The Role of Gibberellin in Stem Elongation (Cambridge International A-Level Biology)

Spec point — 16.2.7 · 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

The role of gibberellin in stem elongation

• In some plants species their height is partially controlled by their genes

• The Le gene dictates the height of some plants

• It has two alleles: Le and le

  • The dominant allele ****Le produces tall plants when present
  • The recessive allele**** le produces shorter plants when present (in a homozygous individual)

• The gene regulates the production of an enzyme that is involved in a pathway that forms active gibberellin GA₁

• Active gibberellin is a hormone that helps plants grow by stimulating cell division and elongation in the stem

• The recessive allele le results in non-functional enzyme

  • It is only one nucleotide different to the dominant allele
  • This causes a single amino acid substitution (threonine -> alanine) in the primary structure of the enzyme
  • This change in primary structure occurs at the active site of the enzyme, making it non-functional

• Without this enzyme no active gibberellin is formed and plants are unable to grow tall

• Plants that are homozygous for the recessive allele le are dwarves

• Some farmers apply active gibberellin to shorter plants to stimulate growth