Plant growth regulators and their discovery

A plant's development is controlled by two kinds of factors: intrinsic (inside the plant: its genes, and chemicals called plant growth regulators, PGRs) and extrinsic (outside: light, temperature, water, oxygen, nutrition).

Plant growth regulators are small, simple molecules of different chemical types:

• Indole compounds: IAA (an auxin).
• Adenine derivatives: kinetin (a cytokinin).
• Carotenoid derivatives: abscisic acid (ABA).
• Terpenes: gibberellic acid (GA₃).
• A gas: ethylene.

By action, auxins, gibberellins and cytokinins mostly promote growth. Abscisic acid mostly inhibits it. Ethylene can do both, but mostly acts as an inhibitor.

Each one was discovered almost by accident:

• Auxin: Charles and Francis Darwin noticed that young shoots (coleoptiles) of canary grass bent towards light, and that the signal came from the tip. Later F.W. Went isolated auxin from the tips of oat coleoptiles.
• Gibberellin: rice seedlings infected by the fungus Gibberella fujikuroi grew abnormally tall and weak: the bakanae ("foolish seedling") disease. E. Kurosawa (1926) produced the same effect in healthy seedlings using just the fungus's filtrate.
• Cytokinin: F. Skoog found that pieces of tobacco stem grew into a mass of cells (callus) only if, besides auxin, he added coconut milk, yeast extract or DNA. Miller and others (1955) crystallised the active substance, kinetin.
• Abscisic acid: in the mid-1960s three groups separately found inhibitors, called inhibitor-B, abscission II and dormin. All three turned out to be the same chemical: ABA.
• Ethylene: H.H. Cousins (1910) noticed that ripe oranges gave off a gas that made stored bananas ripen faster.