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

Control and Coordination in Plants – Gibberellin in Barley Germination (Topic 15, sub‑topic 16.3.4)

Learning objectives

• Describe where and how gibberellin (GA) is synthesised in a barley seed.
• Explain the GA‑GID1‑DELLA signalling cascade that leads to hydrolytic‑enzyme production.
• Analyse the antagonistic and synergistic interactions between GA and other plant hormones.
• Relate environmental factors to GA biosynthesis and action.
• Interpret experimental data and design simple experiments that test GA function.
• Connect the role of GA in germination to its wider functions in plant growth (stem elongation, flowering, fruit development).

1. Major plant hormones at a glance (AO1)

2. GA biosynthesis and its regulation in barley (AO1 & AO2)

2.1 Precursors and core pathway

• Isopentenyl diphosphate (IPP) → geranylgeranyl diphosphate (GGDP) (plastidic MEP pathway).
• GGDP is converted to the diterpene ent‑copalyl diphosphate (CDP) by CPS (copalyl diphosphate synthase).
• CDP → ent‑kaurene via KS (kaurene synthase).
• Oxidation steps:
  • KO (kaurene oxidase) and KAO (kaurenoic acid oxidase) convert ent‑kaurene to GA₁₂.
  • GA₁₂ → GA₁₅ → GA₂₀ → GA₁₉ → GA₂₀ (inactive) → GA₁ (active) via GA20‑oxidase (GA20ox) and GA3‑oxidase (GA3ox).
  • Deactivation occurs through GA2‑oxidase (GA2ox) which converts active GAs to inactive forms.
• In barley the most biologically active form during germination is **GA₃** (derived from GA₁₉).

2.2 Spatial and temporal control

• Site of synthesis: Scutellum and embryo axis (the “embryo” compartment).
• Regulation by water: Imbibition triggers transcription of GA20ox and GA3ox genes.
• Feedback inhibition: High GA levels suppress GA20ox/GA3ox expression and up‑regulate GA2ox (negative feedback loop).
• Environmental cues: Optimal temperature (15‑20 °C) and adequate oxygen promote enzyme activity; darkness has little direct effect on GA synthesis in barley seeds.

3. GA action during barley germination (AO1, AO2 & AO3)

3.1 Timeline of events (hours after imbibition)

3.2 GA‑GID1‑DELLA signalling cascade (diagram suggestion)

1. Perception: GA₃ binds the intracellular receptor GID1.
2. Complex formation: GA‑GID1 interacts with DELLA proteins (e.g., RGA, SLR1).
3. Ubiquitination: The complex recruits the SCF^SLY1 E3‑ligase; DELLA is poly‑ubiquitinated.
4. Proteolysis: 26S proteasome degrades DELLA, removing repression.
5. Transcriptional activation: GA‑responsive transcription factors (e.g., GAMYB) switch on genes encoding hydrolytic enzymes.
6. Secondary messengers: Ca²⁺ spikes and cGMP amplify the signal, enhancing gene expression.

4. Hydrolytic enzymes induced by GA (AO2)

• α‑Amylase: Endo‑hydrolyses starch → maltose & glucose (major source of carbon).
• β‑Amylase: Exo‑hydrolyses maltose from starch fragments.
• Proteases (e.g., cysteine proteases): Break down storage proteins → amino acids.
• Lipases: Release fatty acids from minor oil bodies (more important in wheat/maize).

5. Hormonal interactions (AO2)

5.1 GA ↔ Abscisic acid (ABA) – antagonism

• ABA dominates in mature, dormant seeds; it inhibits GA‑biosynthetic genes and stabilises DELLA proteins.
• During imbibition ABA is catabolised by **ABA‑8′‑hydroxylase**, allowing GA levels to rise.
• Experimental pattern: exogenous ABA blocks α‑amylase synthesis even when GA is supplied; high GA can partially overcome low‑dose ABA.

5.2 GA ↔ Ethylene – synergism

• Ethylene production spikes after water uptake.
• Ethylene enhances GA‑induced α‑amylase activity and promotes aleurone cell expansion.
• Inhibitors such as AVG (aminoethoxyvinylglycine) slow radicle emergence despite normal GA levels.

