Armillaria root rot fungi host single-stranded RNA viruses

Abstract

Species of Armillaria are distributed globally and include some of the most important pathogens of forest and ornamental trees. Some of them form large long-living clones that are considered as one of the largest organisms on earth and are capable of long-range spore-mediated transfer as well as vegetative spread by drought-resistant hyphal cords called rhizomorphs. However, the virus community infecting these species has remained unknown. In this study we used dsRNA screening and high-throughput sequencing to search for possible virus infections in a collection of Armillaria isolates representing three different species: Armillaria mellea from South Africa, A. borealis from Finland and Russia (Siberia) and A. cepistipes from Finland. Our analysis revealed the presence of both negative-sense RNA viruses and positive-sense RNA viruses, while no dsRNA viruses were detected. The viruses included putative new members of virus families Mymonaviridae, Botourmiaviridae and Virgaviridae and members of a recently discovered virus group tentatively named “ambiviruses” with ambisense bicistronic genomic organization. We demonstrated that Armillaria isolates can be cured of viruses by thermal treatment, which enables the examination of virus effects on host growth and phenotype using isogenic virus-infected and virus-free strains.

Towards the Forest Virome: Next-Generation-Sequencing Drastically Expands our Understanding on Virosphere in Temperate Forest Ecosystems

“…From the total of 33 (+)ssRNA viruses detected, 23 belong to the family Mitoviridae; 11 novel mitoviruses have been detected during the last five years applying NGS, while Cryphonectria parasitica mitovirus 1 [35], Gremmeniella mitoviruses 1 and 2 [36], Hymenoscyphus fraxineus mitovirus 1 [37] and Ophiostoma mitoviruses [38] were known earlier. Other (+)ssRNA viruses revealed by NGS methodology include a virga-like virus (unclassified Virgaviridae) and a botourmia-like virus (family Botourmiaviridae) in Armillaria spp [39]. NGS methodology has also allowed for first time the detection of (-)ssRNA RNA viruses in our target pathogens: Armillaria mellea negative strand RNA virus 1 [39] and Cryphonectria parasitica sclerotimonavirus 1 [40] (both putative members of Mymonaviridae).…”

“…Other (+)ssRNA viruses revealed by NGS methodology include a virga-like virus (unclassified Virgaviridae) and a botourmia-like virus (family Botourmiaviridae) in Armillaria spp [39]. NGS methodology has also allowed for first time the detection of (-)ssRNA RNA viruses in our target pathogens: Armillaria mellea negative strand RNA virus 1 [39] and Cryphonectria parasitica sclerotimonavirus 1 [40] (both putative members of Mymonaviridae). Finally, new unclassified ambisense viruses were very recently identified and characterized: Cryphonectria parasitica ambivirus 1-NB631 [40] and Armillaria ambi-like viruses [39].…”

“…NGS methodology has also allowed for first time the detection of (-)ssRNA RNA viruses in our target pathogens: Armillaria mellea negative strand RNA virus 1 [39] and Cryphonectria parasitica sclerotimonavirus 1 [40] (both putative members of Mymonaviridae). Finally, new unclassified ambisense viruses were very recently identified and characterized: Cryphonectria parasitica ambivirus 1-NB631 [40] and Armillaria ambi-like viruses [39]. Considering the soilborne pathogen fungus Rosellinia necatrix that infects many fruit trees and related natural trees, NGS has been shown to be a very efficient tool for the detection of viral diversity and led to the discovery of members of Hypoviridae, as well as unclassified "fusagraviruses" and "megatotiviruses" [41].…”

“…Furthermore, birch may be infected with Armillaria root rot fungi, which typically attack trees weakened by drought or other pathogens. A recent NGS study detected a novel virus group called "ambiviruses" in Armillaria borealis growing on birch wood [39].…”

“…Recently, NGS analysis revealed a Siberian isolate of A. borealis from Populus spp. to be infected by multiple viruses, including a novel ssRNA(+) virus named Armillaria borealis mycovirgavirus 1 and three ambi-like viruses [39].…”