Vertical and horizontal gene transfer shaped plant colonization and biomass degradation in the fungal genus Armillaria

Members of the fungal genus Armillaria are necrotrophic pathogens with efficient plant biomass-degrading strategies. Armillaria species are some of the largest terrestrial organisms on Earth that cause tremendous losses in diverse ecosystems. Despite their global importance, how Armillaria evolved pathogenicity in a clade of dominantly non-pathogenic wood-degraders (Agaricales) remains elusive. Here, using new genomic data, we show that Armillaria species, in addition to widespread gene duplications and de novo gene origins, appear to have acquired at least 1025 genes via 124 horizontal gene transfer (HGT) events, primarily from Ascomycota donors. Functional and expression data suggest that HGT might have affected plant biomass-degrading and virulence abilities of Armillaria, two pivotal traits in their lifestyle. HGT provides an explanation for their soft-rot like biomass degrading strategy too, which is markedly different from the primarily white rot decay mechanism of related species. Combined multi-species expression data revealed putative virulence factors, extensive regulation of horizontally acquired and wood-decay related genes as well as novel noserved pathogenicity-induced small secreted proteins (PiSSPs). Two PiSSPs induced necrosis in live plants, suggesting they are potential virulence effectors conserved across Armillaria. Overall, this study details how evolution knitted together horizontally and vertically inherited genes in complex adaptive traits, such as plant biomass degradation and pathogenicityin one of the most influential fungal pathogens of temperate forest ecosystems. In vitro invasion assays using freshly cut conifer stems were set up to explore potential virulence factors in Armillaria analyzing the gene expression in highly and less-virulent isolates of the conifer-specific Armillaria ostoyae.