A good number of bacteria have the ability to associate with plants at various levels, e.g. such as saprophytes, symbionts or pathogens. These bacteria can live on the aerial surfaces of the plant (epiphytes), in the root environment (rhizobacteria) or inside the plant tissues (endophytes). We have shown that Azoarcus sp. CIB can colonize the root of rice as an endophyte. In addition, the CIB strain possesses various plant growth promoting properties such as the ability to fix nitrogen, to solubilize phosphates or to produce phytohormones (indolacetate). We are analyzing the ability of the CIB strain to promote the growth of rice grown under stress conditions and to be useful as a tool in phytoremediation technologies. We are also conducting global studies, using transcriptomics, to understand the molecular mechanisms that control the ecophysiological changes that transform a free-living bacterium, such as CIB strain, into a rice endophyte.