Transport of Auxins in plants.

 Transport of Auxin

The transport of auxin in plant is predomi­nantly polar. In stems, polar transport of auxin is basipetal i.e., it takes place from apex towards base. In roots also, the auxin transport is polar but is primarily acropetal.

The main axes of shoots and roots, along with their branches, exhibit apex-base structural polarity, and this structural polarity is dependent on the polarity of auxin transport. Soon after Went developed the coleoptile curvature test for auxin, it was discovered that IAA moves mainly from the apical to the basal end (basipetally) in excised oat coleoptile sections. This type of unidirectional transport is termed polar transport.

Auxin is the only plant growth hormone that has been clearly shown to be transported polarly, and polar transport of this hormone is found in almost all plants. Because the shoot apex serves as the primary source of auxin in the plant, polar transport has long been believed to be the principal cause of an auxin gradient extending from the shoot tip to the root tip. The major sites of polar auxin transport in the stems, leaves, and roots of most plants are the vascular parenchyma tissues, most likely those associated with the xylem. In grass coleoptiles, basipetal polar transport may also occur in nonvascular parenchyma tissues. Embryonic polar auxin transport is initially described as entirely basipetal, as the embryo has no root.

The downward direction of auxin transport in the embryonic vascular parenchyma is maintained in the root vascular cylinder throughout the life of the plant. A chemiosmotic model for polar auxin transport proposes that auxin uptake is driven by the proton motive force across the plasma membrane, while auxin efflux is driven by the membrane potential.

The first step in polar transport is auxin influx. Auxin enters plant cells nondirectionally via passive diffusion of the protonated form (IAAH) across the phospholipid bilayer or via secondary active transport of the dissociated form (IAA-) through a 2H+-IAA-symporter. Once IAA enters the cytosol, which has a pH of approximately 7.2, nearly all of it dissociates to the anionic form. Because the membrane is impermeable to the anion, auxin accumulates inside the cell or along membrane surfaces unless it is exported by transport proteins on the plasma membrane. According to the chemiosmotic model, transport of IAA- out of the cell is driven by the negative membrane potential inside the cell.

Auxin transport in shoot tends to be predominantly Basipetal. (Very Imp)