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). Plays a central role in auxin regulation of root growth, in gravitropism, and in lateral root formation (PubMed:9895319). Regulated by an auxin-induced protein turnover (PubMed:14617065). Formation of heterodimers with ARF proteins may alter their ability to modulate early auxin response genes expression. When activated by cytokinin, restricts the expression of the PIN genes to the vascular transition zone (PubMed:19039136). Induction of SHY2 in the vascular transition zone restricts BRX expression to down-regulate PIN3 and thus limit meristem growth, but proper SHY2 expression requires BRX (PubMed:21149702). Involved in meristem growth and in determining its size (PubMed:20605455). May participate in strigolactone signaling to regulate meristem size and lateral root formation (PubMed:23425316). Homodimers and heterodimers (By similarity). Interacts with TPL (PubMed:18258861). Interacts with TIR1, the F-box component of the Skp1-Cdc53/cullin-F-box (SCFTIR1) E3 ubiquitin ligase complex (PubMed:14617065). Highly expressed in stems and flowers (PubMed:7658471). Expressed in hypocotyls, cotyledons and leaves, but barely detected in roots (PubMed:11884676). Expressed in root tips (PubMed:14534134). In the root meristem, specifically detected at the vascular tissue transition zone (PubMed:19039136). The level of expression increases during meristem growth, reaches a maximum at 5 days post germination and subsequently remaines constant in time. By auxin (PubMed:7658471). Up-regulated by cytokinin treatment through the primary cytokinin-response transcription factor ARR1 (PubMed:19039136). Down-regulated by light in the presence of sucrose, but up-regulated in the absence of sucrose (PubMed:11884676). 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. Phosphorylated by phytochrome A in vitro. Belongs to the Aux/IAA family.