Fig. 1. Model for auxin transport and action in the plant cell. Auxin is moved into cells through influx carriers encoded by the AUX1 gene. It is expelled from the cell by localized auxin-efflux carriers associated with PIN auxin facilitators. Auxin efflux carriers are asymmetrically targeted through a mechanism that involves the presence/absence of the PINOID kinase. Auxin-efflux carriers cycle to and from the membrane through the endosomal compartment. At some threshold concentration, auxin triggers the activity of the SCF^TIR ubiquitin E3 ligase which marks the AUX/IAA transcriptional represssor (IAA) for destruction via the proteosome. The auxin-response factor is then free to act as a transcriptional activator.