Summary Ectomycorrhizal fungi are thought to have a key role in mobilizing organic nitrogen that is trapped in soil organic matter (SOM). However, the extent to which ectomycorrhizal fungi decompose SOM and the mechanism by which they do so remain unclear, considering that they have lost many genes encoding lignocellulose‐degrading enzymes that are present in their saprotrophic ancestors.Spectroscopic analyses and transcriptome profiling were used to examine the mechanisms by which five species of ectomycorrhizal fungi, representing at least four origins of symbiosis, decompose SOM extracted from forest soils.In the presence of glucose and when acquiring nitrogen, all species converted the organic matter in the SOM extract using oxidative mechanisms. The transcriptome expressed during oxidative decomposition has diverged over evolutionary time. Each species expressed a different set of transcripts encoding proteins associated with oxidation of lignocellulose by saprotrophic fungi. The decomposition ‘toolbox’ has diverged through differences in the regulation of orthologous genes, the formation of new genes by gene duplications, and the recruitment of genes from diverse but functionally similar enzyme families.The capacity to oxidize SOM appears to be common among ectomycorrhizal fungi. We propose that the ancestral decay mechanisms used primarily to obtain carbon have been adapted in symbiosis to scavenge nutrients instead.

Many trees form ectomycorrhizal symbiosis with fungi. During symbiosis, the tree roots supply sugar to the fungi in exchange for nitrogen, and this process is critical for the nitrogen and carbon cycles in forest ecosystems. However, the extents to which ectomycorrhizal fungi can liberate nitrogen and modify the soil organic matter and the mechanisms by which they do so remain unclear since they have lost many enzymes for litter decomposition that were present in their free-living, saprotrophic ancestors. Using time-series spectroscopy and transcriptomics, we examined the ability of two ectomycorrhizal fungi from two independently evolved ectomycorrhizal lineages to mobilize soil organic nitrogen. Both species oxidized the organic matter and accessed the organic nitrogen. The expression of those events was controlled by the availability of glucose and inorganic nitrogen. Despite those similarities, the decomposition mechanisms, including the type of genes involved as well as the patterns of their expression, differed markedly between the two species. Our results suggest that in agreement with their diverse evolutionary origins, ectomycorrhizal fungi use different decomposition mechanisms to access organic nitrogen entrapped in soil organic matter. The timing and magnitude of the expression of the decomposition activity can be controlled by the below-ground nitrogen quality and the above-ground carbon supply.

SummarySoil is a dynamic system in which microorganisms perform important tasks in organic matter transformations and nutrient cycles. Recently, some studies have started to focus on soil metaproteomics as a tool for understanding the function and the role of members of the microbial community. The aim of our work was to provide a review of soil proteomics by looking at the methodologies used in order to illustrate the challenges and gaps in this field, and to provide a broad perspective about the use and meaning of soil metaproteomics. The development of soil metaproteomics is influenced strongly by the extraction methods. Several methods are available but only a few provide an identification of soil proteins, while others extract proteins and are able to separate them by electrophoresis but do not provide an identification. The extraction of humic compounds together with proteins interferes with the latter's separation and identification, although some methods can avoid these chemical interferences. Nevertheless, the major problems regarding protein identification reside in the fact that soil is a poor source of proteins and that there is not enough sequencedatabase information for the identification of proteins by mass spectrometric analysis. Once these pitfalls have been solved, the identification of soil proteins may provide information about the biogeochemical potential of soils and pollutant degradation and act as an indicator of soil quality, identifying which proteins and microorganisms are affected by a degradation process. The development of soil metaproteomics opens the way to proteomic studies in other complex substrates, such as organic wastes. These studies can be a source of knowledge about the possibility of driven soil restoration in polluted and degraded areas with low organic matter content and even for the identification of enzymes and proteins with a potential biotechnological value.

Ectomycorrhizal fungi play a key role in mobilizing nutrients embedded in recalcitrant organic matter complexes, thereby increasing nutrient accessibility to the host plant. Recent studies have shown that during the assimilation of nutrients, the ectomycorrhizal fungus Paxillus involutus decomposes organic matter using an oxidative mechanism involving Fenton chemistry (Fe2+ + H2O2 + H+ → Fe3+ + ˙OH + H2O), similar to that of brown rot wood-decaying fungi. In such fungi, secreted metabolites are one of the components that drive one-electron reductions of Fe3+ and O2, generating Fenton chemistry reagents. Here we investigated whether such a mechanism is also implemented by P. involutus during organic matter decomposition. Activity-guided purification was performed to isolate the Fe3+-reducing principle secreted by P. involutus during growth on a maize compost extract. The Fe3+-reducing activity correlated with the presence of one compound. Mass spectrometry and nuclear magnetic resonance (NMR) identified this compound as the diarylcyclopentenone involutin. A major part of the involutin produced by P. involutus during organic matter decomposition was secreted into the medium, and the metabolite was not detected when the fungus was grown on a mineral nutrient medium. We also demonstrated that in the presence of H2O2, involutin has the capacity to drive an in vitro Fenton reaction via Fe3+ reduction. Our results show that the mechanism for the reduction of Fe3+ and the generation of hydroxyl radicals via Fenton chemistry by ectomycorrhizal fungi during organic matter decomposition is similar to that employed by the evolutionarily related brown rot saprotrophs during wood decay.

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