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Gibberellins

Coordination, Response and Gene Technology · Plant responses · note 3 of 4

Spec 8.11
A Level only (what this means)A Level only: only in the full A Level, not the AS Level. AS Level students can skip it. What the labels mean
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GibberellinsSpec 8.11

In short

Gibberellins are plant growth substances that stimulate stem elongation and seed germination. In a germinating cereal grain, the embryo releases gibberellin, which switches on transcription of the amylase gene in the aleurone layer. Amylase digests starch to sugars that the embryo respires and uses to grow.

Gibberellins are plant growth substances made in young leaves, shoot tips and embryos. They stimulate stem elongation, partly by promoting cell elongation and cell division, and trigger germination of seeds. Dwarf varieties of plants often make little gibberellin.

Germination of a cereal grain (such as barley)

  1. The seed absorbs water and the embryo releases gibberellin.
  2. Gibberellin diffuses to the aleurone layer, the outer layer of the endosperm, and binds to a receptor.
  3. This causes breakdown of a repressor protein that normally stops transcription factors working, so the transcription factors can bind to the amylase gene and switch it on. Gibberellin therefore acts at the level of transcription.
  4. The gene is transcribed and translated, so amylase is made and secreted into the endosperm.
  5. Amylase hydrolyses starch in the endosperm to maltose, which is converted to glucose.
  6. Glucose is transported to the embryo and used in respiration, releasing ATP for growth, and as a source of carbon for cellulose.
Longitudinal section of a barley grain showing seed coat, aleurone layer, endosperm, embryo and scutellum, with numbered arrows for gibberellin, amylase, starch breakdown and sugars moving to the embryo. (opens full size in a new tab)
Gibberellin from the embryo switches on amylase production in the aleurone layer.
Exam tip:

A full answer links hormone, target cell, gene and enzyme: gibberellin, aleurone layer, transcription of the amylase gene, starch digestion.

Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026

Frequently asked questions

What is phytochrome and what does it do?

Phytochrome is a light-sensitive protein pigment in plants that exists as Pr and Pfr. Red light converts Pr to the active Pfr, and far-red light converts it back. Pfr moves into the nucleus and changes which genes are transcribed, controlling responses such as seed germination, stem elongation and flowering.

How does auxin cause phototropism?

Auxin (IAA) is moved to the shaded side of a shoot by carrier proteins. There it increases proton pumping into cell walls and switches on genes for elongation, so the cells on the shaded side elongate more than those on the lit side. The shoot therefore bends towards the light.

How do gibberellins affect seed germination?

In a cereal grain, the embryo releases gibberellin, which diffuses to the aleurone layer and switches on transcription of the amylase gene. The amylase digests starch in the endosperm to maltose and glucose, which the embryo uses in respiration for energy and growth.

All 4 questions on Plant responses