Rhizobia are nitrogen fixing bacteria living either free in the soil or in symbiotic association with legume plants. Their unique capacity to fix atmospheric nitrogen to the benefit of their host plants plays a major role in agriculture, as it makes possible to cultivate legumes without addition of nitrogen fertilizers, provided the bacteria are present in the soil or inoculated at the time of sowing. However, either in the soil or in the root nodules of legumes, rhizobia are exposed to various stress and starvation conditions which threaten their survival and the efficiency of symbiotic nitrogen fixation. It is therefore important, both from an academic point of view and with the perspective of agronomical improvements, to understand how these bacteria respond and adapt to stress. To this end, two kinds of rhizobial stress responses are studied in the group: i) the general stress response, and ii) the response to nitric oxide (NO). We use as a model the α-proteobacterium Sinorhizobium meliloti, the microsymbiont of alfalfa (Medicago sativa) and of the model legume M. truncatula.