The diverse roles of ubiquitin and the 26S proteasome in the life of plants

The tagging of proteins with ubiquitin and their subsequent degradation by the 26S proteasome provides a highly regulated proteolysis system that is necessary in all eukaryotes.The ubiquitylation of a target protein involves the sequential action of E1, E2 and E3 enzymes; E3 ubiquitin ligase provides target specificity.Plants use ubiquitin and the 26S proteasome to control a wide variety of developmental processes through the degradation of key cellular regulators.Approximately 5% of the Arabidopsis thaliana genome encodes components of the ubiquitin/26S proteasome system.We discuss the roles of ubiquitin in the regulation of floral development, responses to plant hormones and to pathogens, and in the regulation of photomorphogenesis.The COP9 signalosome (CSN) is intimately connected to the ubiquitin/26S proteasome system and might regulate the activity of SCF-type (and possibly other) E3 ligases.Recent advances, particularly in the fields of hormone signalling and responses to pathogens, have shown just how frequently plants use ubiquitin to regulate cellular processes and indicate that ubiquitin and the 26S proteasome act as regulators in many more, as yet unidentified, plant developmental processes.