It has been proposed that receptors recognizing auxin will transduce that binding signal into a physiological response; these receptors are known as auxin-binding proteins (ABP). One model for auxin binding is that each receptor is thought to have a binding conformation that includes a naphthalene-binding platform (Figure 3a), a carboxylic acid binding site (Figure 3b) and a hydrophobic transition zone (Figure 3c) located between the two binding elements (Figure 3d). The animation ’Auxin and Auxinic Herbicides Mechanism of Action’ illustrates the binding process by describing the receptor as well as the various protein carriers that help transport auxin from cell to cell. Other models exist as well (Woo et al., 2002, EMBO J. 21:2877-2885).
 

Animation 1: Auxin and Auxinic Herbicide Mechanisms of Action

Fig. 3a: Planar aromatic binding platform	Fig. 3b: Carboxylic acid binding
Fig. 3c: Hydrophobic transition region	Fig. 3d: Receptor binding

Fig. 4: Major auxin binding proteins

Major auxin binding proteins (ABP) include auxin influx carriers, auxin efflux carriers and the auxin receptors responsible for transducing auxin signals (Figure 4). Auxin influx and efflux carriers are the major mode of transport for auxin from cell to cell toward the base of the plant, also known as basipetal or polar transport. The H⁺/ IAA^- influx carriers (Figure 5)

Fig. 5: IAA- influx carrier

are evenly distributed in the plasma membrane and recognize the conjugate base of auxin (the anion, IAA^-) and two hydrogen ions (2 H⁺). This saturable carrier is able to actively transport IAA^- against an electrochemical gradient across the plasma membrane by coupling IAA^-’s entry to the transport of two H⁺ down their electrochemical gradient. Once inside the alkaline cytoplasm, the IAA^- is recognized by a series of saturable efflux carriers (Figure 6) located in the membrane on the lower side of each cell. These efflux carriers move IAA^- out of the cell and into the cell wall region where IAA^- diffuses to the auxin influx carriers located on the cell below.

Fig. 6: IAA- efflux carrier

Another contributor to basipetal or polar transport of auxin, is the simple diffusion of auxin across the plasma membrane in its lipophilic, undissociated acid form (IAAH) (Figure 7). This form of transport does not require a carrier.

Fig. 7: Diffusion of IAAH

Because of its low pKa of 4.7, the acid form of auxin (IAAH) predominates in the acidic cell wall region creating a large concentration gradient to drive diffusion of IAAH across the membrane into the cell. Click on the image below to view the ’Ionization, pH, pKa Processes’ animation.

 

Animation 2: Ionization, pH, pKa Processes

Fig. 8: IAAH dissociation to IAA- and H+ in alkaline pH

The proton motive force (PMF) of the cell is maintained because H⁺-ATPases located in the plasma membrane hydrolyze ATP and pump H⁺ out, into the cell wall space. Once in the alkaline cytoplasm, IAAH dissociates to the anion, IAA^- and IAA^- accumulates because of its lower membrane permeability (Figure 8). The anion, IAA^-, can also efflux from the cell via the IAA^- efflux carriers at the base of the cell.