Studies performed in our Agroecology Department show that the immune response of plants is linked to their iron nutrition and is modulated by pyoverdine, a siderophore produced by the plant beneficial rhizobacteria Pseudomonas fluorescens C7R12. Accordingly, Arabidopsis thaliana plantlets exposed to iron deficiency and treated with pyoverdine in its iron non‐chelated structure (apo‐pyo) show an enhanced growth but a decreased immune response capacity. We hypothesize that nitric oxide (NO), a universal signaling molecule, is a key component of the regulation of the immune response in plants exposed to apo‐pyo and to the C7R12 strain. We checked by fluorescence microscopy that NO is actually produced in response to iron deficiency in presence or in absence of apo-pyo. To verify a role of NO in these processes, three tasks will be developed: ‐ identification of S‐nitrosylated proteins in response to apo‐pyo and P. fluorescens C7R12. S-nitrosylation is a major NO‐dependent posttranslational protein modification which targets cysteine residues (Cys). ‐ analysis of the impact of NO on the structure and function of the selected proteins. The influence of NO on the structure and function of the proteins will be investigated in vitro and the Cys residues regulated by NO will be identified by directed mutagenesis. ‐ study of the incidence of the S‐nitrosylation of the proteins of interest in planta. A. thaliana plants impaired in the expression of the proteins of interest or expressing them in a non‐nitrosylable structure will be generated and their interaction with pathogenic or beneficial micro‐organisms will be analyzed.