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The importance of the ubiquitin-proteasome pathway to cellular regulation in eukaryotes has become increasingly apparent during the last several years. This fact was formally acknowledged recently by the awarding of the 2004 Nobel Prize in Chemistry to Aaron Ciechanover, Avram Hershko, and Irwin Rose for the discovery of ubiquitin-mediated protein degradation. In plants, regulated protein degradation by the ubiquitin/26S proteosome contributes significantly to development by affecting a wide range of processes, including embryogenesis, hormone signaling, and senescence. In Arabidopsis thaliana more than 1400 genes (; 5% of the proteome) encode components of the ubiquitin/26S proteasome (Ub/26S) pathway (Smalle and Vierstra, 2004). Approximately 90% of these genes encode subunits of the E3 ubiquitin ligases, which confer substrate specificity to the pathway. Two of the primary objectives in the …