Polar auxin transport

In other words, the flow and relative concentrations of auxin informs each plant cell where it is located and therefore what it should do or become.

To explain the mechanism behind unique character of auxin transport through living cell files of the plant, the so-called chemiosmotic model was formulated.

[1][2][3][4] The mechanism was first proposed in the 1970s by Ruberry and Sheldrake[1][5] and this visionary[5] prediction was finally proven in the 21st century.

Auxin enters plant cells by two methods, first by passive diffusion as non-ionized protonated indole-3-acetic acid (IAAH) across the phospholipid bilayer, or second by active co-transport in the anionic form IAA−.

While the PGP auxin efflux carriers are evenly distributed, the PIN proteins normally maintain polar (i.e. asymmetric) localisation on the plasma membrane.

As a result, the PIN proteins generate a directional flow of auxin at the tissue and organ scale.

For example, the cells located in the vasculature (at the center) of the root all show PIN1 proteins on their basal membrane only (i.e. on their lower side).

This positive feedback regulation auxin on its own transport also plays an essential role in vascular development, which process is called canalization.

PIN proteins are so named because mutant plants lacking the founding member of this family, PIN1, cannot develop flowers.

The formation of flowers is triggered by regularly spaced local auxin accumulation at the surface of the shoot apical meristem and, for this PIN1 is required.

Several mathematical models making different assumptions on the way auxin influences PIN localisation explain different observations.

Some models assume PIN proteins polarize towards the neighbouring cell containing the highest cytosolic auxin concentration.

Other models assume that PIN proteins localise on the side of the cell where the efflux of auxin is the highest.

A study suggests that phosphatase inhibition can alter the activities of acropetal and basipetal auxin transport.

D6PK and its homologs localize at the basal side of plasma membrane, modulating the rootward auxin fluxes and subsequent developmental processes.