Complete genome sequence of a novel mycovirus isolated from the phytopathogenic fungus Corynespora cassiicola in China

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.

Six Novel Mycoviruses Containing Positive Single-Stranded RNA and Double-Stranded RNA Genomes Co-Infect a Single Strain of the Rhizoctoniasolani AG-3 PT

Six novel mycoviruses that collectively represent the mycovirome of Rhizoctonia solani anastomosis group (AG)-3 PT strain ZJ-2H, which causes potato black scurf, were identified through metatranscriptome sequencing and putatively designated as Rhizoctonia solani fusarivirus 4 [RsFV4, positive single-stranded RNA (+ssRNA)], Rhizoctonia solani fusarivirus 5 (RsFV5, +ssRNA), Rhizoctonia solani mitovirus 40 (RsMV40, +ssRNA), Rhizoctonia solani partitivirus 10 [RsPV10, double-stranded RNA (dsRNA)], Rhizoctonia solani partitivirus 11 (RsPV11, dsRNA), and Rhizoctonia solani RNA virus 11 (RsRV11, dsRNA). Whole genome sequences of RsFV4, RsMV40, RsPV10, RsPV11, and RsRV11, as well as a partial genome sequence of RsFV5, were obtained. The 3'- and 5'- untranslated regions of the five mycoviruses with complete genome sequences were folded into stable stem-loop or panhandle secondary structures. RsFV4 and RsFV5 are most closely related to Rhizoctonia solani fusarivirus 1 (RsFV1), however, the first open reading frame (ORF) of RsFV4 and RsFV5 encode a hypothetical protein that differs from the first ORF of RsFV1, which encodes a helicase. We confirmed that RsPV10 and RsPV11 assemble into the spherical virus particles (approximately 30 nm in diameter) that were extracted from strain ZJ-2H. This is the first report that +ssRNA and dsRNA viruses co-infect a single strain of R. solani AG-3 PT.

Complete genome sequence of a novel fusarivirus from the phytopathogenic fungus Corynespora cassiicola

Corynespora cassiicola is an important phytopathogenic fungus that severely impairs crop production. Here, we report the molecular characterization of a novel positive-sense single-stranded RNA (+ssRNA) mycovirus, Corynespora cassiicola fusarivirus 1 (CcFV1), isolated from C. cassiicola strain 20200826-3-1. Excluding the poly(A) tail, the genome of the virus is 6491 nt in length and contains three putative open reading frames (ORFs). The large ORF1 encodes a polypeptide of 1524 aa with a conserved RNA-dependent RNA polymerase (RdRp) domain and a helicase (Hel) domain. BLASTp analysis showed that CcFV1 ORF1 has the highest similarity to Setosphaeria turcica fusarivirus 1 (StFV1, 50.45% identity, E-value 0.0). ORF2 encodes a polypeptide with a conserved chromosome segregation ATPase (Smc) domain. The smaller ORF3 encodes a polypeptide with an unknown function. Phylogenetic analysis based on the ORF1- encoded polypeptide showed that CcFV1 is phylogenetically related to members of the newly proposed family "Fusariviridae". Thus, we suggest that CcFV1 might be a novel member of the family "Fusariviridae", and is the first to be discovered in C. cassiicola.

Complete nucleotide sequence of a novel mycovirus infecting Rhizoctonia fumigata AG-Ba isolate C-314 Baishi

The complete nucleotide sequence of a novel mycovirus, designated as "Rhizoctonia fumigata bipartite virus 1" (RfBV1), from Rhizoctonia fumigata AG-Ba isolate C-314 Baishi was determined. The genome of RfBV1 is composed of two double-stranded RNAs (dsRNA). dsRNA-1 (2311 bp) contains one open reading frame (ORF), which codes for the putative RNA-dependent RNA polymerase (RdRp) of the virus. dsRNA-2 (1690 bp) contains one ORF, which encodes a putative protein whose function is unknown. Phylogenetic analysis indicated that the RdRp of RfBV1 clustered with several unassigned bipartite viruses belonging to the CThTV-like viruses group, but not the family Amalgaviridae or Partitiviridae. Our study suggests that RfBV1 is a novel mycovirus related to the CThTV-like viruses.

Molecular characterization of two dsRNAs that could correspond to the genome of a new mycovirus that infects the entomopathogenic fungus Beauveria bassiana

The entomopathogenic fungus Beauveria bassiana is used worldwide for biological control of insects. Seven dsRNA segments were detected in a single B. bassiana strain, RCEF1446. High-throughput sequencing indicated the presence of three mycoviruses in RCEF1446. Two were identified as the known mycoviruses Beauveria bassiana victorivirus 1 and Beauveria bassiana polymycovirus 1, and the novel mycovirus was designated as "Beauveria bassiana bipartite mycovirus 1" (BbBV1). The complete sequence of the BbBV1 is described here. The mycovirus contains two dsRNA segments. The RNA 1 (dsRNA 4) of BbBV1 is 2,026 bp in length, encoding a RNA-dependent RNA polymerase (RdRp) (68.54 kDa), while the RNA 2 (dsRNA 6) is 1,810 bp in length, encoding a hypothetical protein (35.55 kDa) with unknown function. Moreover, the amino acid sequence of RdRp showed the highest sequence identity of 62.31% to Botryosphaeria dothidea bipartite mycovirus 1. Phylogenetic analysis based on RdRp sequences revealed that BbBV1 represents a distinct lineage of unassigned dsRNA mycoviruses infecting fungi.

A novel mycovirus isolated from the plant-pathogenic fungus Botryosphaeria dothidea

A novel virus, Botryosphaeria dothidea bipartite mycovirus 1 (BdBMV1), was isolated from the plant-pathogenic fungus Botryosphaeria dothidea strain HNDT1, and the complete nucleotide sequence of its genome was determined. BdBMV1 consists of two genomic segments. The first segment is 1,976 bp in length and contains a single open reading frame (ORF) encoding the RNA-dependent RNA polymerase (RdRp) (68.95 kDa). The second segment is 1,786 bp in length and also contains a single ORF encoding a hypothetical protein of 35.19 kDa of unknown function. Based on the sequence of its RdRp, BdBMV1 is phylogenetically related to several other unclassified dsRNA mycoviruses, including Cryphonectria parasitica bipartite mycovirus 1 (CpBV1), and has a distant relationship to members of the family Partitiviridae.