A highly active synthetic auxin response element (AuxRE), referred to as DR5, was created by performing site-directed mutations in a natural composite AuxRE found in the soybean GH3 promoter. DR5 consisted of tandem direct repeats of 11 bp that included the auxin-responsive TGTCTC element. The DR5 AuxRE showed greater auxin responsiveness than a natural composite AuxRE and the GH3 promoter when assayed by transient expression in carrot protoplasts or in stably transformed Arabidopsis seedlings, and it provides a useful reporter gene for studying auxin-responsive transcription in wild-type plants and mutants. An auxin response transcription factor, ARF1, bound with specificity to the DR5 AuxRE in vitro and interacted with Aux/IAA proteins in a yeast two-hybrid system. Cotransfection experiments with natural and synthetic AuxRE reporter genes and effector genes encoding Aux/IAA proteins showed that overexpression of Aux/IAA proteins in carrot protoplasts resulted in specific repression of TGTCTC AuxRE reporter gene expression.

The plant hormone auxin regulates plant physiology by modulating the interaction of transcription factors with auxin response elements (AuxREs) of the affected genes. A transcription factor, Auxin Response Factor 1 (ARF1), that binds to the sequence TGTCTC in AuxREs was cloned from Arabidopsis by using a yeast one-hybrid system. ARF1 has an amino-terminal DNA-binding domain related to the carboxyl terminus of the maize transactivator Viviparous-1. Sequence requirements for ARF1 binding in vitro are identical to those that confer auxin responsiveness in vivo. The carboxyl terminus of ARF1 contains two motifs found in the Aux/IAA class of proteins and appears to mediate protein-protein interactions.

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A highly active synthetic auxin response element (AuxRE), referred to as DR5, was created by performing site-directed mutations in a natural composite AuxRE found in the soybean GH3 promoter. DR5 consisted of tandem direct repeats of 11 bp that included the auxin-responsive TGTCTC element. The DR5 AuxRE showed greater auxin responsiveness than a natural composite AuxRE and the GH3 promoter when assayed by transient expression in carrot protoplasts or in stably transformed Arabidopsis seedlings, and it provides a useful reporter gene for studying auxin-responsive transcription in wild-type plants and mutants. An auxin response transcription factor, ARF1, bound with specificity to the DR5 AuxRE in vitro and interacted with Aux/IAA proteins in a yeast two-hybrid system. Cotransfection experiments with natural and synthetic AuxRE reporter genes and effector genes encoding Aux/IAA proteins showed that overexpression of Aux/IAA proteins in carrot protoplasts resulted in specific repression of TGTCTC AuxRE reporter gene expression.

Summary Auxin response factors (ARFs) are transcription factors that bind with specificity to TGTCTC auxin response elements (AuxREs) found in promoters of primary/early auxin response genes. ARFs are encoded by a multi‐gene family, consisting of more than 10 genes. Ten ARFs have been analyzed by Northern analysis and were found to be expressed in all major plant organs and suspension culture cells of Arabidopsis. The predicted amino acid sequences indicate that the 10 ARFs contain a novel amino‐terminal DNA binding domain and a carboxyl‐terminal dimerization domain, with the exception of ARF3 which lacks this dimerization domain. All ARFs tested bind with specificity to the TGTCTC AuxRE, but there are subtle variations in the sequence requirements at positions 5 (T) and 6 (C) of the AuxRE. While the amino‐terminal domain of about 350 amino acids is sufficient for binding ARF1 to TGTCTC AuxREs, this domain is not sufficient for the binding of some other ARFs to palindromic AuxREs. Our results suggest that ARFs must form dimers on palindromic TGTCTC AuxREs to bind stably, and this dimerization may be facilitated by conserved motifs found in ARF carboxyl‐terminal domains. Dimerization in at least some cases may dictate which ARF(s) are targeted to AuxREs.

The auxin-responsive soybean GH3 gene promoter is composed of multiple auxin response elements (AuxREs), and each AuxRE contributes incrementally to the strong auxin inducibility of the promoter. Two independent AuxREs of 25 bp (Dl) and 32 b p (D4) contain the sequence TGTCTC. Results presented here show that the TGTCTC element in D1 and D4 is required but not sufficient for auxin inducibility in carrot protoplast transient expression assays. Additional nucleotides upstream of TGTCTC are also required for auxin inducibility. These upstream sequences showed constitutive activity and no auxin inducibility when part or all of the TGTCTC element was mutated or deleted. In D1, the constitutive element overlaps the 5' portion of TGTCTC; in D4, the constitutive element is separated from TGTCTC. An 11-bp element in D1, CCTCGTGTCTC, conferred auxin inducibility to a minimal cauliflower mosaic virus 35s promoter in transgenic tobacco seedlings as well as in carrot protoplasts (i.e., transient expression assays). 60th constitutive elements bound specifically to plant nuclear proteins, and the constitutive element in D1 bound to a recombinant soybean basic leucine zipper transcription factor with G-box specificity. To demonstrate further the composite nature of AuxREs and the ability of the TGTCTC element to confer auxin inducibility, we created a novel AuxRE by placing a yeast GAL4 DNA binding site adjacent to the TGTCTC element. Expression of a GAL4-c-Rel transactivator in the presence of this novel AuxRE resulted in auxin-inducible expression. Our results indicate that at least some AuxREs have a composite structure consisting of a constitutive element adjacent to a conserved TGTCTC element that confers auxin inducibility.

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Auxin response factors or ARFs are a recently discovered family of transcription factors that bind with specificity to auxin response elements (AuxREs) in promoters of primary or early auxin-responsive genes. ARFs have an amino-terminal DNA-binding domain related to the carboxyl-terminal DNA-binding domain in the maize transactivator VIVIPAROUS1. All but one ARF identified to date contain a carboxyl-terminal protein-protein interaction domain that forms a putative amphipathic alpha-helix. A similar carboxyl-terminal protein-protein interaction domain is found in the Aux/IAA class of auxin-inducible proteins. Some ARFs contain transcriptional activation domains, while others contain repression domains. ARFs appear to play a pivotal role in auxin-regulated gene expression of primary response genes.