explain the role of the dominant allele, Le, that codes for a functional enzyme in the gibberellin synthesis pathway, and the recessive allele, le, that codes for a non-functional enzyme

Understanding the Role of the Le/le Alleles in Gibberellin Synthesis

Learning Objectives

  • Explain how the dominant allele Le and the recessive allele le affect the activity of a key enzyme in the gibberellin (GA) biosynthetic pathway.

  • Describe the downstream effects of altered GA levels on plant growth, development and agricultural productivity.

  • Connect the Le/le case‑study to the major Cambridge International AS & A‑Level Biology (9700) syllabus topics, from cell structure to genetics, physiology and biotechnology.

  • Apply knowledge of this system to practical plant‑breeding and biotechnological strategies.

Key Vocabulary

TermDefinition (Cambridge context)
AlleleA variant form of a gene located at a specific locus on a chromosome.
Dominant allele (Le)Encodes a fully functional GA‑synthase enzyme that catalyses a crucial step in gibberellin biosynthesis.
Recessive allele (le)Contains a point mutation that produces a non‑functional enzyme, dramatically reducing GA synthesis.
Gibberellins (GA)Plant hormones (diterpenoid acids) that promote stem elongation, seed germination, flowering and other growth processes.
PhenotypeThe observable characteristics of an organism resulting from the interaction of its genotype with the environment.

Syllabus Mapping – How the Le/le System Fits the 9700 Programme

Syllabus Section (AS)Content Covered in These NotesAdditional Points to Teach Elsewhere
1. Cell structure

  • Location of the Le gene in the nucleus.

  • Organelle involvement in protein synthesis – nucleus, ribosome, rough ER, plastid.

  • Prokaryote vs. eukaryote comparison (size, organelles, membrane systems).

  • Microscopy skills – preparing leaf sections, calculating magnification, interpreting photomicrographs.

2. Biological molecules

  • Enzyme (protein) encoded by Le.

  • Gibberellins – diterpenoid hormones derived from the isoprenoid pathway.

  • Structures, functions and tests for carbohydrates, lipids, nucleic acids and water.

  • Link GA biosynthesis to precursor molecules (e.g., geranylgeranyl diphosphate).

3. Enzymes

  • Functional vs. non‑functional GA‑synthase.

  • Factors influencing activity – temperature, pH, cofactors, feedback inhibition.

  • Lock‑and‑key & induced‑fit models.

  • Michaelis–Menten kinetics (Vmax, Km).

  • Design of a colourimetric GA‑production assay and data analysis.

4. Cell membranes & transportNone directly.

  • GA is a relatively hydrophobic hormone – crosses the plasma membrane by simple diffusion and can move cell‑to‑cell via plasmodesmata.

  • Practical activity: uptake of radiolabelled GA by excised leaf discs.

5. Mitotic cell cycleNone.

  • DNA replication of the Le locus occurs in S‑phase; transcription of the gene is highest in G1/G2 when GA‑dependent growth is needed.

  • Diagram linking cell‑cycle phases to gene dosage effects in heterozygotes.

6. Nucleic acids & protein synthesis

  • Gene → pre‑mRNA → mRNA → GA‑synthase (functional or defective).

  • Point mutation in le (e.g., G→A) causes a missense change that destroys the enzyme’s active site.

  • DNA/RNA structure, replication, transcription, translation, splicing (if applicable).

  • Types of mutations and their effects on protein structure.

7. Transport in plants

  • GA stimulates cell division and expansion in the shoot apical meristem, influencing internode length and vascular development.

  • Structure and function of xylem and phloem, water potential, transpiration stream.

  • Practical: measuring stem elongation after exogenous GA application.

8–11. Transport in mammals, Gas exchange, Infectious diseases, ImmunityNot applicable to this case study.Covered elsewhere in the syllabus.
A‑Level extensions (12‑19)Genetic case study, hormone regulation, plant‑breeding applications.

  • Respiration, photosynthesis, homeostasis, control mechanisms.

  • Evolutionary significance of dwarfing alleles (Green Revolution).

  • Advanced biotechnological tools – CRISPR, over‑expression, marker‑assisted selection.

1. Cellular Basis of the Le/le System

  • Gene location: The Le locus is a nuclear gene on chromosome 5 (example).

  • Transcription: Both alleles are transcribed by RNA polymerase II into a pre‑mRNA that receives a 5′ cap, poly‑A tail and (if required) intron splicing.

  • Translation: Cytoplasmic ribosomes translate the mature mRNA into a polypeptide of ~55 kDa. In le a single nucleotide substitution (e.g., G→A at codon 210) replaces a conserved serine with a stop codon, truncating the protein and abolishing the active site.

  • Post‑translational processing: The nascent polypeptide folds in the rough ER, acquires a plastid‑targeting peptide, and is imported into the plastid where GA synthesis occurs.

