Figure 3.

Model for Regulation of Transcription by Auxin.

(A) A nucleus of a hypocotyl cell in a dark-grown seedling with only basal levels of auxin present. ARF transcription factors, such as NPH4/ARF7 (red), are bound to DNA target sequences (AuxRE) within the nuclear DNA as heteromeric complexes with a dominant transcriptional repressor protein, such as AUX/IAA19 (gold) and a corepressor, such as TPL (orange). This complex is transcriptionally inactive, and, as such, transcription of auxin-regulated genes is repressed (red X). Also present in the nucleus is the SCF^TIR1/AFB auxin receptor complex (blue-violet and light green) in its inactive ligand (auxin)-free state.

(B) A nucleus of a hypocotyl cell that is in the shaded portion of a seedling exposed to directional BL where auxin has accumulated. Elevated auxin levels stimulate the binding of AUX/IAA proteins, such as IAA19, to the SCF^TIR1/AFB complex (green and light blue), which in turn promotes the polyubiquitination of the AUX/IAA protein and its subsequent degradation by a 26S proteasome (gray). Removal of AUX/IAA proteins releases the corepressor TPL and allows for homodimerization of ARF proteins, which stimulates RNA polymerase core protein activation and transcription of target genes, such as TSI genes.