LAUSR

The diversity of bacteria inhabiting nodules, roots and rhizosphere of Vicia faba grown in three different agro-systems was analyzed by a cultivation-independent approach using the T-RFLP technique. The main objective was to unravel the extent of nodule colonization by rhizospheric bacteria and to highlight the difference, if any, with roots. A clear separation between nodule, root and rhizosphere bacterial populations was found. The similarity in TRF structure between nodules and roots was around 30%. The TRF structure of bacterial communities was further explored by the MICA software which permits the putative taxonomic assignation of TRFs according to their relative size. Nodules were largely dominated by Proteobacteria with a quite equal representation of alpha, beta, gamma and delta/epsilon classes. Species belonging to Firmicutes, Actinobacteria and Cyanobacteria were weakly represented in nodules. Roots were significantly inhabited by Actinobacteria Besides to Proteobacteria. However, rhizosphere was dominated by Proteobacteria and Firmicutes. Nodule associated bacteria belonged to 49 genera and 68 species. More than 30% of the hits constituted bacterial species known for their plant growth promoting abilities. Around 15% were sulfate-reducers or sulfur-oxidizers. Nodule associated bacteria encompassed also methane oxidizers and potential human or plant pathogens. More than 16% of nodule associated bacteria were not revealed in the rhizosphere. The most plausible explanation is that they were not sufficiently frequent to be detected by PCR, which suggests that nodule colonization by rhizospheric bacteria is not a simple passive entry based on a density gradient. The biological significance of the nodule microbiome and the potential implications on plant functioning will certainly be the focus of future research.