A carboxyl group that is separated by one or two C-atoms is required. The receptor has accordingly to have two separate contact sites. The distribution of auxin within plant tissues is subject to clear and recognizable rules that indicate a transport simultaneously active and polar. The transport velocity is higher than expected from simple diffusion. Transport can occur against a concentration gradient.
Transport is energy-consuming and is drastically reduced in the absence of oxygen.
The transport system is substrate-specific. It transports certain auxin molecules like IES or naphtylacetic acid faster than 2,4-dichlorphenoxy acetic acid, for example. The transport system can be blocked by specific inhibitors. In lower concentrations do they aid the coleoptile s elongation, that of the shoot and the roots. If the concentration becomes higher, the effect reverses and elongation of root and shoot is inhibited.