explain how gibberellin activates genes by causing the breakdown of DELLA protein repressors, which normally inhibit factors that promote transcription

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology 9700 – Gene Control: Gibberellin and DELLA Proteins

Gene Control – Gibberellin Activation of Transcription

Learning Objective

Explain how gibberellin (GA) activates genes by causing the breakdown of DELLA protein repressors, which normally inhibit factors that promote transcription.

Key Concepts

  • Gibberellin – a plant hormone that promotes growth and development.

  • DELLA proteins – nuclear repressors that block transcription of GA‑responsive genes.

  • GA receptor GID1 – a soluble protein that binds GA and initiates DELLA degradation.

  • SCFSLY1/GID2 complex – an E3 ubiquitin ligase that tags DELLA for proteasomal destruction.

  • Transcription factors (e.g., PIFs, MYB) – become active once DELLA repression is removed.

Step‑by‑Step Mechanism

  1. GA perception: GA diffuses into the nucleus and binds to the GID1 receptor, forming a GA–GID1 complex.

    \$\text{GA} + \text{GID1} \rightleftharpoons \text{GA–GID1}\$

  2. DELLA recognition: The GA–GID1 complex undergoes a conformational change that creates a high‑affinity surface for DELLA proteins.

  3. Ubiquitination: The GA–GID1–DELLA ternary complex recruits the SCFSLY1/GID2 E3 ubiquitin ligase. SCF adds ubiquitin chains to DELLA.

    \$\text{DELLA} + \text{Ub}n \xrightarrow{\text{SCF}^{\text{SLY1/GID2}}} \text{DELLA–Ub}n\$

  4. Proteasomal degradation: Poly‑ubiquitinated DELLA is recognised by the 26S proteasome and degraded, removing its repressive effect.

  5. Transcription activation: With DELLA removed, transcription factors that were previously bound or inhibited by DELLA can now bind to GA‑responsive promoters, recruiting RNA polymerase II and initiating transcription of growth‑related genes.

Resulting Cellular Effects

Genes activated by GA include those involved in:

  • Stem elongation (e.g., expansins, cell‑wall‑loosening enzymes)

  • Seed germination (e.g., α‑amylase synthesis)

  • Flowering time regulation

  • Fruit development

Summary Table

ComponentRole in GA SignallingOutcome When Functional
GA (gibberellin)Hormone ligandTriggers DELLA degradation
GID1 (GA‑INSENSITI \cdot E DWARF1)Soluble GA receptorForms GA–GID1 complex that binds DELLA
DELLA proteinsTranscriptional repressorsInhibit GA‑responsive transcription when present
SCFSLY1/GID2 complexE3 ubiquitin ligaseUbiquitinates DELLA for proteasomal degradation
Transcription factors (e.g., PIFs)Positive regulators of GA‑responsive genesActivate target genes once DELLA is removed

Key Points to Remember

  • DELLA proteins act as “brakes” on growth; GA removes these brakes.

  • The GA–GID1 interaction is reversible, allowing fine‑tuned control of signalling intensity.

  • Ubiquitin‑mediated proteolysis provides a rapid and irreversible switch from repression to activation.

  • Mutations that stabilise DELLA (e.g., loss of SCF function) lead to dwarf phenotypes, illustrating the pathway’s importance.

Suggested diagram: Flowchart of gibberellin signalling showing GA binding to GID1, formation of the GA–GID1–DELLA complex, recruitment of SCF ubiquitin ligase, DELLA ubiquitination and degradation, and subsequent activation of transcription factors.