One of the most fundamental requirements for forest trees to flourish is the ability to acquire limited nutrients, such as nitrogen and phosphorus, and water from soil. As the levels of bioavailable inorganic nutrients in for- est soils are often too low to sustain plant growth, most trees rely on mycorrhizal fungal symbioses for their nutrition 1,2 . As such, the establishment of the mycor- rhizal lifestyle was a pivotal event in the evolutionary history of land plants 3,4 . Subsequently, soil-borne mycor- rhizal fungi, such as arbuscular mycorrhizal fungi and ectomycorrhizal fungi (BOX 1), helped to shape plant communities through mutualistic relationships with rhizoid-based rooting systems and roots 5–8 (BOX 2). A remarkable number of ectomycorrhizal basidiomy- cetes and ascomycetes (more than 20,000 species) have established symbioses with ~6,000 tree species, includ- ing pines, beeches, oaks, eucalypts, dipterocarps and poplars, whereas arbuscular mycorrhizal glomeromycetes have established symbioses with ~200,000 plant species, including poplars, eucalypts and some gymnosperms 2,6,9 . Thus, these symbioses have a broad influence on forest ecosystems. For example, extensive forests across the tem- perate, boreal, subtropical and mountainous ecoregions of the Northern Hemisphere and Southern Hemisphere are composed of tree species that have been colonized by ectomycorrhizal fungi 1,2,10 . In each of these forests, tril- lions of plant rootlets are colonized and interconnected by the mycelium of hundreds of different species of ecto- mycorrhizal fungi, forming extraradicular mycorrhizal net- works that have been informally termed the ‘wood-wide web’ (REFS 11–13). It is important to note that ectomycorrhizal fungi occupy a dual niche; that is, the soil and the host root. Similarly to their saprotrophic ancestors, ecto- mycorrhizal fungi have access to mineral nutrients in the soil that are efficiently absorbed by the perennial absorbing mycelial network and partly translocated to the host root 1,9,14 . However, ectomycorrhizal fungi have lost much of the ability of saprotrophic fungi to efficiently decay the lignocellulose that accumulates in wood and soil organic matter 15 . Adaptation to the ectomycorrhizal lifestyle has not only involved loss of functions; ecto- mycorrhizal fungi have also gained some of the mech- anisms that are used by biotrophic plant pathogens to colonize root tissues and capture host glucose 16–18 (although ectomycorrhizal fungi lack the parasitic mor- phological structures that are specific to pathogens, such as the haustorium). These adaptations suggest that ecto- mycorrhizal symbiosis provides a useful model for the study of the evolution of nutritional modes in fungi 19,20 . Although the vast majority of ectomycorrhizal fungi share a typical anatomical pattern — a hyphal network (known as the Hartig net) that forms inside root cells, a sheathing mantle around rootlets and extramatri- cal hyphae that explore the rhizosphere and nearby soil niches 14,21 (FIG. 1) — the phenotypic diversity of these fungi is broad, owing to variation in morphology, anatomy, physiology, host species and ecological specialization 2,6,9,21 . However, only a small number of ectomycorrhizal sym- bioses have been studied at a molecular level, as studies have mainly focused on the model associations between Laccaria bicolor and poplars, Hebeloma cylindrosporum Unearthing the roots of ectomycorrhizal symbioses Francis Martin 1 , Annegret Kohler 1 , Claude Murat 1 , Claire Veneault-Fourrey 2 and David S. Hibbett 3 Abstract | During the diversification of Fungi and the rise of conifer-dominated and angiosperm- dominated forests, mutualistic symbioses developed between certain trees and ectomycorrhizal fungi that enabled these trees to colonize boreal and temperate regions. The evolutionary success of these symbioses is evident from phylogenomic analyses that suggest that ectomycorrhizal fungi have arisen in approximately 60 independent saprotrophic lineages, which has led to the wide range of ectomycorrhizal associations that exist today. In this Review, we discuss recent genomic studies that have revealed the adaptations that seem to be fundamental to the convergent evolution of ectomycorrhizal fungi, including the loss of some metabolic functions and the acquisition of effectors that facilitate mutualistic interactions with host plants. Finally, we consider how these insights can be integrated into a model of the development of ectomycorrhizal symbioses. 1 Institut national de la recherche agronomique (INRA), Unité Mixte de Recherche 1136 Interactions Arbres/Microorganismes, Laboratoire d’excellence Recherches Avancés sur la Biologie de l’Arbre et les Ecosystèmes Forestiers (ARBRE), Centre INRA-Lorraine, 54280 Champenoux, France. 2 Université de Lorraine, Unité Mixte de Recherche 1136 Interactions Arbres/ Microorganismes, Laboratoire d’excellence Recherches Avancées sur la Biologie de l’Arbre et les Ecosystèmes Forestiers (ARBRE), 54500 Vandoeuvre-lès-Nancy, France. 3 Biology Department, Clark University, Lasry Center for Bioscience, 950 Main Street, Worcester, Massachusetts 01610, USA. Correspondence to F.M. francis.martin@inra.fr doi:10.1038/nrmicro.2016.149 Published online 31 Oct 2016 NATURE REVIEWS | MICROBIOLOGY ADVANCE ONLINE PUBLICATION | 1 REVIEWS ©2016MacmillanPublishersLimited,partofSpringerNature.Allrightsreserved.