describe the role of gibberellin in the germination of barley (see 16.3.4)

Control and Coordination in Plants

Objective

Describe the role of gibberellin (GA) in the germination of barley (see 16.3.4).

Key Concepts

• Gibberellins are a class of plant hormones that promote cell elongation, seed germination, and flowering.
• In barley (Hordeum vulgare) the most biologically active gibberellin is GA₃.
• Germination is a coordinated process that requires the breakdown of stored food reserves and the emergence of the radicle.

Mechanism of GA Action in Barley Germination

1. Water uptake (imbibition): The dry seed absorbs water, swelling the aleurone layer and the embryo.
2. Synthesis of GA in the embryo:

   • GA is synthesised in the scutellum and the embryo axis.
   • The biosynthetic pathway involves conversion of geranylgeranyl diphosphate to GA₁₂, then to GA₃.

3. Transport of GA to the aleurone layer: GA diffuses from the embryo to the surrounding aleurone cells.
4. Activation of hydrolytic enzymes:

   • GA binds to specific receptors in aleurone cells, triggering a signal cascade (cGMP and Ca²⁺ dependent).
   • This induces transcription of genes encoding α‑amylase, β‑amylase, proteases and lipases.
   • Enzyme synthesis peaks 24–48 h after imbibition.

5. Mobilisation of stored reserves:

   • α‑Amylase hydrolyses starch in the starchy endosperm to maltose and glucose.
   • Proteases degrade storage proteins into amino acids.
   • The soluble sugars and amino acids are taken up by the growing embryo.

6. Radicle emergence: The increase in osmotic potential and turgor pressure in the embryo drives the radicle through the seed coat.

Experimental Evidence

Key experiments that demonstrate GA’s role:

• Application of GA₃ to dormant barley seeds breaks dormancy and initiates germination.
• Removal of the embryo (which produces GA) prevents α‑amylase synthesis in the aleurone, halting germination.
• Use of GA biosynthesis inhibitors (e.g., paclobutrazol) suppresses enzyme production and delays germination.

Summary Table

Key Points to Remember

• GA is produced in the embryo and acts on the aleurone layer, not directly on the endosperm.
• The hormone’s primary role is to induce synthesis of hydrolytic enzymes that release stored nutrients.
• Without GA, the aleurone remains inactive, and the seed cannot mobilise its reserves, preventing germination.
• GA action is tightly regulated by both internal signals (e.g., ABA antagonism) and external conditions (temperature, moisture).