Plant growth regulators and their discovery

Plant growth regulators (PGRs) are small, simple molecules of varied chemistry, also called plant growth substances, plant hormones or phytohormones.

Chemical families: IAA (indole-3-acetic acid) is an indole compound; kinetin (N⁶-furfurylamino purine) is an adenine derivative; ABA (abscisic acid) is derived from carotenoids; GA₃ (gibberellic acid) is a terpene; ethylene (C₂H₄) is a gas.

Promoters drive division, enlargement, pattern formation, tropic bending, flowering, fruiting and seed formation. Three groups belong here: auxins, gibberellins and cytokinins.

Growth inhibitors act in responses to wounds and to biotic and abiotic stresses, and in dormancy and abscission: abscisic acid. Ethylene could fit either group but is largely an inhibitor.

The discovery of each of the five major groups was accidental. Charles Darwin and his son Francis saw canary grass coleoptiles bend towards one-sided light (phototropism) and concluded that the tip was the source of a transmittable influence causing the bend. F.W. Went later isolated auxin from oat coleoptile tips.

Gibberellin: the bakanae (foolish seedling) disease of rice is caused by the fungus Gibberella fujikuroi. E. Kurosawa found that sterile filtrates of the fungus produced the symptoms in healthy seedlings; the active substance was later identified as gibberellic acid.

Cytokinin: F. Skoog and co-workers found that callus from tobacco internode segments proliferated only if, besides auxin, the medium had vascular tissue extract, yeast extract, coconut milk or DNA. Miller and co-workers later crystallised the active substance promoting cytokinesis and named it kinetin.

Abscisic acid: in the mid-1960s three independent groups purified inhibitor-B, abscission II and dormin, which proved to be chemically identical and were named abscisic acid.

Ethylene: H.H. Cousins confirmed that ripe oranges release a volatile substance that hastens the ripening of stored unripe bananas; it was later identified as ethylene.