Phenotypic Recovery of a Heterobasidion Isolate Infected by a Debilitation-Associated Virus Is Related to Altered Host Gene Expression and Reduced Virus Titer

Muhammad Kashif,Eeva J Vainio,Jaana Jurvansuu,Rafiqul Hyder,Jarkko Hantula

doi:10.3389/fmicb.2021.661554

Muhammad Kashif, Eeva J Vainio + Show 3 more

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https://doi.org/10.3389/fmicb.2021.661554

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Abstract

The fungal genus Heterobasidion includes forest pathogenic species hosting a diverse group of partitiviruses. They include the host debilitating Heterobasidion partitivirus 13 strain an1 (HetPV13-an1), which was originally observed in a slowly growing H. annosum strain 94233. In this study, a relatively fast-growing sector strain 94233-RC3 was isolated from a highly debilitated mycelial culture of 94233, and its gene expression and virus transcript quantities as well as the genomic sequence of HetPV13-an1 were examined. The sequence of HetPV13-an1 genome in 94233-RC3 was identical to that in the original 94233, and thus not the reason for the partial phenotypic recovery. According to RNA-seq analysis, the HetPV13-an1 infected 94233-RC3 transcribed eight genes differently from the partitivirus-free 94233-32D. Three of these genes were downregulated and five upregulated. The number of differentially expressed genes was considerably lower and the changes in their expression were small compared to those of the highly debilitated original strain 94233 with the exception of the most highly upregulated ones, and therefore viral effects on the host transcriptome correlated with the degree of the virus-caused debilitation. The amounts of RdRp and CP transcripts of HetPV13-an1 were considerably lower in 94233-RC3 and also in 94233 strain infected by a closely related mildly debilitating virus HetPV13-an2, suggesting that the virus titer would have a role in determining the effect of HetPV13 viruses on their hosts.

Highlights

Fungal viruses are found in a broad range of fungal taxa including Ascomycota and Basidiomycota as well as early diverging fungal lineages, such as Chytridiomycota, Blastocladiomycota, Neocallimastigomycota, Zoopagomycota, and Mucoromycota (Ghabrial and Suzuki, 2009; Pearson et al, 2009; Ghabrial et al, 2015; Sutela et al, 2019, 2020; Myers et al, 2020)
We aimed to investigate how the host can recover from the virus-induced growth retardation and the relevance of this phenomenon to the potential use of the virus as a biocontrol agent by (1) sequencing of the virus genome to determine the possibility of viral mutations, (2) analyzing the sector isolate’s gene expression to find changes that had occurred in the host, (3) quantifying viral genomes and transcripts in the recovered and debilitated isolates to determine whether changes had occurred in the virus quantity, and (4) analyzing the ratio of RNA-dependent RNA polymerase (RdRp) and coat protein (CP) transcripts of HetPV13-an1 among the isolates to test whether excess amount of viral polymerase transcripts correlates with a debilitated host phenotype as suggested earlier (Jurvansuu et al, 2014)
The presence of HetPV13-an1 in this fastgrowing isolate was verified by RT-qPCR with specific primers 13an1RdRpF and 13an1RdRpRev as described earlier (Vainio et al, 2015a)

Summary

Introduction

Fungal viruses (mycoviruses) are found in a broad range of fungal taxa including Ascomycota and Basidiomycota as well as early diverging fungal lineages, such as Chytridiomycota, Blastocladiomycota, Neocallimastigomycota, Zoopagomycota, and Mucoromycota (Ghabrial and Suzuki, 2009; Pearson et al, 2009; Ghabrial et al, 2015; Sutela et al, 2019, 2020; Myers et al, 2020). Viruses are transmitted among fungal strains by cell to cell contacts (hyphal anastomosis) and sexual or asexual spores Hollings (1962) reported for the first time, debilitated growth of Agaricus bisporus mycelium caused by mycovirus infection on artificial agar media. Mycoviruses have been reported to cause phenotypic alterations and hypovirulence (reduced virulence) in their hosts (Huang and Ghabrial, 1996; Preisig et al, 2000; Márquez et al, 2007; Yu et al, 2010; Hyder et al, 2013; Xiao et al, 2014; Vainio et al, 2018b). Different fungal strains may react differently to the same mycovirus strain (Hyder et al, 2013; Kim et al, 2015; Vainio et al, 2018b)

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