5.3 GA ↔ Auxin & Cytokinin – modulation

• Auxin transport from embryo to aleurone increases GA sensitivity; exogenous IAA lowers the GA concentration needed for α‑amylase induction.
• Cytokinins up‑regulate GA20ox/GA3ox expression, reinforcing the germination signal.

5.4 GA ↔ Brassinosteroids – supportive role

• Brassinosteroids (BR) promote cell expansion and can act additively with GA in coleoptile elongation after germination.
• While BR do not directly induce α‑amylase, they enhance the growth response once reserves are mobilised.

6. Environmental regulation of the GA pathway (AO2)

• Temperature: 15‑20 °C yields maximal GA₃ synthesis; < 10 °C or > 30 °C suppresses enzyme activity and delays germination.
• Moisture: Adequate water is required for GA biosynthesis, receptor activation, and secondary‑messenger spikes.
• Oxygen: Aerobic respiration supplies ATP for biosynthetic steps; hypoxia reduces GA production.
• Light: Little direct effect on GA during barley germination, but photoreceptors modulate downstream growth (e.g., coleoptile phototropism) after emergence.

7. Experimental evidence (AO2)

1. GA₃ application: Soaking dormant barley seeds in 10 µM GA₃ breaks dormancy; α‑amylase mRNA appears within 6 h and enzyme activity peaks at 24 h.
2. Embryo removal: Isolating the aleurone layer from the embryo abolishes α‑amylase synthesis, confirming the embryo as the GA source.
3. GA‑biosynthesis inhibitors: Paclobutrazol (10 µM) or uniconazole suppress GA production, leading to < 30 % α‑amylase activity and delayed radicle emergence.
4. DELLA mutants: Barley lines with reduced DELLA protein levels (e.g., *sly1* mutants) show premature α‑amylase expression and faster germination.
5. ABA antagonism test: Adding 10 µM ABA to GA‑treated seeds cuts α‑amylase activity by ~60 %; simultaneous application of ABA‑8′‑hydroxylase restores activity to control levels.
6. Ethylene synergy: 5 µM ethephon (ethylene donor) together with sub‑optimal GA (2 µM) restores normal α‑amylase levels, whereas AVG (1 µM) reduces them.

8. Skills focus for A‑Level examinations

• Diagram interpretation: Students may be asked to label a flow‑chart of the GA‑GID1‑DELLA pathway or to explain how a mutation in the GID1 receptor would affect α‑amylase production.
• Quantitative reasoning: Given a graph of α‑amylase activity versus time for control, GA‑treated and ABA‑treated seeds, calculate the percentage inhibition caused by ABA.
• Experimental design: Propose an experiment to test whether a new chemical inhibits GA biosynthesis; include controls, expected results, and a method of measuring GA (e.g., ELISA or HPLC).
• Data interpretation: Analyse a table showing radicle length after 48 h under different temperature regimes and relate the trend to GA synthesis efficiency.

9. Summary tables

9.1 GA concentration, enzyme activity and physiological effect

9.2 Hormonal interactions in seed germination

10. Key points to remember (AO1)

• GA is synthesised in the embryo (scutellum & axis) and acts on the aleurone layer, not directly on the starchy endosperm.
• The GA‑GID1‑DELLA signalling cascade removes DELLA repression, allowing transcription of hydrolytic‑enzyme genes.
• α‑Amylase induction is the primary mechanism by which GA mobilises stored starch for radicle growth.
• ABA antagonises GA by suppressing its biosynthesis and stabilising DELLA; the GA : ABA balance decides whether a seed remains dormant or germinates.
• Ethylene, auxin, cytokinin and brassinosteroids modulate the GA response, illustrating integrated hormonal control.
• Environmental factors (water, temperature, oxygen) influence GA synthesis and downstream signalling.
• Beyond germination, GA also promotes coleoptile/ stem elongation, flowering and fruit development, often acting together with brassinosteroids.