Heterobasidion Partitivirus 13 Mediates Severe Growth Debilitation and Major Alterations in the Gene Expression of a Fungal Forest Pathogen

Abstract

The fungal genus <i>Heterobasidion</i> includes some of the most devastating conifer pathogens in the boreal forest region. In this study, we showed that the alphapartitivirus Heterobasidion partitivirus 13 from <i>Heterobasidion annosum</i> (HetPV13-an1) is the main causal agent of severe phenotypic debilitation in the host fungus. Based on RNA sequencing using isogenic virus-infected and cured fungal strains, HetPV13-an1 affected the transcription of 683 genes, of which 60% were downregulated and 40% upregulated. Alterations observed in carbohydrate and amino acid metabolism suggest that the virus causes a state of starvation, which is compensated for by alternative synthesis routes. We used dual cultures to transmit HetPV13-an1 into new strains of <i>H. annosum</i> and <i>Heterobasidion parviporum</i> The three strains of <i>H. parviporum</i> that acquired the virus showed noticeable growth reduction on rich culturing medium, while only two of six <i>H. annosum</i> isolates tested showed significant debilitation. Based on reverse transcription-quantitative PCR (RT-qPCR) analysis, the response toward HetPV13-an1 infection was somewhat different in <i>H. annosum</i> and <i>H. parviporum</i> We assessed the effects of HetPV13-an1 on the wood colonization efficacy of <i>H. parviporum</i> in a field experiment where 46 Norway spruce trees were inoculated with isogenic strains with or without the virus. The virus-infected <i>H. parviporum</i> strain showed considerably less growth within living trees than the isolate without HetPV13-an1, indicating that the virus also causes growth debilitation in natural substrates.<b>IMPORTANCE</b> A biocontrol method restricting the spread of <i>Heterobasidion</i> species would be highly beneficial to forestry, as these fungi are difficult to eradicate from diseased forest stands and cause approximate annual losses of €800 million in Europe. We used virus curing and reintroduction experiments and RNA sequencing to show that the alphapartitivirus HetPV13-an1 affects many basic cellular functions of the white rot wood decay fungus <i>Heterobasidion annosum</i>, which results in aberrant hyphal morphology and a low growth rate. Dual fungal cultures were used to introduce HetPV13-an1 into a new host species, <i>Heterobasidion parviporum</i>, and field experiments confirmed the capability of the virus to reduce the growth of <i>H. parviporum</i> in living spruce wood. Taken together, our results suggest that HetPV13-an1 shows potential for the development of a future biocontrol agent against <i>Heterobasidion</i> fungi.