Insights into the Molecular Mechanisms of Litter Decomposition and Assimilation of Nitrogen by Ectomycorrhizal Fungi

Sammanfattning: Ectomycorrhizae is the dominant type of mycorrhiza found in association with tree roots in boreal and northern temperate forests. In this symbiosis, the fungal partner derives energy from photosynthates provided by the host trees and in return delivers soil-derived nutrients such as nitrogen (N). The majority of N in forest soils is embedded in recalcitrant organic matter–protein complexes. Ectomycorrhizal fungi (EMF) are thought to have a key role in decomposing and mobilizing nitrogen from such complexes. However, little is known about the mechanisms governing these decomposing processes, how they are regulated by C and N availability, and the mechanisms of organic N uptake. The work described in the thesis uses spectroscopic methods, chemical analysis and transcriptome profiling to examine the mechanisms by which the model ectomycorrhizal fungus Paxillus involutus degrades soil organic matter (SOM) while assimilating the organic nitrogen from plant litter. Finally, the decomposing ability of seven other species of EMF was examined that differ in ecology and evolutionary history. The EMF P. involutus degraded SOM, while assimilating N, by a radical-based oxidation involving Fenton chemistry similar to the mechanism used by saprophytic brown-rot (BR) fungi. The key indications were the apparition of a C=O peak in the signature of cellulose, the side chain modifications of lignin residues, and the increase in the Fe3+-reducing activity in the culture filtrate. The set of enzymes expressed during the degradation of SOM was similar to the set of enzymes involved in the oxidative degradation of wood by BR fungi. Secondary metabolites are key components for Fe3+-reduction and the generation of Fenton reagent in BR oxidative degradation of lignocellulose. The Fe3+-reducing activity of P. involutus was caused by the pigment involutin. The saprotrophic activity of P. involutus is reduced to a radical-based biodegradation system that efficiently disrupts the organic matter and thereby mobilizes the entrapped N. The decomposition of plant litter and assimilation of nitrogen was triggered by the addition of glucose while ammonium addition had minor effects. P. involutus secreted peptidase activity, mostly contributed by aspartic peptidases while degrading proteins. The expression levels of extracellular peptidases were regulated in parallel with transporters and enzymes involved in the assimilation and metabolism of the released peptides and amino acids. Finally, all the examined EMF species catalyzes oxidative degradation of complex organic components in the litter extract with a mechanism similar to that of BR fungi. The ability to modify complex organic material by oxidation is not restricted to rapidly-growing, long-distance exploration types of EMF, but it is also found in slow-growing, medium- and short-distance EMF exploration types. All examined EMF species expresses distinctively different sets oxidative enzymes to oxidize the litter material. Thus, EMF can degrade plant litter by oxidative mechanisms similar to BR while variation in gene expression might reflect adaptations of the decomposing mechanisms to different environmental conditions.