The Arabidopsis F-box protein TIR1 is an auxin receptor.

Despite 100 years of evidence showing a pivotal role for indole-3-acetic acid (IAA or auxin) in plant development, the mechanism of auxin perception has remained elusive. Central to auxin response are changes in gene expression, brought about by auxin-induced interaction between the Aux/IAA transcriptional repressor proteins and the ubiquitin-ligase complex SCF(TIR1), thus targeting for them proteolysis. Regulated SCF-mediated protein degradation is a widely occurring signal transduction mechanism. Target specificity is conferred by the F-box protein subunit of the SCF (TIR1 in the case of Aux/IAAs) and there are multiple F-box protein genes in all eukaryotic genomes examined so far. Although SCF-target interaction is usually regulated by signal-induced modification of the target, we have previously shown that auxin signalling involves the modification of SCF(TIR1). Here we show that this modification involves the direct binding of auxin to TIR1 and thus that TIR1 is an auxin receptor mediating transcriptional responses to auxin.