB and mycotoxin contamination at the field level. This study not only describes the first mycovirus-derived VIGS system but also proves that the VIGS vector can be used to establish multiple hypovirulent strains to control pathogenic fungi.

RNA viruses, M satellites, chromosomal killer genes, and killer/nonkiller phenotypes in the 100-genomes S. cerevisiae strains

We characterized previously identified RNA viruses (L-A, L-BC, 20S, and 23S), L-A-dependent M satellites (M1, M2, M28, and Mlus), and M satellite-dependent killer phenotypes in the Saccharomyces cerevisiae 100-genomes genetic resource population. L-BC was present in all strains, albeit in 2 distinct levels, L-BChi and L-BClo; the L-BC level is associated with the L-BC genotype. L-BChi, L-A, 20S, 23S, M1, M2, and Mlus (M28 was absent) were in fewer strains than the similarly inherited 2µ plasmid. Novel L-A-dependent phenotypes were identified. Ten M+ strains exhibited M satellite-dependent killing (K+) of at least 1 of the naturally M0 and cured M0 derivatives of the 100-genomes strains; in these M0 strains, sensitivities to K1+, K2+, and K28+ strains varied. Finally, to complement our M satellite-encoded killer toxin analysis, we assembled the chromosomal KHS1 and KHR1 killer genes and used naturally M0 and cured M0 derivatives of the 100-genomes strains to assess and characterize the chromosomal killer phenotypes.

BdCV1-Encoded P3 Silencing Suppressor Identification and Its Roles in Botryosphaeria dothidea, Causing Pear Ring Rot Disease

Pear ring rot disease is an important branch disease, caused by Botryosphaeria dothidea. With the discovery of fungal viruses, the use of their attenuated properties for biological control provides a new strategy for the biological control of fungal disease. RNA silencing is a major antiviral defense mechanism in plants, insects, and fungi. Viruses encode and utilize RNA silencing suppressors to suppress host defenses. Previous studies revealed that Botryosphaeria dothidea chrysovirus 1 (BdCV1) exhibited weak pathogenicity and could activate host gene silencing by infecting B. dothidea. The aim of our study was to investigate whether BdCV1 can encode a silencing suppressor and what effect it has on the host. In this study, the capability of silencing inhibitory activity of four BdCV1-encoded proteins was analyzed, and the P3 protein was identified as a BdCV1 RNA silencing suppressor in the exotic host Nicotiana benthamiana line 16c. In addition, we demonstrated that P3 could inhibit local silencing, block systemic RNA silencing, and induce the necrosis reaction of tobacco leaves. Furthermore, overexpression of P3 could slow down the growth rate and reduce the pathogenicity of B. dothidea, and to some extent affect the expression level of RNA silencing components and virus-derived siRNAs (vsiRNAs). Combined with transcriptomic analysis, P3 had an effect on the gene expression and biological process of B. dothidea. The obtained results provide new theoretical information for further study of interaction between BdCV1 P3 as a potential silencing suppressor and B. dothidea.

Comparative acetylomic analysis reveals differentially acetylated proteins regulating fungal metabolism in hypovirus-infected chestnut blight fungus

Cryphonectria parasitica, the chestnut blight fungus, and hypoviruses are excellent models for examining fungal pathogenesis and virus-host interactions. Increasing evidence suggests that lysine acetylation plays a regulatory role in cell processes and signalling. To understand protein regulation in C. parasitica by hypoviruses at the level of posttranslational modification, a label-free comparative acetylome analysis was performed in the fungus with or without Cryphonectria hypovirus 1 (CHV1) infection. Using enrichment of acetyl-peptides with a specific anti-acetyl-lysine antibody, followed by high accuracy liquid chromatography-tandem mass spectrometry analysis, 638 lysine acetylation sites were identified on 616 peptides, corresponding to 325 unique proteins. Further analysis revealed that 80 of 325 proteins were differentially acetylated between C. parasitica strain EP155 and EP155/CHV1-EP713, with 43 and 37 characterized as up- and down-regulated, respectively. Moreover, 75 and 65 distinct acetylated proteins were found in EP155 and EP155/CHV1-EP713, respectively. Bioinformatics analysis revealed that the differentially acetylated proteins were involved in various biological processes and were particularly enriched in metabolic processes. Differences in acetylation in C. parasitica citrate synthase, a key enzyme in the tricarboxylic acid cycle, were further validated by immunoprecipitation and western blotting. Site-specific mutagenesis and biochemical studies demonstrated that the acetylation of lysine-55 plays a vital role in the regulation of the enzymatic activity of C. parasitica citrate synthase in vitro and in vivo. These findings provide a valuable resource for the functional analysis of lysine acetylation in C. parasitica, as well as improving our understanding of fungal protein regulation by hypoviruses from a protein acetylation perspective.