The interactions between plants and soil microorganisms are mainly based on trophic relationships. The compounds exchanged represent cost for the organism produces them but a benefit for the one who receive those compounds. A mutualistic relation leads to stability in the cost-benefit balance resulting from a co-evolution between the two organisms. The cost corresponding for the release of carbon compounds by the plant would be offset by benefits in return corresponding for the activities of microorganisms that use them. We tested by an experimental way the effect of CO2 concentration on the interaction between M. truncatula and the bacterium P. fluorescens C7R12. The results allowed a best understanding of the rhizosphere ecology.