Insights into the effects of serpentine soil conditions on the community composition of fungal symbionts in the roots of Onosma echioides

In this study, we investigated the impact of the extreme edaphic conditions of serpentine soils on the community structure of fungal symbionts in the roots of the perennial herb Onosma echioides (Boraginaceae). Sixteen root systems of O. echioides were obtained from two serpentine and two non-serpentine sites in the region of Tuscany (Italy) and the associated fungal communities were characterized by 454 pyrosequencing of fungal internal transcribed spacer 1 (ITS1) amplicon libraries. Clustering analysis of 173,639 sequence reads yielded a total of 699 non-singleton operational taxonomic units (OTUs), with the number of non-singleton OTUs per fungal community varying between 65 and 225. The richness of the fungal communities did not appear to be affected by the edaphic conditions of serpentine soils but the compositions of the serpentine and non-serpentine communities differed significantly, even though community structures were mainly influenced by strong spatial effects at low-to-medium distance scales. Whereas non-serpentine communities were dominated, at least in terms of sequence read abundance, by OTUs assigned to the genera Rhizophagus and Glomus (Glomeromycota), serpentine communities were dominated by OTUs assigned to the genera Ceratobasidium and Coprinellus (Basidiomycota). The functional roles of these basidiomycetous species in the roots of serpentine O. echioides specimens are, however, unclear and although their presence may affect the serpentine tolerance of the host plant, they may also be opportunistic symbionts that benefited from the reduced abundance of arbuscular mycorrhizal fungi to colonize the roots of O. echioides in serpentine environments.