gibberellin

A gibberellin is a major type of plant hormone (auxins are the other major type). Gibberellins are produced in comparatively large quantities by plant seeds, algae, mosses, ferns and fungi (especially fungi in the genus Gibberella). Gibberelic acid is commonly used in seed germination experiments to induce growth.

Gibberellins can produce dramatic growth, especially in plant stems. If a plant under-produces gibberellins, it becomes a dwarf. Gibberellins are relatively expensive, but have been used in the production of seedless grapes and navel oranges and have been found to reverse the effects of some herbicides.

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This writeup is based partly on work I did for the science dictionary at http://biotech.icmb.utexas.edu/

Gibberellins

A gibberellin is any of several growth-regulating plant hormones which are produced by seeds, mosses, ferns, algae and fungi. All gibberellins which have so far been discovered are gibberellic acids, and this family of phytohormones are thought to have a similar effect to that had by auxins: they are involved in the promotion of cell division and elongation, the mobilization of food reserves in seeds, flowering and fruit growth.

Gibberellin Deficiency and Dwarfism

Plant development is controlled by several groups of hormones, which are highly active substances present in plants at very low concentrations. Although gibberellins do not in themselves inhibit growth, their absence can result in dwarfism in some plant varieties. Plants that are unable to produce sufficient gibberellin or to respond to it are termed developmental dwarfs.

Many such dwarfs are known, some of which are utilised in agriculture and horticulture since they are more manageable than taller genotypes. For example, dwarf peas and French beans are commonly grown as garden varieties. Since dwarfs with suitable characteristics are not available for many agriculturally important species, it is common practice to apply growth retardants, particularly on cereals and ornamental species. These chemicals act by blocking specific steps in the biosynthetic pathway by which gibberellins are produced in plants.

Gibberellic Acid and Germination

Gibberellic acid, the most commonly found member of the gibberellin family, is thought to be responsible for promoting growth in the embryo of a seed by the following mechanism:

• Gibberellin released by the embryo travels towards the aleurone layer, a tissue situated in the endosperm of the seed.
• Gibberellin acts as a catalyst for the production of amylase, which is then able to break down starch into its component sugars
• Sugar produced in this way is used by the growing embryo to synthesise proteins and break out of dormancy.

Gibberellins initiate this process in summer, when the external environment exhibits favourable conditions for plant growth.