l e t t e r sTo elucidate the genetic bases of mycorrhizal lifestyle evolution, we sequenced new fungal genomes, including 13 ectomycorrhizal (ECM), orchid (ORM) and ericoid (ERM) species, and five saprotrophs, which we analyzed along with other fungal genomes. Ectomycorrhizal fungi have a reduced complement of genes encoding plant cell walldegrading enzymes (PCWDEs), as compared to their ancestral wood decayers. Nevertheless, they have retained a unique array of PCWDEs, thus suggesting that they possess diverse abilities to decompose lignocellulose. Similar functional categories of nonorthologous genes are induced in symbiosis. Of induced genes, 7-38% are orphan genes, including genes that encode secreted effector-like proteins. Convergent evolution of the mycorrhizal habit in fungi occurred via the repeated evolution of a 'symbiosis toolkit', with reduced numbers of PCWDEs and lineage-specific suites of mycorrhiza-induced genes.Fungi are often described as either saprotrophs, which degrade complex organic substrates, or biotrophs, which obtain carbon compounds from living hosts. Among the latter, ECM fungi provide crucial ecological services in interacting with forest trees. They are portrayed as mutualists trading host photoassimilates for nutrients and having limited capacity to decompose soil lignocellulose 1-3 , as a result of their reduced repertoire of PCWDEs 4-6 . However, recent studies are challenging this view [7][8][9][10] . An improved understanding of the ability of ECM fungi to decompose lignocellulose is needed to resolve mechanisms of nutrient cycling in forests. The ECM lifestyle in Laccaria bicolor is associated with the expression of new mycorrhizainduced small secreted proteins (MiSSPs) that are required for establishment of symbiosis 11,12 . Mycorrhizal symbioses have arisen repeatedly during fungal evolution and include not only ECM associations but also those with ERM and ORM mycorrhizae 13 . It is not known whether these symbioses share the genomic features found in L. bicolor 4 and Tuber melanosporum 5 . Here we assess whether there Convergent losses of decay mechanisms and rapid turnover of symbiosis genes in mycorrhizal mutualists

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Environmental pollution and scarcity of natural resources lead to an increased interest in developing more sustainable materials. For example, the traditional construction industry, which is largely based on the extraction of fossil fuels and raw materials, is called into question. A solution can be found in biologically augmented materials that are made by growing mycelium-forming fungal microorganisms on natural fibres rich in cellulose, hemicellulose and lignin. In this way, organic waste streams, such as agricultural waste, are valorised while creating a material that is biodegradable at the end of its life cyclea process that fits in the spirit of circular economy. Mycelium-based materials have properties that are promising for a wide range of applications, including the use as construction materials. Despite this promise, the applicability and the practicality of these materials are largely unexplored and moreover, individual studies use a wide range of different experimental approaches and non-standardized procedures. In this review, we critically evaluate existing data on the composition of mycelium-based materials and process variables with the aim of providing a comprehensive framework of the production process. The framework illustrates the many input factors during the production that have an impact on the final characteristics of the material, and the unique potential to deploy more tuneable levels in the fabrications process that can serve to prototype a diversity of new unprecedented applications. Furthermore, we determine the applicability of existing data and identify knowledge gaps. This framework is valuable in identifying standardized approaches for future studies and in informing the design and process of new applications of mycelium-based materials.

Summary While there has been significant progress characterizing the ‘symbiotic toolkit’ of ectomycorrhizal (ECM) fungi, how host specificity may be encoded into ECM fungal genomes remains poorly understood. We conducted a comparative genomic analysis of ECM fungal host specialists and generalists, focusing on the specialist genus Suillus. Global analyses of genome dynamics across 46 species were assessed, along with targeted analyses of three classes of molecules previously identified as important determinants of host specificity: small secreted proteins (SSPs), secondary metabolites (SMs) and G‐protein coupled receptors (GPCRs). Relative to other ECM fungi, including other host specialists, Suillus had highly dynamic genomes including numerous rapidly evolving gene families and many domain expansions and contractions. Targeted analyses supported a role for SMs but not SSPs or GPCRs in Suillus host specificity. Phylogenomic‐based ancestral state reconstruction identified Larix as the ancestral host of Suillus, with multiple independent switches between white and red pine hosts. These results suggest that like other defining characteristics of the ECM lifestyle, host specificity is a dynamic process at the genome level. In the case of Suillus, both SMs and pathways involved in the deactivation of reactive oxygen species appear to be strongly associated with enhanced host specificity.

Understanding how environments shape evolutionary change is a long-standing topic in biology (Exposito-Alonso et al., 2018; Grant

Zinc (Zn) is an essential micronutrient but may become toxic when present in excess. In Zn-contaminated environments, trees can be protected from Zn toxicity by their root-associated micro-organisms, in particular ectomycorrhizal fungi. The mechanisms of cellular Zn homeostasis in ectomycorrhizal fungi and their contribution to the host tree’s Zn status are however not yet fully understood. The aim of this study was to identify and characterize transporters involved in Zn uptake in the ectomycorrhizal fungus Suillus luteus, a cosmopolitan pine mycobiont. Zn uptake in fungi is known to be predominantly governed by members of the ZIP (Zrt/IrtT-like protein) family of Zn transporters. Four ZIP transporter encoding genes were identified in the S. luteus genome. By in silico and phylogenetic analysis, one of these proteins, SlZRT1, was predicted to be a plasma membrane located Zn importer. Heterologous expression in yeast confirmed the predicted function and localization of the protein. A gene expression analysis via RT-qPCR was performed in S. luteus to establish whether SlZRT1 expression is affected by external Zn concentrations. SlZRT1 transcripts accumulated almost immediately, though transiently upon growth in the absence of Zn. Exposure to elevated concentrations of Zn resulted in a significant reduction of SlZRT1 transcripts within the first hour after initiation of the exposure. Altogether, the data support a role as cellular Zn importer for SlZRT1 and indicate a key role in cellular Zn uptake of S. luteus. Further research is needed to understand the eventual contribution of SlZRT1 to the Zn status of the host plant.

Summary The ectomycorrhizal symbiosis is a predominant tree–microbe interaction in forest ecosystems sustaining tree growth and health. Its establishment and functioning implies a long‐term and intimate relationship between the soil‐borne fungi and the roots of trees. Mycorrhiza‐induced Small‐Secreted Proteins (MiSSPs) are hypothesized as keystone symbiotic proteins, required to set up the symbiosis by modifying the host metabolism and/or building the symbiotic interfaces. L. bicolor MiSSP8 is the third most highly induced MiSSPs in symbiotic tissues and it is also expressed in fruiting bodies. The MiSSP8‐RNAi knockdown mutants are strongly impaired in their mycorrhization ability with Populus, with the lack of fungal mantle and Hartig net development due to the lack of hyphal aggregation. MiSSP8 C‐terminus displays a repetitive motif containing a kexin cleavage site, recognized by KEX2 in vitro. This suggests MiSSP8 protein might be cleaved into small peptides. Moreover, the MiSSP8 repetitive motif is found in other proteins predicted secreted by both saprotrophic and ectomycorrhizal fungi. Thus, our data indicate that MiSSP8 is a small‐secreted protein involved at early stages of ectomycorrhizal symbiosis, likely by regulating hyphal aggregation and pseudoparenchyma formation.