Figure 4. Function and regulation of auxin-transport proteins.

Auxin enters the cell primarily through carrier-mediated influx via AUX1 (blue cylinder) as the anionic form, with a small diffusional contribution of the uncharged species (broken line). Auxin efflux is mediated via PIN proteins (green cylinders) with some contribution from AtPGP1/19 (red), possibly via direct protein–protein interactions. AUX1 regulation by the ER protein AXR4 (light-blue box) is shown as affecting the trafficking of AUX1 to the plasma membrane (dotted blue arrow). PIN polarity controlled by PINOID (yellow box) and an unconfirmed phosphatase are depicted by green arrows, whereas the endosomal cycling of PIN1 and PIN2 is shown schematically by the intracellular vesicle. AtPGP1/19 interactions with TWD (pink box) and the putative import of IAA through AtPGP4 (orange) are also depicted. The overall direction of polar auxin transport is shown by the large arrow to the right-hand side.