Gibberellins are a plant growth substance (phytohormone) involved in promotion of stem elongation, mobilisation of food reserves in seeds and other processes. Its absence results in the dwarfism of some plant varieties. Chemically all known gibberellins are gibberellic acids, a family of diterpene acids that are synthesized by the terpenoid pathway in plastids and then modified in the endoplasmic reticulum and cytosol until they reach their biologically active form.

Much of our knowledge of the biosynthesis and molecular mechanisms of gibberellins comes from research on their role in triggering α-amylase release by the aleurone layer in seed germination.

Gibberellin was first isolated in 1926 by Japanese scientists. It was derived from the Gibberella fungus.

Examples: Gibberellin 452D

Location, Characteristics and Occasions for Synthesis Induction

• Synthesized in the embryo and germinating seeds
• Synthesized in the roots
• Levels go up in the dark when sugar cannot be manufactured and down in the light
• Synthesized in apical meristems ? and young leaves ?
• Produced in the stem rather than the growing tip ? (opposite finding to above – conflicting sources)
• Transport is non-polar, bidirectional producing general responses
• Released in mature cells (particularly root) when they do not have enough sugar and Oxygen to support both themselves and any dependent cell
• Released by all cells when they are experiencing conditions which would normally cause a mature root cell to produce GA or BA
• Released in response to root environmental, pest, or disease stress
• Directly induced by high levels of CK

Effects

• Stimulates shoot and cell elongation
• Delays senescence of leaves
• Inhibits root growth
• Inhibits adventitious root growth
• Produces seed germination
• Antagonist promotes root growth and GA reverses this
• Promotes root initiation in low concentration in pea cuttings
• Stimulates bolting and flowering in biennials
• Regulates production of hydrolytic enzymes for digesting starches
• Inhibits CK bud growth on calluses
• Inhibits bud formation
• Inhibits leaf formation
• Breaking of dormancy
• Induces extra Chlorophyll production or more efficient methods of photosynthesis (C4Photsynthesis).
• Stimulates root senescence
• Directly or indirectly induces CK at high levels
• (From Theory II) Inhibits the rate of metabolism of cells in the roots (who are not already at their lowest metabolism rates) in response to an decrease in the levels sugar and/or essential gases