| Summary: | The kingdom Fungi is one of the major groups of the plant microbiome(Hardoim et al., 2015; Vandenkoornhuyse et al., 2015; Peay et al., 2016). Of the various plant-fungus interactions, mycorrhizal fungi that form mutualistic associations with host plants are the best studied symbiotic system(Bonfante & Genre, 2010; van der Heijden et al., 2015). Fungal endophytes represent another major type of plant-fungus symbioses(Rodriguez et al., 2009; Porras-Alfaro & Bayman, 2011). Defined as endosymbionts inhabiting a wide range of plant and lichen hosts without causing obvious symptoms, endophytes are now considered both ubiquitous and hyperdiverse (Stone, 2004; Rodriguez et al., 2009; U'Ren et al., 2012). Yet most of these fungi have to be identified using a phylogenetic approach (Arnold et al., 2009; Gazis et al., 2012; Chen et al., 2015) and remain unknown at lower taxonomic ranks (e.g., genus and species) and undefined in terms of their function in their symptomless hosts(Arnold et al., 2003; Busby et al., 2016). It is now understood that some endophytes are capable of switching to pathogenic(Wipornpan Photita et al.; Ávarez-Loayza et al., 2011) or saprotrophic(U'Ren et al., 2010; Zuccaro et al., 2011; Kuo et al., 2014) modes, but the genetic mechanisms of these switches remain unexplored. Bryophytes are a major component of the vegetation in boreal and arctic regions, where ecosystems are most vulnerable to global climate change(Turetsky et al., 2012; Jassey et al., 2013). It has been proposed that early land plants adopted a terrestrial lifestyle with the help of fungi(Heckman et al., 2001; Field et al., 2015). Mosses do not have mutualistic fungal symbionts such as mycorrhizal fungi(Davey & Currah, 2006; Field et al., 2015), but they are known to harbor diverse fungal endophytes of uncertain functions(U'Ren et al., 2010; Davey et al., 2012; Davey et al., 2013). The growth form of the moss Dicranum scoparium provided an ideal system for studying functional transitions between endophytism and ...
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