Recent studies of auxin response have focused on the functions of three sets of 
proteins: the auxin (Aux) response factors (ARFs), the Aux/IAAs, and the F-box 
protein TIR1. The ARF proteins bind DNA and directly activate or repress 
transcription of target genes while the Aux/IAA proteins repress ARF function. 
TIR1 is part of a ubiquitin protein ligase required for degradation of Aux/IAA 
proteins. Here we report the isolation and characterization of a novel mutant of 
Arabidopsis called axr5-1. Mutant plants are resistant to auxin and display a 
variety of auxin-related growth defects including defects in root and shoot 
tropisms. Further, the axr5-1 mutation results in a decrease in auxin-regulated 
transcription. The molecular cloning of AXR5 revealed that the gene encodes the 
IAA1 protein, a member of the Aux/IAA family of proteins. AXR5 is expressed 
throughout plant development consistent with the pleiotropic mutant phenotype. 
The axr5-1 mutation results in an amino acid substitution in conserved domain II 
of the protein, similar to gain-of-function mutations recovered in other members 
of this gene family. Biochemical studies show that IAA1/AXR5 interacts with TIR1 
in an auxin-dependent manner. The mutation prevents this interaction suggesting 
that the mutant phenotype is caused by the accumulation of IAA1/AXR5. Our 
results provide further support for a model in which most members of the Aux/IAA 
family are targeted for degradation by SCFTIR1 in response to auxin.