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

The Roles of Genes in Determining the Phenotype

Learning Objective

Explain how the dominant allele Le, which codes for a functional enzyme in the gibberellin synthesis pathway, and the recessive allele le, which codes for a non‑functional enzyme, influence the observable characteristics of a plant.

Key Concepts

• Allele: A variant form of a gene located at a specific locus on a chromosome.
• Dominant allele (Le): Produces a functional enzyme that catalyses the conversion of precursor molecules into gibberellins.
• Recessive allele (le): Produces a non‑functional enzyme; the pathway is blocked or severely reduced.
• Gibberellins (GA): Plant hormones that promote stem elongation, seed germination, and other growth processes.
• Phenotype: The observable traits resulting from the interaction of genotype with the environment.

Biochemical Basis

The enzyme encoded by Le catalyses the reaction:

\$\text{Precursor} \xrightarrow{\text{Enzyme}_{Le}} \text{Gibberellin (GA)}\$

When the allele is le, the enzyme is either absent or inactive, so the reaction proceeds at a negligible rate:

\$\text{Precursor} \xrightarrow{\text{Enzyme}_{le}\;(\text{inactive})} \text{Very low GA}\$

Genotype → Enzyme Activity → Phenotype

Phenotypic Consequences in Plants

1. Stem Length: Gibberellins stimulate cell division and elongation in the internodes. Reduced GA → shorter stems.
2. Seed Germination: GA breaks seed dormancy. lele plants may exhibit delayed or incomplete germination.
3. Flowering Time: GA influences the transition from vegetative to reproductive growth; dwarf plants often flower later.
4. Environmental Interaction: Under low light or nutrient stress, plants with Lele may retain enough GA to maintain growth, whereas lele plants cannot compensate.

Application: Breeding for Desired Height

Plant breeders can manipulate the Le/le locus to obtain varieties suited to different agricultural needs:

• Dwarf varieties: Introduce le alleles to produce compact plants that are less prone to lodging.
• Tall varieties: Retain Le alleles for crops where height contributes to yield (e.g., certain cereals).

Summary

The dominant allele Le encodes a functional enzyme that enables normal gibberellin synthesis, leading to typical growth patterns. The recessive allele le produces a non‑functional enzyme, drastically reducing gibberellin levels and resulting in dwarfism and related phenotypic changes. Understanding this gene‑enzyme‑phenotype relationship is essential for both basic genetics and practical plant breeding.