15.2.3—Gibberellin role in barley germination

In a dormant barley seed, water uptake starts germination and enables the embryo to produce gibberellin. Gibberellin coordinates the mobilisation of stored starch so the embryo receives soluble sugars for growth.

1. The dry, dormant seed absorbs water. The embryo then produces and releases gibberellin.
2. Gibberellin diffuses to the protein-rich aleurone layer surrounding the starch-containing endosperm.
3. In aleurone cells, gibberellin regulates gene expression, increasing transcription of mRNA coding for the hydrolytic enzyme amylase.
4. Amylase is released into the endosperm and hydrolyses stored starch to soluble maltose. Maltose is converted to glucose, which is transported to the embryo.

The embryo respires the glucose to obtain energy for growth, linking hormone signalling to seedling development. The causal chain is water uptake → embryo gibberellin → aleurone gene expression → amylase → starch hydrolysis → soluble sugar → embryo respiration and growth.

Gibberellin is a signal, not the digestive enzyme: amylase performs the starch hydrolysis. The aleurone layer makes the enzyme, while the endosperm stores the starch and the embryo uses the resulting sugars. This card does not describe auxin elongation or Venus flytrap electrical signalling.