Plant hormones - Gibberellin

Gibberellins are plant growth regulators that are synthesized in most parts of plants (mainly in young leaves and seeds and in stems). They have a role in seed germination and controlling stem elongation.

Stem elongation

The height for some plants is partly controlled by their genes. Tallness in pea plants is affected by a gene with two alleles Le/le

1. dominant allele Le:

• codes for the functional enzyme of the active form of gibberellin GA1 --> stimulates cell division and cell elongation
• plant grows tall

2. recessive allele le:

• caused by a substitution mutation (alanine to theorine amino acid)
• homozygous recessive lele: no active form of gibberellin
• plant remains short

Seed germination: of wheat and barley

Seeds are dormant (waiting for the optimum condition) when first shed from parent plant. It contains very little water and is metabolically inactive.

1. Absorption of water stimulates germination
2. Embryo synthesizes gibberellin in response to water uptake
3. Aleurone layer synthesizes amylase in response to gibberellin
4. Amylase mobilizes energy reserves: hydrolyses startch to maltose. Maltose is converted to glucose and respired to release ATP

15.2  Control and co-ordination in plants Plant co-ordination systems involve rapid responses as in the case of the Venus fly trap, but also complex interactions between plant growth regulators, such as auxin and gibberellin. Plants respond quite differently to different concentrations of plant growth regulators. a) describe the rapid response of the Venus fly trap to stimulation of hairs on the lobes of modified leaves and explain how the closure of the trap is achieved b) explain the role of auxin in elongation growth by stimulating proton pumping to acidify cell walls c) describe the role of gibberellin in the germination of wheat or barley d) explain the role of gibberellin in stem elongation including the role of the dominant allele, Le, that codes for a functioning enzyme in the gibberellin synthesis pathway, and the recessive allele, le, that codes for a non-functional enzyme