The polar auxin transport mechanism undergoes cell-to-cell process, and it transports molecules through plasma membrane and crosses the middle lamella that directly enters the plasma membrane of another plant cell. The mechanism takes place by proton when hormone enters or uptakes the cell by protonation process. On contact with the cytoplasmic pH of the cell, the auxin involves a deprotonation mechanism, and in its charged form consequently, it is readily trapped in the inner side of the cell. Auxin detaches from the activity of auxin efflux carrier that is present in the plasma membrane. The basal localization of auxin efflux occurs through the polar transport. The inhibitor of polar auxin transport known as phytotrophin undergoes a site of action like 1-N-naphthylphthalamic acid (NPA). The polar transport stops the mechanism in a non-competitive manner that breaks the site to undergo efflux activity. The 1-N-naphthylphthalamic acid (NPA) binding sites breakdown the polypeptide that controls the auxin efflux carrier. This 1-N-naphthylphthalamic acid (NPA) was prevented by monoclonal antibodies that bind to the microsomal of pea membrane proteins that are present in the binding site mainly to the pea stem of the plasma membrane. The important feature of polar auxinis transport. The movement of auxin intake is dependent on the pH gradient or PMF to transport through the plasma membrane. The release of auxin efflux is determined by the occurrence of auxin efflux carriers that are present at the auxin conducting cells in the base.

Auxin Influx (Uptake):

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Based on the chemiosmotic model, the plant auxin influx is taken from all the directions, and the indole-3-acetic acid (IAA) takes place in two forms as

Undissociated form or protonated form – The lipophilic compound crosses the plasma membrane.

Dissociated or anionic form – Not able to cross the plasmamembrane.

In highly acidic conditions, the indole-3-acetic acid (IAA) hormone takes place in the cell wall (apoplast). At low pH, the apoplast is controlled by the activities of ATPases that are present in the cell wall and is associated with protonated (H+) that is converted to indole-3-acetic acid (IAA) hormone. The hormone transports to the plasma membrane. The anionic form of indole-3-acetic acid (IAA) transfers across the plasma membrane in the cell wall; this mechanism is called secondary transport. The secondary transport acts as a symporter that transfers 2H+/IAA–called AUX1 that equally spreads in the cell. In 1996, arabidopsis was discovered by Bennet.

Auxin Efflux Carrier:

The process occurs in the cytosol, where the higher pH is comparatively neutral (about 7) in the cell wall. In indole-3-acetic acid, the hormone (IAAH) breaks into H+ and IAA. The auxin dissociates in the cytosol under anionic condition, and indole-3-acetic acid exits in the cell, which presents the auxin efflux carriers known as PIN proteins. The indole-3-acetic acid inside the cell has negative potential with auxin efflux in every cell at the longitudinal pathway.

Regulation of Polar transport:
The mechanism of PIN proteins with most exogenous and endogenous regulators is as follows:

Mechanical stress

In mechanical stress, the signals are regulated with the polarity of PIN.

Vesicle Trafficking

The auxin efflux carrier is present in the cell membrane or plasma membrane, which involves the vesicles and endocytosis that is regulated. The final vesicle is trafficking the actin cytoskeleton.

Inhibitors of the transport

2,3,5-triiodobenzoic acid (TIBA) and 1-N-Naphthylphthalamic acid (NPA) act as particular inhibitors of the auxin efflux carrier. The naturally occurring polar transport inhibitors are Quercetin and Genistein. They prevent the growth of plants bilaterally at the globular stage. These inhibitors are produced by a combination of cotyledons in the shape of globular.


The PIN proteins are located in the plasma membrane that is monitored by the auxin. The model used to design PIN proteins is polarized in the chief concentration of cytosolic auxin known as “up-the-gradient”. The other model is known as the “with-the-flux” model that shows a pattern of vascular strands.