Explain the role of gibberellin in stem elongation including 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

Sourav PanNovember 1, 2024

Answered

Gibberellins are a group of plant hormones that play a crucial role in various growth and developmental processes, including stem elongation. They influence plant height, seed germination, flowering, and fruit development. The process of stem elongation, particularly, is significantly affected by the action of gibberellins, which promote cell division and elongation in the stem tissues.

Role of Gibberellin in Stem Elongation

1. Cell Elongation: Gibberellins stimulate the elongation of stem cells by promoting cell division and expansion. When gibberellins are present, they activate genes that lead to the production of enzymes, such as expansins, which loosen the cell wall structure. This loosening allows cells to take up more water, expand, and elongate.
2. Promoting Growth: Gibberellins are involved in the synthesis of proteins and enzymes that are essential for growth. They increase the activity of certain enzymes that facilitate the breakdown of starches into sugars, providing energy for growth. This increase in available energy contributes to the overall elongation of the stem.
3. Overcoming Dwarfism: In some plant species, genetic mutations can result in dwarfism due to the lack of gibberellin synthesis. In these cases, the application of gibberellins can restore normal stem elongation and overall growth.

Genetic Control of Gibberellin Synthesis

The synthesis of gibberellins is controlled by specific genes, and variations in these genes can influence plant height. In particular, the Le and le alleles play a critical role in determining gibberellin levels and, consequently, stem elongation.

1. Dominant Allele (Le):
   • The Le allele encodes a functional enzyme involved in the gibberellin biosynthesis pathway. This enzyme catalyzes a step in the conversion of precursor molecules into gibberellins.
   • Plants that are homozygous dominant (LeLe) or heterozygous (Lele) for this allele can produce sufficient gibberellins, leading to normal or enhanced stem elongation.
   • As a result, these plants exhibit taller stature because the functional enzyme enables the synthesis of gibberellins that stimulate growth.
2. Recessive Allele (le):
   • The le allele codes for a non-functional enzyme in the gibberellin synthesis pathway. Plants with this allele do not produce enough gibberellins, which results in stunted growth and shorter stems.
   • Plants that are homozygous recessive (lele) for this allele lack the ability to synthesize gibberellins effectively, leading to dwarfism or reduced stem elongation compared to their dominant allele counterparts.
   • This reduced capacity for gibberellin production is often observed in certain mutant strains, where the lack of gibberellins leads to the characteristic short stature.