Genomics and Spectroscopy Provide Novel Insights into the Mechanisms of Litter Decomposition and Nitrogen Assimilation by Ectomycorrhizal Fungi

Abstract

The majority of nitrogen in forest soils is found in organic form, primarily as proteins. This nitrogen is mobilized and becomes available to trees as a result of the depolymerizing activities of symbiotic ectomycorrhizal fungi. To capture the nitrogen, the fungi must at least partly disrupt the recalcitrant organic matter–protein complexes within which the nitrogen is embedded. In this review, we will describe how a combination of spectroscopic methods and transcriptome analyses has provided novel insights into the mechanisms by which the ectomycorrhizal fungus Paxillus involutus decomposes organic matter when acquiring nitrogen from plant litter. The observed chemical changes were consistent with a hydroxyl-radical attack, involving Fenton chemistry similar to that of saprophytic brown-rot fungi. Unlike the saprophytic fungi, P. involutus did not show any expression of genes encoding extracellular enzymes needed to metabolize the released carbon. We suggest that these activities have been lost in ectomycorrhizal fungi as an adaptation to symbiotic growth on host photosynthate. Indeed experiments have shown that the decomposition of plant litter and assimilation of nitrogen are triggered by the addition of glucose. In contrast, the addition of ammonium, the most abundant inorganic N form in forest soils, had relatively minor effects of the decomposition of litter material by P. involutus. The data suggest that the expression of the decomposition and nitrogen assimilation processes can be tightly regulated by the host carbon supply. Finally, the prospects of using novel spectroscopic methods and transcriptomic data to identify specific transcripts or chemical signatures that can be used as biomarkers for probing the activity of mycorrhizal fungi in the field are discussed.