Lesson 2 - Barley

The term aleurone comes from the Greek word aleuron, which means flour. The aleurone layer develops at the same time as the endosperm cells. It produces most of the enzymes that are not present in the kernel before germination.

The aleurone layer runs below the hyaline layer, and above the endosperm. The barley aleurone is around three cells thick (except over the embryo where it is only one cell thick). It is living tissue that respires. The cells of the aleurone layer do not grow or multiply during germination. The aleurone layer helps produce most of the enzymes required for modification and starch conversion. The following enzymes are synthesised from scratch (de novo synthesis) in this layer:

• α-Amylase
• Limit dextrinase
• Endoproteases

The aleurone layer is also involved in the activation of enzymes such as β-amylase and carboxypeptidase, which are located in the endosperm. The endosperm contains an inactive form of β-amylase. During germination the β-amylase is activated by separating it from an insoluble protein. This is thought to be due to chemical reduction processes and the action of proteolytic enzymes which are likely to originate from the aleurone layer.

Aleurone layer composition

• Lipids (30%)
• Protein (20%)
• Phosphate
• Phytic acid
• Pentosan-cellulose
• Minerals
• Sucrose
• B vitamins
• Cell walls: Pentosans (66%) & β-glucans (28%)

Key points:

• The hyaline layer and the aleurone layer together make up 8-15% of the dry grain weight.
• One side of the aleurone layer is fused to the hyaline layer; the bottom is fused to the endosperm.
• Aleurone cells have dense cytoplasm and prominent nuclei.
• The cells do not contain starch.
• This layer is a source of phosphate and other mineral ions.
• The aleurone layer is the site of enzyme synthesis during germination that helps in the modification of the endosperm.

The photograph below shows a barley cross section with the aleurone layer cell walls in electric blue.

Figure: Barley cross section