Bacteria Community Inhabiting Heterobasidion Fruiting Body and Associated Wood of Different Decay Classes

Microbial communities in the phyllosphere and endosphere of Norway spruce under attack by Heterobasidion

Heterobasidion annosum species complex has been regarded as the most destructive disease agent of conifer trees in boreal forests. Tree microbiome can regulate the plant-pathogen interactions by influencing both host resistance and pathogen virulence. Such information would help to improve the future health of forests and explore strategies to enhance ecosystem stability. In this study, using next-generation sequencing technology, we investigated the microbial community in different tree regions (needles, upper stem, and lower stem) of Norway spruce with and without wood decay symptoms. The primary purpose was to uncover signature characteristic microbiome harbored by asymptomatic trees compared to diseased trees. Additionally, the study was to explore the inter-kingdom and intra-kingdom interactions in microbiome (bacteria and fungi) of symptomatic versus asymptomatic trees. The results showed that in upper stem, species richness (Chao1) of fungi and bacteria were both higher in asymptomatic trees than symptomatic trees (P < 0.05). Compared to symptomatic trees, asymptomatic trees harbored a higher abundance of Actinobacteriota, bacterial genera of Methylocella, Conexibacter, Jatrophihabitans, and fungal genera of Mollisia. Fungal communities from the same anatomic region differed between the symptomatic and asymptomatic trees. Bacterial communities from the two stem regions were also distinct between the symptomatic and asymptomatic trees. The symptomatic trees possessed a less stable microbial network with more positive correlations compared to the asymptomatic trees. In the lower stem, at intra-kingdom level, the distribution of correlation numbers was more even in the bacterial network compared to the fungal network. In conclusion, the Heterobasidion attack decreased the microbial community species richness and shifted the community structure and functional structure to varying degrees. The microbial network was enlarged and became more unstable at both inter-kingdom and intra-kingdom level due to the Heterobasidion infection.

Decomposition of Fomes fomentatius fruiting bodies - transition of healthy living fungus into a decayed bacteria-rich habitat is primarily driven by Arthropoda

Fomes fomentarius is a widespread, wood-rotting fungus of temperate, broadleaved forests. Although the fruiting bodies of F. fomentarius persist for multiple years, little is known about its associated microbiome or how these recalcitrant structures are ultimately decomposed. Here we used metagenomics and metatranscriptomics to analyse the microbial community associated with healthy living and decomposing F. fomentarius fruiting bodies to assess the functional potential of the fruiting body-associated microbiome and to determine the main players involved in fruiting body decomposition. F. fomentarius sequences in the metagenomes were replaced by bacterial sequences as the fruiting body decomposed. Most CAZymes expressed in decomposing fruiting bodies targeted components of the fungal cell wall with almost all chitin-targeting sequences, plus a high proportion of beta-glucan-targeting sequences, belonging to Arthropoda. We suggest that decomposing fruiting bodies of F. fomentarius represent a habitat rich in bacteria, while its decomposition is primarily driven by Arthropoda. Decomposing fruiting bodies thus represent a specific habitat supporting both microorganisms and microfauna.

Endosymbiotic bacteria within the nematode-trapping fungus Arthrobotrys musiformis and their potential roles in nitrogen cycling

Endosymbiotic bacteria (ESB) have important effects on their hosts, contributing to its growth, reproduction and biological functions. Although the effects of exogenous bacteria on the trap formation of nematode-trapping fungi (NTF) have been revealed, the effects of ESB on NTF remain unknown. In this study, we investigated the species diversity of ESB in the NTF Arthrobotrys musiformis using high-throughput sequencing and culture-dependent approaches, and compared bacterial profiles to assess the effects of strain source and culture media on A. musiformis. PICRUSt2 and FAPROTAX were used to predict bacterial function. Our study revealed that bacterial communities in A. musiformis displayed high diversity and heterogeneity, with Proteobacteria, Firmicutes, Bacteroidetes and Actinobacteria as the dominant phyla. The ESB between A. musiformis groups isolated from different habitats and cultured in the same medium were more similar to each other than the other groups isolated from the same habitat but cultured in different media. Function analysis predicted a broad and diverse functional repertoire of ESB in A. musiformis, and unveiled that ESB have the potential to function in five modules of the nitrogen metabolism. We isolated nitrogen-fixing and denitrifying bacteria from the ESB and demonstrated their effects on trap formation of A. musiformis. Among seven bacteria that we tested, three bacterial species Bacillus licheniformis, Achromobacter xylosoxidans and Stenotrophomonas maltophilia were found to be efficient in inducing trap formation. In conclusion, this study revealed extensive ESB diversity within NTF and demonstrated that these bacteria likely play important roles in nitrogen cycling, including nematode trap formation.

Interkingdom and intrakingdom interactions in the microbiome of Heterobasidion fruiting body and associated decayed woody tissues

IMPORTANCE

We applied macro- (forest stand and forest management) and micro-scale (bacterial and fungal community) analyses for a better understanding of the Heterobasidion pathosystem and associated wood decay process. The core microbiome, as defined by hierarchy analysis and a consistent model, and environmental factors correlation with the community assembly were found to be novel.