In temperate and boreal forest ecosystems, ectomycorrhizal (ECM) fungi mediate the supply of inorganic and monomeric organic N compounds to fuel plant growth. However, most N in the soils of these ecosystems is contained in soil organic matter (SOM) and is not thought to be available for plant uptake. Recent literature has challenged this idea, suggesting that ECM fungi commonly deploy oxidative extracellular enzymes to degrade SOM, obtain N bound therein, and transfer this N to the host plant. Here, we review and analyze literature concerning the evolutionary history and physiology of ECM fungi; we specifically discuss their capacity to enzymatically obtain N from SOM. Secondly, we advance several methodological approaches that aim to explore how differential evolutionary trajectories result in variation amongst ECM fungi in their ability to obtain, and transfer organic N to the host plant.
We present evidence that the putative ability for ECM fungi to obtain N from SOM and transfer this growth limiting nutrient to their plant hosts, depends foremost upon the evolutionary history of each ECM lineage. Because ECM fungi have repeatedly and differentially evolved from saprotrophic ancestors, some lineages appear to possess greater genetic capacity to produce extracellular enzymes capable of degrading SOM than do others. We suggest that variation amongst ECM lineages must be explicitly incorporated into models that wish to predict the ability of ECM fungi to metabolize SOM and liberate N for plant use.
Further, we highlight several additional conditions that must be experimentally validated before we can conclude that ECM lineages that have retained saprotrophic genes, actively transcribe them into extracellular enzymes that degrade SOM and transfer this N to their plant host for assimilation. Finally, we will present forthcoming results from whole-plant microcosms in which Cortinarius glaucopus and Suillus luteus were grown on intact Pinus strobus seedlings. The genomes of these fungal symbionts respectively contain multiple and no genes with saprotrophic function, presenting the possibility that they will differentially produce enzymes with the potential to operate on SOM.