A quest for microbial indicators of the Tuber melanosporum production using an environmental genomics approach

The "brule" - the area where the truffles tend to grow to a mature stage- is commonly characterized by a drastic drop in the plant diversity and biomass around those trees, which have been mycorrhized by Tuber melanosporum. Despite its presence, which indicates the development as well as the activity of the mycelium network of this symbiont, the "brule" is not systematically associated with the production of ascocarps. To date, understanding how truffle producing ecosystems function has been a puzzling task for researchers, particularly when attempting to explain what elements are involved in the spatio-temporal dynamics of the colonization process, in the intensity of the "brules" and in what influences the yield. In the frame of the ANR SYSTRUF project (2010-2014), and within a partnership with the professional bodies (FFT, CETEF) and some volunteer truffle growers, we have aimed to identify which bacterial and mycorrhizal (Glomeromycetes) markers are intrinsically associated with the productive status of the "brules", in relation to the non-producing "brules" and to the "brules" - deprived oaks. Two "natural"-spontaneous and slightly managed-truffle orchards (truffieres) and two "planted" ones were sampled during November 2010 in the Languedoc-Roussillon region (at the ascocarp maturation stage). These truffieres are composed of the evergreen holm oak (Quercus ilex) and are managed without mechanical soil laboring; in addition, they possess a spontaneous plant cover, which is kept to a minimal extent by the owners. Soil and root samples from plant species persisting on the "brule" have been collected around the oak trees (classified as: with a producing "brule", with a non-producing "brule", and "brule"-deprived ones) in order to (1) determine by pyrosequencing the genetic diversity of the total soil bacterial community (16S rDNA gene) and of the arbuscular mycorrhizae (18S rDNA gene) associated to the oak companion plants, and to (2) potentially identify specific taxa linked to the producing status of the "brule", at the level of a single truffiere or along the four sampling sites. Eventually, the approach of relating sequence data with the producing status and the floral diversity of the studied ecosystems opens up new avenues for the study of the interaction networks established between Tuber melanosporum and certain components of the microbial and floral compartments, characterizing the truffieres managed with low anthropic input. (Resume d'auteur)