Mycoviruses as triggers and targets of RNA silencing in white mold fungus Sclerotinia sclerotiorum

Authors: MOCHAMA, PAULINE - SOUTH DAKOTA STATE UNIVERSITY; JADHAV, PRAJAKTA - SOUTH DAKOTA STATE UNIVERSITY; NEUPANE, ACHAL - SOUTH DAKOTA STATE UNIVERSITY; MARZANO, SHIN-YI LEE - SOUTH DAKOTA STATE UNIVERSITY

Submitted to: Viruses
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/20/2018
Publication Date: 4/22/2018
Citation: Mochama, P., Jadhav, P., Neupane, A., Marzano, S.-Y.L. 2018. Mycoviruses as triggers and targets of RNA silencing in white mold fungus Sclerotinia sclerotiorum. Viruses. 10:214. https://doi.org/10.3390/v10040214.
DOI: https://doi.org/10.3390/v10040214

Interpretive Summary: The fungus, Sclerotinia sclerotiorum, is a pathogen of numerous crop plants. It causes white mold disease, which damages plant tissues and can lead to significant yield losses. There are limited ways to control the fungus, with one proposed strategy being a biological approach in which the fungus is exposed to fungal-infecting viruses that reduce its ability to grow and reproduce. Although some of these viruses are known, we need more information on how the fungus responds to and can sometimes protect itself from these viruses. Thus, we studied how different fungal enzymes are able to silence invading viruses. We showed that when two specific enzymes were impaired in the fungus, the fungus was highly susceptible to viral infection. These results will help us identify new biological control strategies for combatting white mold disease.

Technical Abstract: This study aimed to demonstrate the existence of antiviral RNA silencing mechanisms in Sclerotinia sclerotiorum by infecting wild-type and RNA-silencing-deficient strains of the fungus with an RNA virus and a DNA virus. Key silencing-related genes were disrupted to dissect the RNA silencing pathway. Specifically, dicer genes (dcl-1, dcl-2, and both dcl-1/dcl-2) were displaced by selective marker(s). Disruption mutants were then compared for changes in phenotype, virulence, and susceptibility to virus infections. Wild-type and mutant strains were transfected with a single- stranded RNA virus, SsHV2-L, and copies of a single-stranded DNA mycovirus, SsHADV-1, as a synthetic virus constructed in this study. Disruption of dcl-1 or dcl-2 resulted in no changes in phenotype compared to wild-type S. sclerotiorum; however, the double dicer mutant strain exhibited significantly slower growth. Furthermore, the dcl-1/dcl-2 double mutant, which was slow growing without virus infection, exhibited much more severe debilitation following virus infections including phenotypic changes such as slower growth, reduced pigmentation, and delayed sclerotial formation. These phenotypic changes were absent in the single mutants, dcl-1 and dcl-2. Complementation of a single dicer in the double disruption mutant reversed viral susceptibility to the wild-type state. Virus-derived small RNAs were accumulated from virus-infected wild-type strains with strand bias towards the negative sense. The findings of these studies indicate that S. sclerotiorum has robust RNA silencing mechanisms that process both DNA and RNA mycoviruses and that, when both dicers are silenced, invasive nucleic acids can greatly debilitate the virulence of this fungus.