Chrysoviruses in Magnaporthe oryzae

The Interaction between Hypovirulence-Associated Chrysoviruses and Their Host Fusarium Species

Chrysoviruses are isometric virus particles (35-50 nm in diameter) with a genome composed of double-stranded RNAs (dsRNA). These viruses belonged to the Chrysoviridae family, named after the first member isolated from Penicillium chrysogenum. Phylogenetic classification has divided the chrysoviruses into Alphachrysovirus and Betachrysovirus genera. Currently, these chrysoviruses have been found to infect many fungi, including Fusarium species, and cause changes in the phenotype and decline in the pathogenicity of the host. Thus, it is a microbial resource with great biocontrol potential against Fusarium species, causing destructive plant diseases and substantial economic losses. This review provides a comprehensive overview of three chrysovirus isolates (Fusarium graminearum virus 2 (FgV2), Fusarium graminearum virus-ch9 (FgV-ch9), and Fusarium oxysporum f. sp. dianthi mycovirus 1 (FodV1)) reported to decline the pathogenicity of Fusarium hosts. It also summarizes the recent studies on host response regulation, host RNA interference, and chrysovirus transmission. The information provided in the review will be a reference for analyzing the interaction of Fusarium species with chrysovirus and proposing opportunities for research on the biocontrol of Fusarium diseases. Finally, we present reasons for conducting further studies on exploring the interaction between chrysoviruses and Fusarium and improving the accumulation and transmission efficiency of these chrysoviruses.

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.

Hypovirulence-associated mycovirus epidemics cause pathogenicity degeneration of Beauveria bassiana in the field

BACKGROUND

The entomogenous fungus Beauveria bassiana is used as a biological insecticide worldwide, wild B. bassiana strains with high pathogenicity in the field play an important role in controlling insect pests via not only screening of highly virulent strains but also natural infection, but the pathogenicity degeneration of wild strains severely affected aforementioned effects. Previous studies have showed that multiple factors contributed to this phenomenon. It has been extensively proved that the mycovirus infection caused hypovirulence of phytopathogenic fungi, which has been used for plant disease biocontrol. However, it remains unknown whether the mycovirus epidemics is a key factor causing hypovirulence of B. bassiana naturally in the field.

METHODS

Wild strains of B. bassiana were collected from different geographic locations in Jilin Province, China, to clarify the epidemic and diversity of the mycoviruses. A mycovirus Beauveria bassiana chrysovirus 2 (BbCV2) we have previously identified was employed to clarify its impact on the pathogenicity of host fungi B. bassiana against the larvae of insect pest Ostrinia furnacalis. The serological analysis was conducted by preparing polyclonal antibody against a BbCV2 coat protein, to determine whether it can dissociate outside the host fungal cells and subsequently infect new hosts. Transcriptome analysis was used to reveal the interactions between viruses and hosts.

RESULTS

We surprisingly found that the mycovirus BbCV2 was prevalent in the field as a core virus in wild B. bassiana strains, without obvious genetic differentiation, this virus possessed efficient and stable horizontal and vertical transmission capabilities. The serological results showed that the virus could not only replicate within but also dissociate outside the host cells, and the purified virions could infect B. bassiana by co-incubation. The virus infection causes B. bassiana hypovirulence. Transcriptome analysis revealed decreased expression of genes related to insect epidermis penetration, hypha growth and toxin metabolism in B. bassiana caused by mycovirus infection.

CONCLUSION

Beauveria bassiana infected by hypovirulence-associated mycovirus can spread the virus to new host strains after infecting insects, and cause the virus epidemics in the field. The findings confirmed that mycovirus infection may be an important factor affecting the pathogenicity degradation of B. bassiana in the field.

Detection of the Xanthi Chryso-like Virus in New Geographical Area and a Novel Arthropod Carrier

Here, we report on a serendipitous finding of a chryso-like virus associated with Culex pipiens mosquitos in the course of study aimed to detect and characterize West Nile virus (WNV) circulating in mosquitos in Serbia, Southern Europe. Upon initial detection of unexpected product in a PCR protocol for partial WNV NS5 gene amplification, further confirmation and identification was obtained through additional PCR and Sanger sequencing experiments. Bioinformatic and phylogenetic analysis identified the obtained sequences as Xanthi chryso-like virus (XCLV). The finding is particular for the fact that it associates XCLV with a new potential vector species and documents a novel geographical area of its distribution.

Transcriptomic analysis of entomopathogenic fungus Beauveria bassiana infected by a hypervirulent polymycovirus BbPmV-4

Polymycoviridae is a recently established family of mycoviruses. Beauveria bassiana polymycovirus 4 (BbPmV-4) was previously reported. However, the effect of the virus on host fungus B. bassiana was not clarified. Here, a comparison between virus-free and virus-infected isogenic lines of B. bassiana revealed that BbPmV-4 infection of B. bassiana changes morphology and could lead to decreases in conidiation and increases in virulence against Ostrinia furnacalis larvae. The differential expression of genes between virus-free and virus-infected strains was compared by RNA-Seq and was consistent with the phenotype of B. bassiana. The enhanced pathogenicity may be related to the significant up-regulation of genes encoding mitogen activated protein kinase, cytochrome P450, and polyketide synthase. The results enable studies of the mechanism of interaction between BbPmV-4 and B. bassiana.

