Plant Growth Regulators: Characteristics and Discovery

Plant growth regulators (PGRs), also called phytohormones, are small, chemically diverse organic molecules that a plant synthesises in tiny amounts and that, even at very low concentration, regulate its own growth, differentiation and developmental responses -- speeding some processes up, slowing others down, and coordinating the activity of cells and tissues that may lie some distance from where the regulator was actually produced. They are chemically diverse rather than belonging to one family of compounds: auxin is built on an indole ring, gibberellins are terpenes (specifically diterpenoids), cytokinins are derivatives of the purine base adenine, abscisic acid is a derivative of a carotenoid pigment, and ethylene is simply a small, unsaturated hydrocarbon gas -- yet all five are grouped together functionally because each acts, at minute concentration, as an internal chemical signal governing some aspect of growth or development.

By their overall physiological effect, the five classical PGRs are broadly sorted into two functional groups. Auxins, gibberellins and cytokinins are generally described as plant growth promoters, because their best-known and most consistent effects are to stimulate cell division, cell elongation, or both, and to promote processes such as flowering and fruit development. Abscisic acid, by contrast, is generally described as a plant growth inhibitor, because its best-known effects -- inducing and maintaining seed and bud dormancy, closing stomata under water stress, and generally slowing growth -- act in the opposite direction. Ethylene sits somewhat apart from this two-way split: it promotes some processes (notably fruit ripening and the rapid internode elongation seen in some flooded plants) while inhibiting others (notably further vegetative growth once ripening or senescence has begun), so it is best understood on its own terms rather than forced into either the 'promoter' or 'inhibitor' category. …