Decomposition of Fomes fomentarius fruiting bodies is primarily driven by Arthropoda

Background. Fomes fomentarius is a white-rot fungus, with a long-lasting fruiting body, that plays a major role in the decomposition of deadwood in Northern Hemisphere forests. It is of scientific interest for its ecological role, potential as a structural material and use as a medicinal supplement. However, little is know about the metatranscriptome of the actively-growing fungus and how this changes during fruiting body decomposition.

Objectives. We hypothesised that the fresh fruiting body metatranscriptome would show the expression of deadwood decomposition-related enzymes, whereas the rotten fruiting bodies would serve as a growth substrate for fungivorous microbes.

Methods. Fresh and rotten fruiting bodies of F. fomentarius growing on beech trees (Fagus sylvatica) in Žofínský Prales National Nature Reserve in the Czech Republic were harvested in 2017. RNA was extracted using a NucleoSpin RNA plant kit and sequenced on an Illumina HiSeq 2500. Transcriptomes were assembled with Trinity and analysed in R. The gene expression of six fresh and three rotten transcriptomes was assessed with a focus on Carbohydrate Active enZymes (CAZymes).

Results. In fresh samples, 60-70% of the transcripts belonged to F. fomentarius with high expression of short, secreted proteins of unknown function. In rotten samples, fewer than 1% of the transcripts came from F. fomentarius and chitinases were produced by Tenebrionidae. Tenebrionidae and Opiidae both produced CAZymes targetting beta-glucans and were present in all rotten samples. We conclude that decomposition of F. fomentarius fruiting bodies is primarily due to insects, particularly Tenebrionidae beetles and Opiidae mites.