Plant Growth regulators are chemical compounds and are also called plant hormones. PGRs include auxin, giberrellin, cytokinin, abscisic acid and ethylene. They are divided into two groups – plant growth promoters and plant growth inhibitors. Plant growth promoters enhance growth activities like cell division, cell enlargement, flowering, fruiting and seed formation. Auxin, gibberellin and cytokinin act as plant growth promoters. Plant growth inhibitors respond to wounds and stress from biotic and/or abiotic factors by inhibiting growth.
The discovery of PGRs was accidental. The discovery of auxin was initiated when Charles Darwin and his son Francis Darwin observed the coleoptile of canary grass exhibiting phototropism towards blue light. After several experiments, they found that the tip of the coleoptile is responsible for the bending of the full coleoptile. Later, F W Went discovered that a substance at the tip of the plant was responsible for its growth and called it ”auxin”, which means ‘to grow’ in Greek. But the chemical structure of auxin was identified when it was isolated from the urine of a patient by Kogl and Thimann.
The discovery of gibberellin is linked to E Kurosawa who observed abnormal tallness in rice seedlings struck by Bakane or Foolish Seedling Disease. This disease is caused by the fungus Gibberalla fujikuroi. Later, Yabuta and Sumiki isolated the substance secreted by the fungus and called it ”gibberellin”.
The discovery of cytokinin is linked to F Skoog and his co-workers, who noticed that cells extracted from the internodal segment of the tobacco stem proliferated to callus when the nutrient medium is supplemented with extracts of vascular tissues, yeast and coconut milk in addition to auxin. Later, it was Skoog and Miller who isolated and crystallised the substance that promoted cytokinesis in plant cells. It was then named “kinetin”.
The discovery of abscisic acid dates back to the 1960s when three independent researchers purified three different inhibitors of plant growth, namely inhibitor-B, abscission II and dormin. Later, they were chemically identical and were called “abscisic acid”. The falling of leaves due to scars; and dormancy, are due to the action of plant growth inhibitors.
The discovery of ethylene is credited to a scientist named Cousins who observed the release of a volatile substance from ripened oranges. He also found that the volatile substance promoted ripening of other unripened oranges. Later, this volatile substance was identified as”ethylene”.