FIGURE 4.

Summary of the auxin perception by ABP1 and TIR1 receptors. Rapid auxin responses are thought to be mediated by ABP1. Auxin is perceived by ABP1 at the outer surface of the Plasma membrane. In this case, ABP1 is anchored by an unknown membrane-associated protein (Protein?). In flowering plants, ABP1 is mainly located at the endoplasmic reticulum (ER) due to an ER retention motif (KDEL). Currently, it is not known how ABP1 is exported to the plasma membrane. Binding of auxin to ABP1 induces several events including activation of proton pumps, which culminates in acidification of the outer space and contributes to cell wall loosening. There is also activation of potassium inward channels, which increase the intracellular K⁺ and, consequently, lead to increased water uptake, allowing cell expansion. For slower responses, auxin is perceived by the F-box TIR1, which directs the auxin repressors Aux/IAAs for degradation and releases the auxin response factors (ARFS) to induce auxin-related gene expression.