Understanding virulence of Heterobasidion annosum s.l., a root rot pathogen of conifers

Heterobasidion annosum sensu lato (s.l.) species are destructive pathogens causing root and butt rot in conifers. The species complex consists of five species: H. annosum sensu stricto (s.s.), H. abietinum, H. parviporum, H. irregulare and H. occidentale. The aim of this thesis was to improve the understanding of fungal virulence in this species complex. The comparison of H. irregulare and H. occidentale transcriptomes revealed differences in the consistently significant up-regulated genes (CUGs) in Norway spruce (Picea abies) bark. It appears that more CUGs involved in detoxification and in the production of secondary metabolites are activated in H. irregulare. By contrast, H. occidentale emphasizes carbohydrate degradation. This enrichment of CUGs in particular gene ontology terms may be driven by their host preferences and by their evolutionary history. In H. irregulare, an endo-rhamnogalacturonase gene (HIRHG) from a virulence QTL was up-regulated during infection and the protein was mainly produced during growth on complex carbon sources. Although the HIRHG gene had been lost in most of the biotrophic and hemibiotrophic plant pathogens investigated, it was common in the necrotrophic pathogens and saprotrophs. Expression of HIRHG in Magnaporthe oryzae increased its capacity to grow on pectin, but did not significantly affect its virulence in our experimental set up. In parallel, the evolution of RNA interference (RNAi) was investigated to lay a foundation for the establishment of reverse genetics study tools. Dicer and argonaute are central to the functioning of the RNAi machinery required for gene silencing applications. The evolution of argonauteand dicer-encoding genes in 43 fungal genomes indicated an ancient duplication of dicer and argonaute genes concurrent with the early diversification of the Basidiomycota, followed by additional species-specific duplications and losses of a more recent origin. The quelling pathway possibly exists in most Basidiomycota; however, to date, no evidence for the meiotic silencing (MSUD) pathway has been found. Given that both argonaute and dicer are present, it should be possible to apply RNAi to study virulence genes in H. annosum s.l.