Molecular and biological characteristics of a novel chrysovirus infecting the fungus phytopathogenic Setosphaeria turcica f.sp. sorghi

A new double-stranded RNA (dsRNA) virus has been identified in the filamentous fungus Setosphaeria turcica f.sp. sorghi, whose genome consists of four segments (dsRNA1-4). Each dsRNA carries single open reading frame (ORF) flanked by 5' and 3' untranslated regions (UTRs) containing strictly conserved termini. The putative protein encoded by dsRNA1 showed 80.50% identity to the RNA-dependent RNA polymerase (RdRp) of the most closely related virus, Alternaria alternata chrysovirus 1 (AaCV1), belonging to the Chrysoviridae. dsRNA2 encodes the putative coat protein, while dsRNA3 and dsRNA4 respectively encode the hypothetical proteins of unknown functions. Phylogenetic analysis based on the RdRp protein indicated the virus clustered with members of the genus Betachrysovirus in the family Chrysoviridae. Based on the dsRNA profile, amino acid sequence comparisons, and phylogenetic analyses, the mycovirus is thought to be a new member of the family Chrysoviridae and designated as Setosphaeria turcica chrysovirus 1 (StCV1). Moreover, obvious differences were observed in the colony, mycelial and spore morphology between StCV1-infected and virus-cured strains of S. turcica f.sp. sorghi. StCV1 infection strongly reduced colony growth rate, spore production ability and virulence on host fungus. To our knowledge, this is the first report about mycovirus infecting S. turcica f.sp. sorghi and also the first chrysovirus infecting S. turcica.

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

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

Research Progress on Phytopathogenic Fungi and Their Role as Biocontrol Agents

Phytopathogenic fungi decrease crop yield and quality and cause huge losses in agricultural production. To prevent the occurrence of crop diseases and insect pests, farmers have to use many synthetic chemical pesticides. The extensive use of these pesticides has resulted in a series of environmental and ecological problems, such as the increase in resistant weed populations, soil compaction, and water pollution, which seriously affect the sustainable development of agriculture. This review discusses the main advances in research on plant-pathogenic fungi in terms of their pathogenic factors such as cell wall-degrading enzymes, toxins, growth regulators, effector proteins, and fungal viruses, as well as their application as biocontrol agents for plant pests, diseases, and weeds. Finally, further studies on plant-pathogenic fungal resources with better biocontrol effects can help find new beneficial microbial resources that can control diseases.

ICTV Virus Taxonomy Profile: Chrysoviridae

Members of the family Chrysoviridae are isometric, non-enveloped viruses with segmented, linear, dsRNA genomes. There are 3–7 genomic segments, each of which is individually encapsidated. Chrysoviruses infect fungi, plants and possibly insects, and may cause hypovirulence in their fungal hosts. Chrysoviruses have no known vectors and lack an extracellular phase to their replication cycle; they are transmitted via intracellular routes within an individual during hyphal growth, in asexual or sexual spores, or between individuals via hyphal anastomosis. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on the taxonomy of the family Chrysoviridae, which is available at ictv.global/report/chrysoviridae.

Morphological Characterization and Genetic Diversity of Rice Blast Fungus, Pyricularia oryzae, from Thailand Using ISSR and SRAP Markers

Rice blast disease is caused by the ascomycete fungus Pyricularia oryzae and is one of the most destructive rice diseases in the world. The objectives of this study were investigating various fungal morphological characteristics and performing a phylogenetic analysis. Inter-simple sequence repeat (ISSR) and sequence-related amplified polymorphism (SRAP) markers were used to examine the genetic variation of 59 rice blast fungus strains, including 57 strains collected from different fields in Thailand and two reference strains, 70-15 and Guy11. All isolates used in this study were determined to be P. oryzae by internal transcribed spacer (ITS) sequence confirmation. A total of 14 ISSR primers and 17 pairs of SRAP primers, which produced clear and polymorphic bands, were selected for assessing genetic diversity. A total of 123 polymorphic bands were generated. The similarity index value for the strains ranged from 0.25 to 0.95. The results showed that the blast fungus population in Thailand has both morphological and genetic variations. A high level of genetic variation, or genome adaptation, is one of the fungal mechanisms that could overcome host resistance to avoid host recognition. Results from this research study could bring substantial benefits and ultimately help to understand the blast fungal pathogen genome and the population structure in Thai blast fungus.

Chrysoviruses Inhabited Symbiotic Fungi of Lichens

A lichen body is formed most often from green alga cells trapped in a net of ascomycetous fungi and accompanied by endolichenic or parasitic fungi, other algae, and symbiotic or free-living bacteria. The lichen’s microcosmos is inhabited by mites, insects, and other animals for which the lichen is a source of food or a place to live. Novel, four-segmented dsRNA viruses were detected in saxicolous Chrysothrixchlorina and Lepraria incana lichens. Comparison of encoded genome proteins revealed classification of the viruses to the genus Alphachrysovirus and a relationship to chrysoviruses from filamentous ascomycetous fungi. We propose the names Chrysothrix chrysovirus 1 (CcCV1) and Lepraria chrysovirus 1 (LiCV1) as acronyms for these viruses. Surprisingly, observation of Chrysothrix chlorina hybridization with fluorescent-labelled virus probe by confocal microscope revealed that the CcCV1 virus is not present in the lichen body-forming fungus but in accompanying endolichenic Penicilliumcitreosulfuratum fungus. These are the first descriptions of mycoviruses from a lichen environment.