Class 11 Biology Plant Growth Development Plant growth regulators

Plant growth regulators

  • The plant growth regulators (PGRs) are small, simple molecules of diverse chemical composition.

Examples- Indole compounds (indole-3-acetic acid, IAA); adenine derivatives (N6-furfurylamino purine, kinetin), derivatives of carotenoids (abscisic acid, ABA); terpenes (gibberellic acid, GA3); gases (ethylene, C2H).

  • The PGRs can be broadly divided into two groups based on their functions
  • Plant growth promoter
  • Plant growth inhibitor
  • Plant growth promoters are involved in growth promoting activities, such as cell division, cell enlargement, pattern formation.

For example- auxins, gibberellins and cytokinins.

  • Plant growth inhibitors are involved in various growth inhibiting activities such as dormancy and abscission.

For example- abscisic acid

  • Ethylene acts as both promoter and inhibitor, though it is largely an inhibitor of growth activities.
  • Discovery of plant growth regulators started with the observation of Charles Darwin and his son Francis Darwin when they observed that the coleoptiles of canary grass responded to unilateral illumination by growing towards the light source, and finally concluded that the tip of coleoptile was the site of transmittable influence that caused the bending of the entire coleoptile.
  • Auxin was isolated from tips of coleoptiles of oat seedlings.
  • The ‘bakane’ (foolish seedling) a disease of rice seedlings, was caused by a fungal pathogen Gibberalla fujikuroi, symptoms of the disease in uninfected rice seedlings appeared when they were treated with sterile filtrates of the fungus, which is now known as gibberellic acid.
  • It was seen that the callus proliferated only if, in addition to auxins, the nutrients medium was supplemented with extracts of vascular tissues or yeast extract or coconut milk or DNA; and later the cytokinesis promoting active substance was identified and crystallised that was termed as kinetin.
  • Purification and chemical characterisation of three different kinds of inhibitors, inhibitor-B, abscission II and dormin, showed them to be chemically identical, which was named abscisic acid (ABA).
  • The release of a volatile substance from ripened oranges hastened the ripening of stored unripened bananas, which was identified as ethylene.



Fig. Gibberalla fujikuroi 



Fig. coconut milk


Fig. yeast extract

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