Co-infection of a hypovirulent isolate of Sclerotinia sclerotiorum with a new botybirnavirus and a strain of a mitovirus.
BACKGROUND
Sclerotinia sclerotiorum, a notorious plant fungal pathogen, causes yield loss of many crops and vegetables, and is a natural host of a diverse viruses with positive-sense RNA (+ssRNA), negative-sense RNA (-ssRNA), double-stranded RNA (dsRNA), or DNA genomes. Mixed-infection with multiple related or unrelated mycoviruses is a common phenomenon in S. sclerotiorum. However, a single strain co-infected with dsRNA and + ssRNA viruses has not been reported in S. sclerotiorum.


RESULTS
We report two unrelated viruses, Sclerotinia sclerotiorum botybirnavirus 2 (SsBRV2) with a bipartite dsRNA genome and Sclerotinia sclerotiorum mitovirus 4 (SsMV4/AH16) with a + ssRNA genome, which were originally detected in a single hypovirulent strain AH16 of S. sclerotiorum. SsMV4/AH16 has a typical genome of mitovirus and is a strain of mitovirus SsMV4. The genome of SsBRV2 consists of two separated dsRNA segments. The large dsRNA segment is 6159 bp in length and only has a single open reading frame (ORF) encoding a putative 1868-aa polyprotein with a conserved RNA dependent RNA polymerase (RdRp) domain. The small dsRNA segment is 5872 bp in length and encodes a putative 1778-aa protein. Phylogenetic analysis using RdRp conserved domain sequences revealed that SsBRV2 is phylogenetically related to the previously reported three bipartite viruses SsBRV1, Botrytis porri RNA virus 1 (BpRV1), and soybean leaf-associated botybirnavirus 1 (SlaBRV1). Electron microscopy demonstrated that SsBRV2 forms rigid spherical virions with a diameter of approximately 40 nm in infected mycelia. The virion of SsBRV2 was successfully introduced into a virus-free strain, which provides conclusive evidence that SsBRV2 confers hypovirulence on phytopathogenic fungus S. sclerotiorum.


CONCLUSIONS
A bisegmented dsRNA virus (SsBRV2/AH16) and a nonsegmented + ssRNA virus (SsMV4/AH16) were characterized in a hypovirulent strain AH16 of S. sclerotiorum. SsMV4/AH16 is a strain of a reported mitovirus, whereas SsBRV2 is a new botybirnavirus. SsBRV2 is the causal agent of hypovirulence on S. sclerotiorum. Our findings supplied a first evidence that a single S. sclerotiorum strain is co-infected by dsRNA and + ssRNA mycoviruses.
State Key Laboratory of Agricultural Microbiology, the Provincial Key Lab of Plant Pathology of Hubei Province, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, 430070, China.
Hongchang
H
Ran
State Key Laboratory of Agricultural Microbiology, the Provincial Key Lab of Plant Pathology of Hubei Province, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, 430070, China.
Lijiang
L
Liu
State Key Laboratory of Agricultural Microbiology, the Provincial Key Lab of Plant Pathology of Hubei Province, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, 430070, China.
Bo
B
Li
State Key Laboratory of Agricultural Microbiology, the Provincial Key Lab of Plant Pathology of Hubei Province, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, 430070, China.
Jiasen
J
Cheng
State Key Laboratory of Agricultural Microbiology, the Provincial Key Lab of Plant Pathology of Hubei Province, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, 430070, China.
Yanping
Y
Fu
State Key Laboratory of Agricultural Microbiology, the Provincial Key Lab of Plant Pathology of Hubei Province, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, 430070, China.
Daohong
D
Jiang
State Key Laboratory of Agricultural Microbiology, the Provincial Key Lab of Plant Pathology of Hubei Province, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, 430070, China. jiataoxie@mail.hzau.edu.cn.
Jiatao
J
Xie
D001203:Ascomycota / Q000254:growth & development / Q000821:virology*; D000067148:Fungal Viruses / Q000145:classification* / Q000235:genetics / Q000302:isolation & purification*; D016679:Genome, Viral; D016366:Open Reading Frames; D012328:RNA Viruses / Q000145:classification* / Q000235:genetics / Q000302:isolation & purification*; D012367:RNA, Viral / Q000235:genetics; D017422:Sequence Analysis, DNA; D014764:Viral Proteins / Q000235:genetics
D012367:RNA, Viral; D014764:Viral Proteins
Botybirnavirus; Hypovirulence; Mitovirus; Mixed-infection; Mycovirus; Sclerotinia sclerotiorum