Auxin Action in a Cell-Free System

The plant hormone auxin regulates diverse aspects of plant growth and development [1]. Despite its importance, the mechanisms of auxin action remain poorly understood. In particular, the identities of the auxin receptor and other signaling proteins are unknown. Recent studies have shown that auxin acts by promoting the degradation of a family of transcriptional regulators called the Aux/IAA proteins 2, 3 and 4. These proteins interact with another large family of plant-specific transcription factors called Auxin Response Factors (ARF) and negatively regulate their activity [5]. Auxin stimulates Aux/IAA degradation by promoting the interaction between a ubiquitin protein ligase (E3) called SCFTIR1 and the Aux/IAA protein [2]. In this report, we demonstrate that auxin promotes the interaction between the Aux/IAA proteins and SCFTIR1 in a soluble extract free of membranes, indicating that this auxin response is mediated by a soluble receptor. In addition, we show that the response is not dependent on protein phosphorylation or dephosphorylation but rather is prevented by an inhibitor of peptidyl-prolyl isomerases

Molecular cloning and characterization of a heat-shock induced calmodulin binding protein gene and cDNAs encoding glutamate decarboxylase from tobacco

Thesis (Ph. D.)--University of Hawaii at Manoa, 1995.Includes bibliographical references (leaves 118-133).Microfiche.xiii, 133 leaves, bound ill. 29 cmTwo individual heat shock induced/enhanced tobacco (Nicotiana tabacum L. cv Wisconsin-38) CaMBP cDNAs, pTCB48 and pTCB15, were characterized. Both of these clones were partial cDNAs. The 5' end of pTCB48 was obtained using 5' RACE technique. The ORF of this cDNA codes for a protein with a molecular weight of 50 kD. Using several deletions coupled with CaM gel overlay, CaM binding domain of pTCB48 was localized to the very end of the C-terminus of the protein. Structural prediction of this domain revealed a possibility of forming a BAA structure but an alternative β-strand and β-turn structure could not be excluded. Polyclonal antibody was raised against purified TCB48 fusion protein and used in Western analysis. The gene (TG48) corresponding to the pTCB48 cDNA was isolated and characterized. TG48 contains a total of 6149 bp including 1064 bp of 5' flanking region and 364 bp of 3' flanking sequence. The TG48 gene is consists of 6 exons and 5 introns. The 5' flanking sequence contains common promoter elements such as TATA and CAAT boxes, and 5 putative HSEs, characteristic of heat shock induced genes. These HSEs may be involved in heat shock induction of this gene. The structure of both TG48 gene and full length cDNA indicate that of two heat shock induced mRNAs, the 1.8 kb transcript is encoded by this gene. Northern analysis indicates this gene is subjected to tissue specific, developmental and environmental regulation. Sequence analysis of pTCB15 indicated that it encodes a CaM binding glutamate decarboxylase (GAD). Two full length GAD clones were isolated by screening a N. tabacum L. cv Xanthi cDNA library. Both clones had similar nucleotide sequences except that GAD9 had a shorter 3' UTR. CaM binding domain of the tobacco GAD may be located at C-terminus and possibly a BAA structure. Results presented here indicate GAD transcript level is subjected to developmental, environmental and tissue specific regulation

The RUB/Nedd8 conjugation pathway is required for early development in Arabidopsis

The r elated-to- ub iquitin (RUB) protein is post-translationally conjugated to the cullin subunit of the SCF (SKP1, Cullin, F-box) class of ubiquitin protein ligases. Although the precise biochemical function of RUB modification is unclear, studies indicate that the modification is important for SCF function. In Arabidopsis , RUB modification of CUL1 is required for normal function of SCF TIR1 , an E3 required for response to the plant hormone auxin. In this report we show that an Arabidopsis protein called RCE1 functions as a RUB-conjugating enzyme in vivo . A mutation in the RCE1 gene results in a phenotype like that of the axr1 mutant. Most strikingly, plants deficient in both RCE1 and AXR1 have an embryonic phenotype similar to mp and bdl mutants, previously shown to be deficient in auxin signaling. Based on these results, we suggest that the RUB-conjugation pathway is required for auxin-dependent pattern formation in the developing embryo. In addition, we show that RCE1 interacts directly with the RING protein RBX1 and is present in a stable complex with SCF. We propose that RBX1 functions as an E3 for RUB modification of CUL1

Arabidopsis AXR6 encodes CUL1 implicating SCF E3 ligases in auxin regulation of embryogenesis

The AXR6 gene is required for auxin signaling in the Arabidopsis embryo and during postembryonic development. One of the effects of auxin is to stimulate degradation of the Aux/IAA auxin response proteins through the action of the ubiquitin protein ligase SCF TIR1 . Here we show that AXR6 encodes the SCF subunit CUL1. The axr6 mutations affect the ability of mutant CUL1 to assemble into stable SCF complexes resulting in reduced degradation of the SCF TIR1 substrate AXR2/IAA7. In addition, we show that CUL1 is required for lateral organ initiation in the shoot apical meristem and the inflorescence meristem. These results indicate that the embryonic axr6 phenotype is related to a defect in SCF function and accumulation of Aux/IAA proteins such as BDL/IAA12. In addition, we show that CUL1 has a role in auxin response throughout the life cycle of the plant