In Mediterranean area, legumes occupy an important place due to their agro-economic and environmental interests. These plants contribute to nitrogen incorporation in ecosystems by symbiotic nitrogen fixation (SNF) involving rhizobial strains. This association provides the necessary nitrogen for plant growth and contributes to improvement of the soil nitrogen balance to the benefit of other crops in intercropping systems or in rotation. Thus, the rhizobial symbiosis could, through the cultivation of legumes, allow farmers to save chemical fertilizer costs and alleviate their environment pollution effects. However, environmental constraints, such as water deficit and low soil phosphorus availability, prevailing in Mediterranean region, reduce SNF performance, thus causing a decrease in grain and forage legume yields. In this context, our present project aims at exploring legume-rhizobia functional diversity to select and characterize symbioses tolerant to water deficit under limiting phosphorus conditions, and will focus on physiological, biochemical and molecular tools at plant and rhizobia levels.