The history and functions of Auxin hormone

Do you know how auxin hormone was discovered? The below article will provide you with the history of auxin hormones. You will also find the functions of auxin hormone. To know about the detail of the auxin hormone, please read this article.


Auxins are weak organic compounds having an unsaturated ring structure and capable of promoting cell elongation, especially of shoot and roots in low concentration. Auxin is the first plant hormone to be discovered and are perhaps the most important group of phytohormones.

History of Auxins

The discovery of auxins was starting from the classical work of Charles Darwin (1880) while working on Canary grass (Phlaris canariensis). Darwin observed that when unilateral light was given to the tip of grass coleoptile, it exhibited positive phototrophic curvature and when the tip was removed it fails to cause curvature. The tip also failed to show curvature when it was covered by a tin foil cap. On this basis, he predicted that the tip contained a growth substance which was transmitted to the lower portion where it causes curvature. Boysen- Jensen (1910-1913) found that if a transverse slit was caused in the coleoptile of Avena on the dark side and a piece of mica was inserted into the slit, no phototropic response took place. On the other hand, there was a phototropic response (curvature) if the slit and the piece of mica were on the illuminated side. Based on these results they conclude that a substance migrates down the dark side promoting growth curvature towards the light. Paras (1919) found that when the excised tip is replaced on one side of Avena coleoptile stump, accelerated growth beneath the tip, resulting in curvature. F.W. Went (1928) working in Avena coleoptile and succeed in isolating the growth substance. He cut coleoptile tips and placed them on agar blocks. After a particular time, he cut the agar block into small cubes, discarded the coleoptile tips and placed the agar cubes eccentrically on decapitated coleoptile stumps for two hours in dark and found that curvature was produced. He concluded that some substances are responsible for unilateral growth. This substance is synthesized in coleoptile tip and it translocated downwards. He found that the hormone diffused into the agar is diffused into coleoptile and caused growth and curvature like a coleoptile tip. He called this substance as auxin. In his, another experiment, Went separated two agar blocks by a mica plate. He kept the oat coleoptile on te agar block so that half-half portion of the coleoptile remain on the two blocks. When the coleoptile was illuminated, he observed that 65% of auxin from the coleoptile was diffused into the block of unilluminated side. Remaining 35% auxin was found diffused into the illuminated side agar block.

Distribution of Auxins in plants

Auxins are commonly found in plants. Auxins are mostly concentrated on growing apices like coleoptile tips, floral and shoot buds, root apices and leaf apices. The distribution of auxins in the plant body was studied in detail by Thimann (1934) in etiolated Avena coleoptile. He proved successfully that auxins are concentrated on root and shoot tips. Thimann and Skoog observed a decreasing order auxin concentration in apical buds, young leaves and mature leaves.
Apical buds > Young leaves > Mature leaves

Physiological effects of Auxins

The physiological effects of auxins are given below:

  1. Cell division and cell enlargement: Auxins stimulate cell division, cell enlargement and cell elongation in the apical region.

  2. Root and shoot growth: Auxins promote the growth of shoots at a relatively higher concentration and that of the root at a very low concentration. In stem cutting of Rose and other plants, the cut end of the stem is deep into the solution of phenylacetic acid, alpha naphthyl butyric acid, indole acetic acid or other auxins to promote early rooting in the plantlet.

  3. Xylem differentiation: The quality of auxin promoting xylem element differentiation is now well established and is taken as a measure of auxin bioassays. Because of this property, auxins have been commonly used in bud grafting into a callus. The callus is a mass of undifferentiated parenchymatous cells and when treated with auxin, xylem elements get differentiated into the callus tissue. By adding auxin and sugar, the continued growth of callus may be obtained and new shoot s and even new plants can be produced from callus.

  4. Apical dominance and dormancy: Presence of auxin in the tip stimulates the growth of the apical bud but suppresses the growth of lateral buds, this phenomenon is called apical dominance. If the apical buds are removed and auxin is supplied to the decapitated shoot the lateral buds are suppressed confirming that auxins are responsible for apical dominance. The principle of apical dominance is applied by gardeners who use to cut the apices of twigs of hedges at regular interval. This process leads to dense branching in the hedge which looks like a dense attractive shrub.

  5. Prevention of abscission: Very often leaves and floral buds fall before the development of fruits. This happens due to the formation of the abscission layer at their base. When diluting solution of auxins like 2, 3-D, N.A.A, etc. are sprayed on plants leaf and bud fall is stopped. This leads to an increase in the production of fruits.

  6. Weed control: Unwanted or undesirable plants present along with crops is called weed. Weeds cause competition for water, minerals, light and space. This causes poor yield. By the use of 2,4-D as a spray, broad-leaved weeds can be destroyed. It does not affect monocotyledon plants.

  7. Reactivation of cambium activity: Auxins trigger activity in the cambium. Therefore, it is used to join the cambium of stock and scion in grafting.

  8. As a chemical weapon in war: 2,4-D and 2,4,5-T were used in the Vietnam war to destroy the crops of enemies. Hundreds of hectares of forest were also destroyed by the spraying of such hormones.

Kogl and Haagen Smit (1931) have isolated some auxins which are auxin-a, auxin-b, heteroauxin. These are called natural auxins. Some auxins have also been synthesized chemically which function like natural auxins. These are called synthetic auxins. Some synthetic auxins are P.A.A., P.O.A, 2,4-D, 2,4,5-T, etc.


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