The Discovery of Plant Growth Regulators

Interestingly, the discovery of each of the five major groups of PGRs has been accidental — in every case an observation made for some other reason led to the identification of a growth regulator.

• Auxins — It all started with the observations of Charles Darwin and his son Francis Darwin, who noticed that the coleoptiles of canary grass responded to unilateral (one-sided) illumination by growing towards the light source, a response called phototropism. After a series of experiments it was concluded that the tip of the coleoptile was the site of a transmittable influence that caused the bending of the entire coleoptile. Auxin was later isolated by F. W. Went from the tips of coleoptiles of oat seedlings.
• Gibberellins — The bakanae, or foolish seedling, disease of rice seedlings was caused by the fungal pathogen Gibberella fujikuroi. E. Kurosawa (1926) reported the appearance of the symptoms of the disease when rice seedlings were treated with sterile filtrates of the fungus. The active substances were later identified as gibberellic acid.
• Cytokinins — F. Skoog and his co-workers observed that callus (a mass of undifferentiated cells) from the internodal segments of tobacco stems proliferated only if, in addition to auxins, the nutrient medium was supplemented with one of the following: extracts of vascular tissues, yeast extract, coconut milk or DNA. Miller and colleagues (1955) later identified and crystallised the cytokinesis-promoting active substance, which they named kinetin. …