The PoV mycovirus affects extracellular enzyme expression and fruiting body yield in the oyster mushroom, Pleurotus ostreatus.

Isogenic virus-cured and virus-infected fungal strains were previously obtained and compared to investigate mycoviral diseases and, specifically, the influence of viral infection on the vegetative growth of Pleurotus ostreatus. The present study demonstrated that infection with mycovirus PoV-ASI2792 (PoV) caused phenotypic and physiological changes in fungal cells and mycelia. The microscopically determined growth rate of the virus-infected strain was lower than that of the virus-cured strain, due to the conglomerate phenomenon during the mycelial growth process. An exploration of the viral effects of PoV on fruiting bodies yield showed significantly lower than that on virus-cured P. ostreatus. A colorimetric assay of polyphenol oxidase activity in the strains showed very weak activity in the virus-infected strain. To estimate the activity levels of enzymes related to the growth and fruiting body formation, the relative expression levels of genes encoding various extracellular enzymes such as Carbohydrate-Active Enzymes (CAZymes) were measured by quantitative RT-PCR. The expression levels of the assayed genes were significantly lower in virus-infected than in virus-cured P. ostreatus. Together, these results indicate that PoV infection affects the spawn growth and fruiting body formation of P. ostreatus via decreased expression and activity of some extracellular enzymes including lignocellulolytic enzymes.

Curing viruses in Pleurotus ostreatus by growth on a limited nutrient medium containing cAMP and rifamycin.

Oyster mushroom spherical virus (OMSV) and oyster mushroom isometric virus (OMIV) are the causative agents of a fruiting body deformation disease in the edible mushroom Pleurotus ostreatus. The curing of these mycoviruses was facilitated by a serial transfer of infected mycelia onto a limited nutrient medium containing 1mM of cAMP and 75 µg/ml of rifamycin (cAMP-rifamycin plate). The mycelia were grown on cAMP-rifamycin plates for 5 successive passages. ELISA and RT-PCR showed that the amount of mycoviruses inside the mycelia decreased significantly with increasing numbers of passages. The mycelia became free of viruses after 5 successive passages. Cultivation of the virus-cured mycelia on a mushroom compost medium produced a normal harvest, whereas the spawn infected with viruses failed to produce any fruiting bodies.

An application of protein microarray in the screening of monoclonal antibodies against the oyster mushroom spherical virus.

The oyster mushroom spherical virus (OMSV) is a causative agent of dieback disease in the oyster mushroom, Pleurotus ostreatus. Outbreaks of this virus occasionally result in serious disease that is associated with hefty economic losses. Thus, the detection and removal of OMSV-infected spawn is considered to be a crucial step for the stable production of P. ostreatus. For the detection of OMSV, we attempted to generate monoclonal antibodies (mAbs) against an RNA polymerase domain (RPD) of an OMSV protein. In an effort to simplify the laborious multistep mAb screening process, we developed a protein microarray on a slide glass that is chemically modified with the RPD protein. The culture supernatants of 87 hybridoma cells, which were prepared from the fusion of RPD-immunized mouse spleen cells with myeloma cells, were spotted onto the RPD-coated microarray. The binding of mAb to RPD was detected via Alexa 488 dye-labeled anti-mouse immunoglobulin G (IgG) as a secondary antibody. Of 87 samples, 13 evidenced a significant level of fluorescence signal intensity. Subsequent immunoblot analysis revealed that the specificity of each mAb against RPD coincided with the corresponding fluorescence signal intensity, thereby indicating the effectiveness of the protein microarray in mAb screening.

Characterization of a novel single-stranded RNA mycovirus in Pleurotus ostreatus.

A mycovirus, named oyster mushroom spherical virus (OMSV), was isolated from cultivated oyster mushrooms with a severe epidemic of oyster mushroom Die-back disease. OMSV was a 27-nm spherical virus encapsidating a single-stranded RNA (ssRNA) of 5.784 kb with a coat protein of approximately 28.5 kDa. The nucleotide sequence of the virus revealed that its genomic RNA was positive strand, containing 5784 bases with seven open reading frames (ORF). ORF1 had the motifs of RNA-dependent RNA polymerases (RdRp) and helicase. ORF2 encoded a coat protein. ORF3 to 7 could encode putative polypeptides of approximately 12, 12.5, 21, 14.5, and 23 kDa, respectively, but none of them showed significant similarity to any other known polypeptides. The 5' end of the viral RNA was uncapped and the 3' end was polyadenylated with 74 bases. Genomic structure and organization and the derived amino acid sequence of RdRp and helicase domain were similar to those of tymoviruses, a plant virus group.