Mycorrhizal symbioses—the union of roots and soil fungi—are universal in terrestrial ecosystems and may have been fundamental to land colonization by plants^1,2. Boreal, temperate and montane forests all depend on ectomycorrhizae¹. Identification of the primary factors that regulate symbiotic development and metabolic activity will therefore open the door to understanding the role of ectomycorrhizae in plant development and physiology, allowing the full ecological significance of this symbiosis to be explored. Here we report the genome sequence of the ectomycorrhizal basidiomycete Laccaria bicolor (Fig. 1) and highlight gene sets involved in rhizosphere colonization and symbiosis. This 65-megabase genome assembly contains ∼ 20,000 predicted protein-encoding genes and a very large number of transposons and repeated sequences. We detected unexpected genomic features, most notably a battery of effector-type small secreted proteins (SSPs) with unknown function, several of which are only expressed in symbiotic tissues. The most highly expressed SSP accumulates in the proliferating hyphae colonizing the host root. The ectomycorrhizae-specific SSPs probably have a decisive role in the establishment of the symbiosis. The unexpected observation that the genome of L. bicolor lacks carbohydrate-active enzymes involved in degradation of plant cell walls, but maintains the ability to degrade non-plant cell wall polysaccharides, reveals the dual saprotrophic and biotrophic lifestyle of the mycorrhizal fungus that enables it to grow within both soil and living plant roots. The predicted gene inventory of the L. bicolor genome, therefore, points to previously unknown mechanisms of symbiosis operating in biotrophic mycorrhizal fungi. The availability of this genome provides an unparalleled opportunity to develop a deeper understanding of the processes by which symbionts interact with plants within their ecosystem to perform vital functions in the carbon and nitrogen cycles that are fundamental to sustainable plant productivity.

Summary: Arbuscular mycorrhiza (AM) symbiosis occurs between obligate biotrophic fungi of the phylum Glomeromycota and most land plants. The exchange of nutrients between host plants and AM fungi (AMF) is presumed to be the main benefit for the two symbiotic partners. In this review article, we outline the current concepts of nutrient exchanges within this symbiosis (mechanisms and regulation). First, we focus on phosphorus and nitrogen transfer from the fungal partner to the host plant, and on the reciprocal transfer of carbon compounds, with a highlight on a possible interplay between nitrogen and phosphorus nutrition during AM symbiosis. We further discuss potential mechanisms of regulation of these nutrient exchanges linked to membrane dynamics. The review finally addresses the common mycorrhizal networks formed AMF, which interconnect plants from similar and/or different species. Finally the best way to integrate this knowledge and the ensuing potential benefits of AM into sustainable agriculture is discussed.

Summary: Data on the diversity and distribution of enzyme activities in native ectomycorrhizal (ECM) communities are inadequate. A microplate multiple enzymatic test was developed which makes it possible to measure eight enzyme activities on 14 individual, excised ECM root tips. Hydrolytic and oxidative enzymes are involved in the decomposition of lignocellulose, chitin and phosphorus‐containing organic compounds. This test system was used to describe the functional diversity of ECM communities in two forest sites. This set of tests proved to be accurate and sensitive enough to reveal a high diversity of activity profiles, depending on the fungal symbiont and the soil horizon. Ectomycorrhizas can be classified into specialists and generalists, and appear to complement each other in the same horizon to collectively perform all eight activities studied. By including a higher number of different assays for more detailed analyses, ECM activity profiling will provide a valuable tool for studying the functional diversity of ECM communities.