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.