Symbiosis with ectomycorrhizal (ECM) fungi is an advantageous partnership for trees in nutrient-limited environments. Ectomycorrhizal fungi colonize the roots of their hosts and improve their access to nutrients, usually nitrogen (N) and, in exchange, trees deliver a significant portion of their photosynthetic carbon (C) to the fungi. This nutrient exchange affects key soil processes and nutrient cycling, as well as plant health, and is therefore central to forest ecosystem functioning. Due to their ecological importance, there is a need to more accurately understand ECM fungal mediated C and N movement within forest ecosystems such that we can better model and predict their role in soil processes both now and under future climate scenarios. There are a number of hurdles that we must overcome, however, before this is achievable such as understanding how the evolutionary history of ECM fungi and their inter-and intra-species variability affect their function. Further, there is currently no generally accepted universal mechanism that appears to govern the flux of nutrients between fungal and plant partners. Here, we consider the current state of knowledge on N acquisition and transport by ECM fungi and how C and N exchange may be related or affected by environmental conditions such as N availability. We emphasize the role that modern genomic analysis, molecular biology techniques and more comprehensive and standardized experimental designs may have in bringing cohesion to the numerous ecological studies in this area and assist us in better understanding this important symbiosis. These approaches will help to build unified models of nutrient exchange and develop diagnostic tools to study these fungi at various scales and environments.

Ectomycorrhizal (ECM) fungi are key players in forest carbon (C) sequestration, receiving a substantial proportion of photosynthetic C from their forest tree hosts in exchange for plant growth-limiting soil nutrients. However, it remains unknown whether the fungus or plant controls the quantum of C in this exchange, nor what mechanisms are involved. Here, we aimed to identify physiological and genetic properties of both partners that influence ECM C transfer. Using a microcosm system, stable isotope tracing, and transcriptomics, we quantified plant-to-fungus C transfer between the host plant Eucalyptus grandis and nine isolates of the ECM fungus Pisolithus microcarpus that range in their mycorrhization potential and investigated fungal growth characteristics and plant and fungal genes that correlated with C acquisition. We found that C acquisition by P. microcarpus correlated positively with both fungal biomass production and the expression of a subset of fungal C metabolism genes. In the plant, C transfer was not positively correlated to the number of colonised root tips, but rather to the expression of defence- and stress-related genes. These findings suggest that C acquisition by ECM fungi involves individual fungal demand for C and defence responses of the host against C drain.

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<p>Temperate and boreal forest trees are dependent on soil microorganisms for the acquisition of limiting nutrients, including phosphorus. These include ectomycorrhizal fungi, which form a symbiotic association with the roots of the trees, and soil-dwelling bacteria. The exact roles of and mechanisms used by ectomycorrhizal fungi and soil bacteria in plant phosphorus nutrition and phosphorus cycling are unclear, as are the effects of fungal identity and nutrient availability on these processes.</p><p>We compared the effects of inoculation with two species from the ectomycorrhizal fungal genus Pisolithus on the amounts of phosphorus available to and present in Eucalyptus grandis seedlings, under different levels of nitrogen fertilisation and atmospheric CO<sub>2</sub>. We then further explored the phosphorus-solubilising abilities of the fungi and soil bacterial community using in vitro plate assays, soil enzymatic assays and qPCR analyses.</p><p>We show evidence of synergistic interactions between the ectomycorrhizal fungi and soil bacterial community to improve phosphorus nutrition in the soil – interactions that are impacted by both nitrogen and CO<sub>2</sub> levels and the species of the fungus. Our findings expand the current understanding of how ectomycorrhizal fungi and soil bacteria contribute to forest tree phosphorus nutrition and reveal how this interaction has important implications for sustainable forest management practices and estimations of future climate impacts on forest ecosystems.</p>