Auxins are weakly acidic phytohormones with an unsaturated ring structure. They act as growth promoter hormones and help in the elongation of shoot cells. Auxin was the first hormone to be discovered.
Auxins were discovered by Dutch botanist F.W.Went (1928) by experimenting on oat (Avena sativa).
The chemical which works as auxin is Indole Acetic Acid (IAA). It was first isolated from human urine. This auxin is produced naturally inside the plant body, therefore, it is also called as natural auxin.
In addition to IAA, some other natural auxins are indole-3-pyruvic acid, indole-3-ethanol, indole-3-acetaldehyde, etc.
There are also present some synthetic auxins. Example- 2,4-dichlorophenoxyacetic acid (2,4-D), indole butyric acid (IBA), and naphthalene acetic acid.
Out of these various types of auxins, Indole-3-acetic acid is the most important natural auxin. It is farmed at the shoot apical meristems, cotyledons, and young leaves. The IAA is then shifted downwards towards the root apex (Polar transport).
Role of Auxins (IAA) in the Plant Body:
Cell Growth: Auxins promote cell enlargement by loosening cell wall microfibrils, solubilization of carbohydrate reserve, increased respiration, increased membrane permeability, and synthesis of new wall microfibrils.
Cell Division in Cambium: IAA promotes cell division in the vascular cambium and helps in the process of secondary growth.
Tissue Formation: Auxins are useful in callus formation, tissue formation, prevention as well as the promotion of flowering.
Auxins in association with cytokinins also help in cell differentiation.
Auxins help in the formation of roots.
Indole Acetic Acid (IAA) helps in the formation of parthenocarpic fruits.
Auxins prevent premature fruit and leaf fall by stopping the process of formation of the abscission zone.
It initiates the development of xylem. If the undifferentiated mass of parenchymatous cells called callus is treated with auxin, xylem elements get differentiated in vascular tissues.
Role of Auxins in the Apical Dominance: It means that the terminal bud (Bud at the top of the main stem) dominates the lower part of the stem body. It stops the growth of lateral buds into branches. If the tip of the main stem is removed then all the lateral buds grow into branches. The whole process can be reversed if the auxins are applied to the decapitated stem tip. It proves that apical dominance is controlled by the auxins.