13.4 Plant Growth Regulators

• The plant growth regulators (PGRs) are small, simple molecules of diverse chemical composition
• PGRs could be indole compounds (indole-3-acetic acid, IAA); adenine derivatives (N6 -furfurylamino purine, kinetin), derivatives of carotenoids (abscisic acid, ABA); terpenes (gibberellic acid, GA3) or gases (ethylene, C2H4 )
• The group of PGRs involved in growth promoting activities are called plant growth promoters (auxins, gibberellins and cytokinins)
• The PGRs of the growth inhibitor group play an important role in plant responses to wounds and stresses of biotic and abiotic origin. They are involved in various growth inhibiting activities such as dormancy and abscission.
• 1. Discovery of Auxin (Indole derivative):

  • Darwin and Darwin observed bending of coleoptile of canary grass w.r.t light
  • F.W Went isolated auxin from oat seedling
• 2. Discover of Gibberellic Acid (Terpene derivative):

  Kurosawa reported Bakane disease in rice caused by fugus Gibberella Fujikori
• 3. Discovery of Cytokinin (Adenine derivative):

  • F. Skoog and his co-workers observed the internodal segments of tobacco stems
  • The callus (a mass of undifferentiated cells) proliferated only with Vascular tissue, yeast extract, coconut milk or DNA
  • Miller et al. (1955), later identified and crystallised the cytokinesis promoting active substance that they termed kinetin
• 4. Discovery of Abscisic Acid (Carotenoid derivative):

  • Three independent researches reported the purification and chemical characterisation of three different kinds of inhibitors: inhibitor-B, abscission II and dormin
  • They were all proved to be chemically identical
• 5. Discovery of Ethylene (Gases):

  • H.H. Cousins (1910) confirmed the release of a volatile substance from ripened oranges that hastened the ripening of stored unripened bananas