"Eukaryotic protein function is regulated in vivo by diverse mechanisms such as protein turnover, regulation of protein activity, localization and protein-protein interactions. These mechanisms involve constitutive or reversible post-translational modifications of specific amino-acid residues in the target protein by molecules of different nature. Ubiquitin and ubiquitin-like modifiers are polypeptides that are covalently attached to a lysine residue in the target protein. SUMO is a member of the ubiquitin family, but it differs from ubiquitin in its cellular function. Whereas protein ubiquitination results in degradation by the 26S proteasome, sumoylation is involved in regulation of protein activity, cellular localization, or protection from ubiquitination. In plants, SUMO plays an important role in biotic and abiotic stress responses, and regulates abscisic acid (ABA) signaling, plant hormone that mediates environmental stress responses, and flowering. In addition, we have found that a functional sumoylation system is essential during seed development, process that is also regulated by ABA at different stages. Our general goal is to investigate the biological role of SUMO in the context of ABA signaling and stress responses in Arabidopsis. For this purpose we will study different aspects of this novel posttranslational regulatory system involving the analysis of the SUMO biological role during seed development and germination, identification of new SUMO targets and dissection of the biological role of the catalase AtCAT3 sumoylation, and the study of molecular factors that could be responsible for recognition of SUMO conjugates. The data generated will contribute to better understanding of this biological process and, eventually, to a thoughtful design of plants with improved agronomical traits. Also, as sumoylation is an evolutionary conserved regulatory system, our work will greatly contribute to understand its mechanism of action in mammals."