26- 4 Schematic model for polar auxin transport The auxin indole-3-acetic acid (IAA) enters the cell by passive diffusion in a protonated (IAAH) form and/or by secondary active cotransport in its anionic form (IAA-), via an influx carrier (AUX1 protein). The anionic form (IAA-) predominates in the cytosol, which has a neutral pH. The anions exit the cells only at the basal end, via efflux carriers (PIN proteins).