Using electroporation-mediated gene transfer, the gene encoding the Slow (S) migrating polypeptide of the maize (Zea mays L.) alcohol dehydrogenase-1 (Adh1) enzyme has been introduced stably and transiently into maize cells containing an endogenous Fast (F) ADH1 electromorph. In stable transformants an 11.5-kb fragment was sufficient to program normal S expression relative to the endogenous F allele. In transient assays, Adh1-S gene constructs lacking the 9 Adh1-S intervening sequences (introns) were expressed at levels 50- to 100-fold less than the intact gene; the presence of intron 1 alone restored levels of gene expression to those found with the intact gene. The last two introns also stimulate Adh1-S expression, but the level is threefold below that of the intact gene. The expression of a chimeric chloramphenicol acetyltransferase (CAT) gene utilizing the 5' promoter and 3' polyadenylation regions of the Adh1 gene was increased 100-fold by the addition of sequences containing the Adh1 intron 1. The Adh1 intron 1 sequences did not stimulate CAT expression when located outside the transcribed region. When located within the transcribed region, the Adh1 intron 1 region efficiently stimulated CAT expression only when located between the promoter and the CAT coding region. A construct containing the Adh1 intron 1 fragment produced 40-fold more mRNA than a construct containing an equivalent cDNA fragment. Both the Adh1 intron 1 and the intron from a second maize gene, Bronze1, stimulated expression from other promoters (cauliflower mosaic virus 35S and nopaline synthase) and of other coding regions (luciferase and neomycin phosphotransferase II) as well. These results indicated that introns increase both Adh1 and chimeric gene expression in maize and the optimal location for such an intron is near the 5' end of the mRNA.

Approximately 5% of the Arabidopsis (Arabidopsis thaliana) proteome is predicted to be involved in the ubiquitination/26S proteasome pathway. The majority of these predicted proteins have identity to conserved domains found in E3 ligases, of which there are multiple types. The RING-type E3 is characterized by the presence of a cysteine-rich domain that coordinates two zinc atoms. Database searches followed by extensive manual curation identified 469 predicted Arabidopsis RING domain-containing proteins. In addition to the two canonical RING types (C3H2C3 or C3HC4), additional types of modified RING domains, named RING-v, RING-D, RING-S/T, RING-G, and RING-C2, were identified. The modified RINGs differ in either the spacing between metal ligands or have substitutions at one or more of the metal ligand positions. The majority of the canonical and modified RING domain-containing proteins analyzed were active in in vitro ubiquitination assays, catalyzing polyubiquitination with the E2 AtUBC8. To help identity regions of the proteins that may interact with substrates, domain analyses of the amino acids outside the RING domain classified RING proteins into 30 different groups. Several characterized protein-protein interaction domains were identified, as well as additional conserved domains not described previously. The two largest classes of RING proteins contain either no identifiable domain or a transmembrane domain. The presence of such a large and diverse number of RING domain-containing proteins that function as ubiquitin E3 ligases suggests that target-specific proteolysis by these E3 ligases is a complex and important part of cellular regulation in Arabidopsis.

The covalent attachment of ubiquitin to a substrate protein changes its fate. Notably, proteins typically tagged with a lysine48-linked polyubiquitin chain become substrates for degradation by the 26S proteasome. In recent years many experiments have been performed to characterize the proteins involved in the ubiquitylation process and to identify their substrates, in order to understand better the mechanisms that link specific protein degradation events to regulation of plant growth and development. This review focuses on the role that ubiquitin plays in hormone synthesis, hormonal signalling cascades and plant defence mechanisms. Several examples are given of how targeted degradation of proteins affects downstream transcriptional regulation of hormone-responsive genes in the auxin, gibberellin, abscisic acid, ethylene and jasmonate signalling pathways. Additional experiments suggest that ubiquitin-mediated proteolysis may also act upstream of the hormonal signalling cascades by regulating hormone biosynthesis, transport and perception. Moreover, several experiments demonstrate that hormonal cross-talk can occur at the level of proteolysis. The more recently established role of the ubiquitin/proteasome system (UPS) in defence against biotic threats is also reviewed. The UPS has been implicated in the regulation of almost every developmental process in plants, from embryogenesis to floral organ production probably through its central role in many hormone pathways. More recent evidence provides molecular mechanisms for hormonal cross-talk and links the UPS system to biotic defence responses.

The COP9 signalosome is an evolutionary conserved multiprotein complex of unknown function that acts as a negative regulator of photomorphogenic seedling development in Arabidopsis . Here, we show that plants with reduced COP9 signalosome levels had decreased auxin response similar to loss-of-function mutants of the E3 ubiquitin ligase SCF ^TIR1 . Furthermore, we found that the COP9 signalosome and SCF ^TIR1 interacted in vivo and that the COP9 signalosome was required for efficient degradation of PSIAA6, a candidate substrate of SCF ^TIR1 . Thus, the COP9 signalosome may play an important role in mediating E3 ubiquitin ligase–mediated responses.

Analysis of the Arabidopsis thaliana RING-ANK (for Really Interesting New Gene-Ankyrin) family, a subgroup of RING-type E3 ligases, identified KEEP ON GOING (KEG) as essential for growth and development. In addition to the RING-HCa and ankyrin repeats, KEG contains a kinase domain and 12 HERC2-like repeats. The RING-HCa and kinase domains were functional in in vitro ubiquitylation and phosphorylation assays, respectively. Seedlings homozygous for T-DNA insertions in KEG undergo growth arrest immediately after germination, suggestive of increased abscisic acid (ABA) signaling, a major phytohormone that plays a key role in plant development and survival under unfavorable conditions. Here, we show that KEG is a negative regulator of ABA signaling. keg roots are extremely sensitive to the inhibitory effects of ABA and exhibit hypersensitivity to exogenous glucose, consistent with the known interaction between glucose and ABA signaling. The observations that KEG accumulates high levels of ABSCISIC ACID-INSENSITIVE5 (ABI5) without exogenous ABA, interacts with ABI5 in vitro, and that loss of ABI5 rescues the growth-arrest phenotype of keg mutant seedlings indicate that KEG is required for ABI5 degradation. In this capacity, KEG is central to ABA signaling by maintaining low levels of ABI5 in the absence of stress.