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The term auxin is derived from Greek word ‘auxein’ means to grow. First time F.W. Went coined the term ’auxin’ in 1928 meaning‘growth arouser’.The natural auxins are synthesized in the meristematic
tissues of the plants such as in root tips, apical buds, young leaves and flowers. The natural auxins are those auxins which are synthesized into plant such as Indole -3- acetic acid (IAA), Indole acetaldehyde (IA), Indole aceto nitrile (IAN) and Indole pyruvic acid (IPyA). The synthetic auxins are chemicals manufactured outside the plant  such as Indole-3-butyric acid (IBA), Naphthalene acetic acid (NAA), 2,4-Dichlorophenoxy acetic acid (2,4-D), 2,4,5-Trichlorophenoxy acetic acid (2,4,5-T), 2,4,6-Trichlorobenzoic acid (2,4,6-TBA), Indole-3-propionic acid ( ) and Phenylacetic acid (PAA). Auxins move in polar direction (unidirectional) in plants. Its movement is basipetal i.e. from tip to base in shoots and acropetal i.e. from base to tip in roots.


An amino acid ‘Tryptophan’ is precursor of auxins because auxins are synthesized from ‘Tryptophan’. The tryptophan amino acid is either synthesized or derived from the hydrolysis of protein. There are three different pathways of IAA synthesis into the plants:
  1. Indole pyruvic acid pathway: In the path tryptophan is converted into indole pyruvic acid and indole acetaldehyde is synthesized from indole pyruvic acid in the presence of enzymes and indole acetaldehyde is oxidized into indole acetic acid (IAA): Tryptophan →Indole pyruvic acid→ Indole acetaldehyde→ IAA
  2. Tryptamine pathway: In the pathway indole acetaldehyde is synthesized from tryptamine: Tryptophan→ Tryptamine→ Indole acetaldehyde→ IAA
  3. Indole actaldoxime pathway: The pathway observed in Brassicaceae and consists steps:Tryptophan→Indoleacetaldoxime→Glucobrassicin→Indoleacetonitrile→IAA

The auxins are synthesized in the plant parts have higher meristematic activity such as leaves, fruits, coleoptiles, embryos and pollen grains. The biosynthesis of auxins is more in light than dark.

Antagonists of auxins

 The antagonists are compounds interfere with auxins activity indirectly such as Monophenols. The compounds interfere with auxin activity are termed as antiauxins such as TIBA (2,3,5-triiodobenzoic acid), DCA (2,4-dichloraniosole) and NTA (Naphthyl thalamic acid).

Role of auxins

  • Apical dominance
 The auxin synthesizes in the apical duds of the plants inhibits the formation and growth of lateral buds. The suppression of lateral bud’s growth by apical buds is called apical dominance.
  • Cell elongation
 The auxin promotes the cell elongation and rate of elongation is directly proportional to auxin concentration in shoot whereas it promotes root cells elongation at very low concentration and higher concentration of auxin in roots retards the root elongation.
  • Cell division
 Auxin promotes the cell division and callus formation. It also induces the cambium formation and thereby thickness of branches.
  • Adventitious roots initiation
The auxins promote the adventitious roots formation due to which auxins particularly IBA and NAA are used as root promoting hormones in air layering and cuttings of plants.
  • Development of parthenocarpy fruits
 The use if auxins can induce fruit formation without fertilization such as in apple, tomatoes, and bananas.
  • Control of flowers and fruits drop
 The auxins control the premature flowers and fruits drop for example spraying of 2,4 D or 2,4,5-T on citrus and apples checks the flowers and fruits dropping.
  • Weedicides
 Auxins like 2,4 D application at higher concentration eradicates broad leaves weeds.
  • Sex expression
 The spraying of auxins on cucurbits increases the number of female flowers.

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