Co-infection of a hypovirulent isolate of Sclerotinia sclerotiorum with a new botybirnavirus and a strain of a mitovirus.

Hongchang Ran,Lijiang Liu,Jiatao Xie,Daohong Jiang,Yanping Fu,Bo Li,Jiasen Cheng

doi:10.1186/s12985-016-0550-2

Abstract

BackgroundSclerotinia 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.ResultsWe 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.ConclusionsA 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.Electronic supplementary materialThe online version of this article (doi:10.1186/s12985-016-0550-2) contains supplementary material, which is available to authorized users.

Highlights

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, or DNA genomes
When L-double-stranded RNA (dsRNA) elements were resolved on 5 % PAGE gel for 48 h, two similar sized L-dsRNA segments (L1-dsRNA and L2-dsRNA) were obviously separated (Fig. 1b, right figure), revealing that purified virus-like particles (VLPs) contain at least 2 dsRNA species, and strain AH16 is infected by one or more mycoviruses
We subsequently demonstrated that the two L-dsRNA segments from the mycelia were associated with the purified VLPs, and further confirmed that they represented the genome of a new botybirnavirus, whereas SdsRNA was the genome of a mitovirus

Summary

Introduction

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. Mycoviruses usually have double-stranded RNA (dsRNA), single-stranded (ssRNA), and rare ssDNA genomes [4]. An EC-approved ICTV proposed family Botybirnaviridae (https://talk.ictvonline.org/files/ictv_official_taxonomy_updates_since_the_8th_report/m/fungal-official/5871) comprised three potential members of Botrytis porri botybirnavirus 1 (BpRV1), Sclerotinia sclerotiorum botybirnavirus 1 (SsBRV1), and soybean leaf-associated botybirnavirus 1 (SlaBRV1) [11,12,13]. These studies have expanded our knowledge of virus genome, taxonomy, evolution and the interaction between viruses and their hosts

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