"The past decade has seen a remarkable advance in our understanding of the plant circadian system, mostly in Arabidopsis thaliana. It is now well established that Arabidopsis clock genes and their protein products operate via negative feedback loops that promote rhythmic oscillations in cellular, metabolic and physiologic activities. Evidence about the relevant role of transcriptional mechanisms regulating the circadian clock has been provided in a number of different studies. However, the importance of post-translational regulation within the plant circadian clock still remains to be fully elucidated. In this proposal, we attempt to explore the molecular, cellular and biochemical determinants involved in the post-translational regulation of the Arabidopsis circadian clock. We will characterize protein sumoylation and its regulation of the circadian system, deciphering its potential role modulating the light input to the clock. To date, no other studies have demonstrated the importance of this post-translational modification within the plant circadian field, which provide evidence about the novelty and biological relevance of this proposal. Overall, the successful completion of the proposed objectives will provide a substantial advance on our knowledge of circadian clock function in plants."