Plant growth regulators (PGRs), also called plant hormones or phytohormones, are organic compounds produced in small amounts that regulate growth and development by influencing physiological processes. Five major classes: Auxins (IAA = Indole Acetic Acid): promote cell elongation, apical dominance, root initiation, fruit development. Gibberellins (GAs): promote stem elongation, seed germination, bolting, fruit development. Cytokinins (CK): promote cell division, delay senescence, bud formation. Abscisic Acid (ABA): promotes dormancy, stomatal closure, stress response — "inhibitor." Ethylene: promotes fruit ripening, abscission, senescence — gaseous hormone. Additionally, polyamines, brassinosteroids, jasmonates, salicylic acid are also recognised as PGRs.

Gibberellins were first discovered in Japan in the context of the bakanae (foolish seedling) disease of rice caused by the fungus Gibberella fujikuroi (perfect stage of Fusarium moniliforme). Infected plants grew excessively tall, thin, and eventually fell over. Kurosawa (1926) demonstrated the fungal origin of the disease. Yabuta and Sumiki (1938) crystallised the active substance, naming it gibberellin. After World War II, Western scientists (particularly in the UK and USA) independently rediscovered and characterised gibberellins. Currently over 100 structurally related gibberellins are known (GA1 through GA136+), all sharing a common gibban skeleton (a complex tetracyclic diterpene framework). The most abundant and physiologically active gibberellin in higher plants is GA1; GA3 (gibberellic acid) is most commercially produced (by fungal fermentation) and most widely used experimentally and agriculturally.

Gibberellins exert multiple important physiological effects: (1) Stem elongation and bolting: The most dramatic effect — gibberellins promote cell elongation in the internodes by stimulating cell elongation and cell division. In genetically dwarf varieties (which often lack functional GA synthesis genes), exogenous GA application restores normal height. In rosette plants (cabbage, carrot, henbane), GA treatment (or the natural GA surge that occurs in long days) causes dramatic bolting — rapid internode elongation before flowering. (2) Seed germination: Gibberellins promote germination of dormant seeds by stimulating synthesis of hydrolytic enzymes (especially alpha-amylase in cereal aleurone layers) that mobilise endosperm food reserves for the germinating seedling. (3) Fruit development and size: GA treatment can increase fruit size (particularly in grapes — spraying with GA produces larger, elongated, seedless berries) and delay ripening. (4) Flowering: Induces flowering in some long-day plants under short-day conditions. (5) Breaking dormancy: Overcomes dormancy in tubers, bulbs, and seeds requiring cold treatment (vernalisation), replacing the cold requirement with GA treatment in some species.

Gibberellins have several important commercial applications: Grape production: GA3 spraying on Thompson Seedless grapes dramatically elongates the berry clusters, increases individual berry size, and reduces cluster compactness, improving marketability. The grapes become larger, more elongated, and better quality without seeds — this is one of the largest agricultural uses of gibberellins. Malting industry: GA3 is used to accelerate the malting process in barley by stimulating alpha-amylase production in germinating barley seeds, reducing the time and energy needed for malt production for beer brewing. Sugarcane: GA spraying increases internode length (and therefore cane length and sugar yield) in sugarcane without reducing sugar content per unit volume — improving productivity. Seed production: GA application can promote flowering in biennial plants (which normally flower only in their second year) in their first year, speeding up seed production in breeding programmes. Dwarf cereal research: Understanding gibberellin deficiency or insensitivity mutations in wheat and rice (the "Green Revolution" semi-dwarf varieties developed by Norman Borlaug) revealed how reducing plant height through GA pathway manipulation prevents lodging and dramatically increased grain yields, contributing to the Green Revolution that prevented widespread famine in the 1960s-70s.