Polar auxin transport

Polar auxin transport (PAT) is the directional cell-to-cell transport of the plant hormone auxin. It results from the polar/asymmetric localisation at the plasma membrane of the auxin efflux carriers of the PIN family, which are secondary transporters that export auxin out of the cells. Polar auxin transport is one of the main processes determining the spatial distribution of auxin in plants. As such, it plays a central role in the control of plant growth and development and is tightly regulated.

Polar auxin transport (PAT) is the directional flow of auxin molecules through the plant tissues which results from coordinated directional cell-to-cell transport.

The mechanism responsible for polar auxin transport was first proposed in the seventies by Ruberry and Sheldrake and this visionary prediction was finally proven in the 21st century. The proposed mechanism, often referred as to the chemiosmotic model, entirely relies on the properties of auxin as a weak acid, and the fact that the apoplastic pH is acidic while the cytosolic pH is neutral.

According to the chemioosmotic model, PAT results from the combination of three processes 1) The apolar influx of auxin into the cells by passive diffusion of the acidic form of auxin across the membrane; 2) The dissociation and subsequent trapping of auxin under its anionic form in the cell; and 3) the polar efflux of the anionic form of auxin through the activity of polarised auxin efflux carriers.

As weak acids, the protonation state of auxins is dictated by the pH of the environment; a strongly acidic environment inhibits the forward reaction (dissociation), whereas an alkaline environment strongly favors it (see Henderson-Hasselbach equation):

The export of auxins from cells is termed auxin efflux and the entry of auxin in to cells is called auxin influx. The first step in polar transport is auxin influx. Auxin enters plant cells by two methods, first by passive diffusion as non-ionized IAA molecule or the protonated form as IAAH across the phospholipid bilayer, or second by active co-transport in the anionic form IAA⁻. As IAAH is lipophilic, it can easily cross the lipid bilayer.

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