IAA Hormone

IAA or indole-3-acetic acid is a plant growth regulator or hormone. It is a type of auxin. It is a naturally occurring plant hormone. It imparts various physiological effects and regulates plant growth and development. It is also found in some bacteria and fungi and controls gene expression and various physiological responses in them.

Auxins were the first plant hormone discovered. Charles Darwin and his son Francis Darwin observed phototropism in the coleoptiles of canary grass and concluded that some substance present at the tip of the coleoptile influenced the bending of the coleoptile. Auxins were first isolated from human urine. F.W. Went isolated auxin from the coleoptiles of oat seedlings.

Structure and Characteristics

IAA is an indole derivative. It contains an indole ring with carboxymethyl substituent. The structure is similar to tryptophan.

Indole-3-acetic acid

IAA is produced in plants at the growing shoot and root apices. It is synthesised in plants by various biochemical pathways mainly starting from tryptophan (amino acid). Environmental factors such as pH, temperature, etc. influence IAA biosynthesis. IBA or indole butyric acid is also a naturally occurring auxin in plants.

Physiological Effects

IAA is responsible for regulating plant growth and development. It imparts many biological effects such as cell division, elongation, differentiation, etc. The main effects of IAA are summarised here:

  • IAA is the main auxin present in the plants. It regulates cell expansion, differentiation, division, etc.
  • IAA promotes apical dominance by inhibiting lateral bud growth. IAA is present in the apical bud that inhibits the growth of lateral or axillary buds.
  • IAA is responsible for phototropism and response to gravity.
  • IAA initiates primary and lateral root formation.
  • IAA also regulates leaf morphogenesis.
  • IAA is also involved in plant-pathogen interactions and the defence mechanism of plants.
  • IAA is involved in plant gene regulation and has a role to play in stress response.
  • Exogenous application of IAA and other auxins is done extensively in agriculture and horticulture.
  • They promote rooting in the stem cuttings and promote flowering.
  • They prevent premature falling of leaves and fruits but promotes abscission of mature ones.
  • Auxins also regulate xylem differentiation.
  • IAA and other auxins work in tandem with other plant regulators synergistically or antagonistically. E.g. The ratio of auxin to cytokinin in the culture medium decides the initiation of root or shoot buds.

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