Figure 2.

Schematic view of the auxin response cascade. This model involves proteins transcribed by AUX–IAA, ARF, and TIR1–AFB1–3. At high concentrations, auxin interacts with TIR1–AFB1–3 receptors, which are part of the SCF complex, and targets bound and ubiquitinated AUX–IAA for degradation by the 26S proteosome, thereby activating ARF to de-repress early auxin gene expression. (At low IAA concentrations, AUX–IAA proteins form dimers with ARF transcription factors bond to consensus TGTCTC auxin-repressive promoter elements. These AUX–IAA/ARF protein complexes block the transcription of early auxin response genes.) Along with a negative feedback loop due to the expression of AUX–IAA, early auxin gene expression obtains a variety of cell- or tissue-specific auxin responses one among which is the production of V-type H⁺-ATPases that can promote the Golgi-mediated secretion of wall proteins and instigate cell wall loosing when activated by IAA and auxin binding proteins (ABP; see Figure 6 for a logic circuit rendering of auxin-mediated cell wall loosening).