# Co-Infection of Three Novel Deltaflexiviruses in an Isolate of the Edible Fungus Pleurotus ostreatus Reduces Fruiting Body Yield by Inhibiting Cellulase Activity

## TL;DR

Three new viruses found in oyster mushrooms reduce their yield by inhibiting an enzyme needed for growth.

## Contribution

Discovery of three novel Deltaflexiviruses in Pleurotus ostreatus and their impact on cellulase activity and mushroom yield.

## Key findings

- The three novel viruses (PoDFV2, PoDFV3, PoDFV4) belong to the genus Deltaflexivirus.
- These viruses inhibit cellulase activity, leading to malformed fruiting bodies and reduced yield.
- The viruses do not significantly affect mycelial growth rate or laccase activity.

## Abstract

Pleurotus ostreatus, a globally cultivated oyster mushroom, is susceptible to viral infections that threaten yield and quality. This study reports the identification and characterization of three novel viruses from a symptomatic P. ostreatus strain K3: Pleurotus ostreatus deltaflexivirus 2, 3, and 4 (PoDFV2, PoDFV3, PoDFV4). Complete genome sequencing revealed that they are single-stranded, positive-sense RNA viruses with lengths of 7809 nt, 7771 nt, and 7786 nt, encoding 5, 2, and 4 open reading frames (ORFs), respectively. The largest open reading frame (ORF1) encodes a putative replication-associated polyprotein (RP) containing three conserved domains—viral RNA methyltransferase (Mtr), viral RNA helicase (Hel), and RNA-dependent RNA polymerase (RdRp). Based on genomic sequence analysis, multiple sequence alignments, and phylogenetic analysis, PoDFV2–4 were identified as novel viruses of the genus Deltaflexivirus within the family Deltaflexiviridae. PoDFV2–4 had no significant effects on mycelial growth rate, plate mycelial biomass, or laccase activity. However, they significantly inhibited mycelial cellulase activity and resulted in malformed fruiting bodies, as well as a substantial reduction in yield.