Abstract
Three dsRNAs, in sizes of approximately 2.5–5 kbp, were detected in the plant pathogenic fungus Nigrospora oryzae strain CS-7.5-4. Genomic analysis showed that the 5.0 kb dsRNA was a victorivirus named as Nigrospora oryzae victorivirus 2 (NoRV2). The genome of NoRV2 was 5166 bp in length containing two overlapping open reading frames (ORFs), ORF1 and ORF2. ORF1 was deduced to encode a coat protein (CP) showing homology to the CPs of viruses belonging to the Totiviridae family. The stop codon of ORF1 and the start codon of ORF2 were overlapped by the tetranucleotide sequence AUGA. ORF2 was predicted to encode an RNA-dependent RNA polymerase (RdRp), which was highly similar to the RdRps of victoriviruses. Virus-like particle examination demonstrated that the genome of NoRV2 was solely encapsidated by viral particles with a diameter of approximately 35 nm. The other two dsRNAs that were less than 3.0 kb were predicted to be the genomes of two mitoviruses, named as Nigrospora oryzae mitovirus 1 (NoMV1) and Nigrospora oryzae mitovirus 2 (NoMV2). Both NoMV1 and NoMV2 were A-U rich and with lengths of 2865 and 2507 bp, respectively. Mitochondrial codon usage inferred that each of the two mitoviruses contains a major large ORF encoding a mitoviral RdRp. Horizontal transfer experiments showed that the NoMV1 and NoMV2 could be cotransmitted horizontally via hyphal contact to other virus-free N. oryzae strains and causes phenotypic change to the recipient, such as an increase in growth rate. This is the first report of mitoviruses in N. oryzae.
Highlights
Fungal viruses, called mycoviruses, are widespread in diverse fungal species
Several other mycoviruses with their genomes composed of DNA or negative-sense ssRNA have been reported in plant pathogenic fungus Sclerotinia sclerotiorum [4,5]
We report the isolation and molecular characteristic of three mycoviruses co-infecting the N. oryzae, which designated Nigrospora oryzae victorivirus 2 (NoRV2), Nigrospora oryzae mitovirus 1, and Nigrospora oryzae mitovirus 2
Summary
Most mycoviruses cause only cryptic infections, and their fungal hosts show no symptoms [1,2]. Some mycoviruses can cause hypovirulence to their hosts, leading to visible, debilitating symptoms. These viruses were considered to be potential biocontrol agents against plant pathogenic fungi, as has been well exemplified by the Cryphonectria hypovirus 1 (CHV1) in Cryphonectria parasitica, successfully used to control chestnut blight in Europe [3]. Mycoviruses contain either double-stranded (ds) RNA or positive single-stranded (+ss) RNA genomes. Several other mycoviruses with their genomes composed of DNA or negative-sense ssRNA have been reported in plant pathogenic fungus Sclerotinia sclerotiorum [4,5]. Based on current virus taxonomy, dsRNA mycoviruses are ascribed to seven families
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