P LANT G ROWTH R EGULATORS . . Characteristics The plant growth regulators (PGRs) are small, simple molecules of diverse chemical composition. They could be indole compounds (indole- -acetic acid, IAA); adenine derivatives (N -furfurylamino purine, kinetin), derivatives of carotenoids (abscisic acid, ABA); terpenes (gibberellic acid, GA ) or gases (ethylene, C H ).
Plant growth regulators are variously described as plant growth substances, plant hormones or phytohormones in literature. The PGRs can be broadly divided into two groups based on their functions in a living plant body. One group of PGRs are involved in growth promoting activities, such as cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting and seed formation. These are also called plant growth promoters, e.g., auxins, gibberellins and cytokinins.
The PGRs of the other group play an important role in plant responses to wounds and stresses of biotic and abiotic origin. They are also involved in various growth inhibiting activities such as dormancy and abscission. The PGR abscisic acid belongs to this group. The gaseous PGR, ethylene, could fit either of the groups, but it is largely an inhibitor of growth activities.
. . The Discovery of Plant Growth Regulators Interestingly, the discovery of each of the five major groups of PGRs have been accidental. All this 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 (phototropism).
After a series of experiments, it was concluded that the tip of coleoptile was the site of transmittable influence that caused the bending of the entire coleoptile (Figure . ). Auxin was isolated by F.W. Went from tips of coleoptiles of oat seedlings.
Figure . Experiment used to demonstrate that tip of the coleoptile is the source of auxin. Arrows indicate direction of light a b c d The ‘bakanae’ (foolish seedling) disease of rice seedlings, was caused by a fungal pathogen Gibberella fujikuroi . E.
Kurosawa ( ) reported the appearance of symptoms of the disease in rice seedlings when they were treated with