2. Enzyme‑Catalysed Step in Gibberellin Biosynthesis

The functional enzyme, GA‑synthase, catalyses the oxidation of ent‑kaurene to GA12, the first true gibberellin in the pathway.

ent‑kaurene ⟶GA‑synthase (Le) GA12 → GA1, GA3, …

ent‑kaurene ⟶non‑functional enzyme (le) negligible GA

Factors Influencing Enzyme Activity (Cambridge syllabus 3.3)

  • Temperature – optimum ≈ 30 °C; activity falls sharply above 40 °C.

  • pH – optimum 7.0–7.5; deviation reduces catalytic efficiency.

  • Cofactors – NADPH supplies reducing power for the oxidation steps.

  • Feedback inhibition – high GA levels bind a regulatory domain of GA‑synthase, decreasing Vmax (classic end‑product control).

3. From Genotype to Phenotype

GenotypeEnzyme ActivityGA Level (relative)Typical Phenotype (Cambridge AS)
LeLe100 % – both alleles produce functional GA‑synthaseHighTall, normal internode length, rapid stem elongation, vigorous seed germination.
Lele≈ 50 % – one functional copyIntermediateModerately tall; may show slight dwarfism under stress (low light, nutrient deficiency).
lele0 % – no functional enzymeVery lowDwarf phenotype; short internodes, delayed or incomplete germination, possible late flowering.

4. Phenotypic Consequences of Altered GA Levels

  1. Stem elongation & internode length – GA promotes cell division in the shoot apical meristem and cell‑wall loosening in elongating internodes. Low GA → dwarfism.

  2. Seed dormancy & germination – GA induces α‑amylase in the aleurone layer, mobilising starch. lele seeds often require stratification or exogenous GA to germinate.

  3. Flowering time – Adequate GA levels trigger the vegetative‑to‑reproductive transition; dwarf plants frequently flower later.

  4. Response to environment – Heterozygotes (Lele) can compensate under favourable conditions, whereas homozygous recessives cannot, illustrating gene‑environment interaction.

5. Mendelian Inheritance of the Le/le Locus

  • Alleles segregate according to the law of segregation – each gamete receives one allele.

  • Dominance: Le masks the effect of le in heterozygotes.

Typical Monohybrid Cross

Parental GenotypeGametes
LeLe × leleLe × le
F1 (all)Lele (tall)
F1 self‑cross3 : 1 ratio (tall : dwarf)

Linkage & Recombination

The Le locus is single, autosomal and not linked to other genes studied in the syllabus; therefore classic Mendelian ratios apply.

6. Practical Applications in Plant Breeding

  • Dwarf varieties – Introgression of le alleles (e.g., in wheat, rice, barley) reduces lodging, improves harvest index and enables high‑density planting. This was a cornerstone of the Green Revolution.

  • Tall varieties – Retaining Le is advantageous for crops where plant height contributes to yield (e.g., certain legumes or ornamental species).

  • Marker‑Assisted Selection (MAS) – DNA markers tightly linked to the Le locus allow early identification of seedlings carrying the desired allele, saving time and resources.

  • Biotechnological approaches

    • Over‑expression of the functional GA‑synthase gene using a strong constitutive promoter to increase stature.

    • CRISPR‑Cas9 knockout of the recessive allele in dwarf lines to restore normal height.

    • RNAi silencing of competing pathway genes to channel more precursor into GA production.

7. Connecting the Case Study to Wider Biological Themes

  • Gene → Protein → Metabolite → Trait – Demonstrates the central dogma and how a single nucleotide change can cascade to a macroscopic phenotype.

  • Regulation of Hormone Biosynthesis – Shows feedback inhibition, enzyme localisation to plastids, and environmental modulation of hormone levels.

  • Evolutionary Significance – Dwarfing alleles were deliberately selected during the 20th‑century Green Revolution, illustrating human‑driven evolution.

  • Biotechnological Relevance – The Le/le system serves as a model for studying other hormone pathways (auxin, cytokinin) and for developing precision breeding tools.

8. Summary

The dominant Le allele encodes a fully functional GA‑synthase that enables normal gibberellin production, resulting in tall, vigorously growing plants. The recessive le allele carries a point mutation that yields a non‑functional enzyme, drastically lowering GA levels and producing dwarfism, delayed germination and altered flowering. This simple genetic system exemplifies core Cambridge International AS & A‑Level Biology concepts—including cell structure, biomolecules, enzyme kinetics, gene expression, Mendelian inheritance, hormone regulation, and modern plant‑breeding techniques—making it an ideal case study for both classroom teaching and exam preparation.

Suggested diagram: Flowchart linking Le/le genotype → mRNA → GA‑synthase activity → gibberellin synthesis → phenotypic outcomes (tall vs. dwarf). Include feedback inhibition and environmental modifiers (light, nutrients).