Inoculation of Phaseolus vulgaris with the nodule-endophyte Agrobacterium sp. 10C2 affects richness and structure of rhizosphere bacterial communities and enhances nodulation and growth

Agrobacterium sp. 10C2 is a nonpathogenic and non-symbiotic nodule-endophyte strain isolated from root nodules of Phaseolus vulgaris. The effect of this strain on nodulation, plant growth and rhizosphere bacterial communities of P. vulgaris is investigated under seminatural conditions. Inoculation with strain 10C2 induced an increase in nodule number (+54 %) and plant biomass (+16 %). Grains also showed a significant increase in phosphorus (+53 %), polyphenols (+217 %), flavonoids (+62 %) and total antioxidant capacity (+82 %). The effect of strain 10C2 on bacterial communities was monitored using terminal restriction fragment length polymorphism of PCR-amplified 16S rRNA genes. When the initial soil was inoculated with strain 10C2 and left 15 days, the Agrobacterium strain did not affect TRF richness but changed structure. When common bean was sown in these soils and cultivated during 75 days, both TRF richness and structure were affected by strain 10C2. TRF richness increased in the rhizosphere soil, while it decreased in the bulk soil (root free). The taxonomic assignation of TRFs induced by strain 10C2 in the bean rhizosphere revealed the presence of four phyla (Firmicutes, Actinobacteria, Bacteroidetes and Proteobacteria) with a relative preponderance of Firmicutes, represented mainly by Bacillus species. Some of these taxa (i.e., Bacillus licheniformis, Bacillus pumilus, Bacillus senegalensis, Bacillus subtilis, Bacillus firmus and Paenibacillus koreensis) are particularly known for their plant growth-promoting potentialities. These results suggest that the beneficial effects of strain 10C2 observed on plant growth and grain quality are explained at least in part by the indirect effect through the promotion of beneficial microorganisms.