Aux/IAA proteins are short-lived transcriptional factors that function as repressors of early auxin response genes at low auxin concentrations. Repression is thought to result from the interaction with auxin response factors (ARFs), proteins that bind to the auxin-responsive promoter element (AuxRE). Formation of heterodimers with ARF proteins may alter their ability to modulate early auxin response genes expression (By similarity). Homodimers and heterodimers. By auxin. The N-terminal half of the protein contains two conserved domains I and II. Domain I includes a slightly degenerated ERF-associated amphiphilic repression (EAR) motif which seems to be involved in the activity of transcriptional repression. Domain II is required for the correct degradation of the protein through the SCF-mediated ubiquitin-proteasome pathway. Interactions between Aux/IAA proteins and auxin response factors (ARFs) occur through their C-terminal dimerization domains III and IV (By similarity). Phosphorylated by phytochrome A in vitro. Belongs to the Aux/IAA family.