Plants are under the constant threat of microbial pathogens. To defend themselves, plants have developed immune responses (including for example synthesis of antimicrobial secondary metabolites, production of PR proteins or reinforcement of cell wall) that can lead to resistance. However, these plant defense responses are costly and lead plants to continuously face a dilemma regarding the partitioning of their available resources. In order to better understand relationships between plant nutrition and defense, we analyzed the impact of nitrogen (N) nutrition on the capacity of different M. truncatula genotypes to resist against A. euteiches, the causal agent of legume root rot disease. N stands at a crossroad between primary metabolism and defense and we consider that channeling of N metabolism represents a key point in the trade-off between plant growth and immune response. Two nutrition conditions and ten representative plant genotypes have been studied using an in vitro inoculation assay. Plants were phenotyped according to several macroscopic or molecular parameters already described as relevant indicators of plant resistance and principal component analyses were performed in order to estimate the behavior of the different genotypes. First results have shown effects of genotype and nutrition taken individually, and interestingly strong interactions between genotypes and nutrition conditions. Indeed, for the F83005.5 genotype, sensitivity to A. euteiches increases under nitrate deficiency. In contrast, the tolerant A17 genotype, appears more resistant in the same condition. Experiments are in progress to investigate the molecular and biochemical processes underlying the expression of the N-dependent plant resistance. Special attention is given to the interaction between N nutrition, the production and role of Reactive Nitrogen Species such as nitric oxide (NO) and the expression of genes encoding proteins involved in N utilization and in secondary metabolism.