ABSTRACT
     
     Nitric oxide (NO) is a very reactive gaseous molecule that has gained an increasing interest lately due to its well known signaling role. However, its biosynthesis, signaling and mode of action are still largely unknown, thus limiting our knowledge about the molecular basis of NO function in plants. In this Thesis, we have identified, by a genetic approach, the main NO biosynthetic pathways in the model plant Arabidopsis thaliana. Next, we have used the NO deficient mutants we have generated to characterize NO function in developmental and stress-related responses. We have characterized a negative role of NO in ABA sensitivity and further regulation of seed dormancy and germination, seedling establishment and stomata closure. On the other hand, we have also defined the molecular mechanism underlying the positive regulation exerted by NO on photomorphogenesis. Briefly, the production of NO upon darkness to light transition modulates the expression of key photomorphogenesis-related genes such as SLY1 and PIFs, thus ensuring de-etiolation under light conditions. Finally, to gain further insight on the regulatory mode of action of NO, we have optimized a proteomic methodology to characterize a NO-based post-translational modification by nitration of tyrosine residues that help in identifying target proteins of direct NO action. This work has allowed to improve our knowledge on NO regulation of different physiological processes in Arabidopsis and has opened many others for further analysis.