Endophytic and ectomycorrhizal, an overlooked dual ecological niche? Insights from natural environments and Russula species

Ectomycorrhizal (EcM) fungi play key roles in ecosystem functioning, in particular temperate ones. Recent findings suggest that they can endophytically colonize the roots of non-EcM plants. Here we aim at (i) providing new evidence of colonization of non-EcM hosts by EcM fungi, (ii) exploring factors driving such colonization (plant identity, site, root filter), and (iii) providing direct microscopical evidence for endophytism. Using amplicon sequencing (ITS2), we described the root fungal communities of 42 plant species collected at nine locations in France. In two of those sites, we also compared rhizosphere and root fungal communities to identify a potential root filter. Finally, we investigated endophytism in Russula spp. at two Russula-rich sites using fluorescence in situ hybridization (FISH) paired with confocal microscopy. We find a large but variable share of EcM sequences in roots of non-EcM plant species, in particular nearby EcM hosts, suggesting that endophytism is a secondary ecological niche. Though EcM fungi were more abundant in the rhizosphere compared to roots, their composition was similar to that of roots, suggesting a poor root filter. We observed metabolically active hyphae of Russula spp. endophytically colonizing the apoplast of two non-EcM plant species. As shown for other EcM fungi (e.g., Tuber spp., Ascomycota) we demonstrate the dual EcM/endophyte niche for Russula (Basidiomycota). The ecological consequences of this duality still need to be addressed. The ability to colonize two ecological niches may be a trait kept by EcM fungi which evolved from endophytic fungi, as stipulated by the "waiting room hypothesis". ### Competing Interest Statement The authors have declared no competing interest.