Gibberellins

  • Named after fungus Gibberella fujikuroi, which causes rice plants, on which it is parasitic, to grow abnormally tall

  • The disease of rice called ‘foolish seedling’ was investigated in the 1920s by a Japanese plant pathologist Eiichi Kurosawa

  • The substance causing the abnormal growth was isolated from fungus and structural formulae identified

  • The chemical was found to belong to a class of more than 100 naturally occurring plant hormones called gibberellins

  • All are acidic and usually referred to as GI (Giberellic acid)

  • Synthesised in the apical portions of roots and stems

  • Play a leading role in controlling stem elongation in the growing stems of mature trees and shrubs, where its application characteristically promotes internode elongation

    • Elongations effect is also enhanced if auxin is present

  • TGe application of gibberellins to dwarf mutants is known to restore normal growth and development in many plants

  • Gibberellin application can also induce some plants to grow out of the rosette stage of and flower early during their first year of growth

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

  1. Absorption of water stimulated germination

  2. Embryo synthesises gibberellin in response to water uptake

  3. Aleurone layer synthesises amylase in response to gibberellin

  4. Amylase mobilises energy reserves: hydrolyses starch to maltose. Maltose is converted to glucose and respired to release ATP

Questions

  1. Name the plant hormone that initiates seed germination

    • Gibberellin

  2. Which one of the following stimuli does not bring about a directional growth response in plants?

    A. Gravity

    B. Oxygen levels

    C. Light

    D. Water

    • B

  3. When a seed germinates it does not obtain energy through photosynthesis. Explain where the energy for the initial growth of a seed comes from.

    • The gibberellin is synthesised in response to water uptake, which cases the aleurone layer to synthesise amylase, which mobilises the energy reserves, hydrolysing the starch into maltose which is converted into glucose and respired to release ATP.

  4. What is a non-directional response in a plant called?

    • Nastic movement

Explain why gibberellin is classed as a plant hormone:

  • They control many aspects of plant growth and development

  • Chemical messengers

  • Produced in one part of plant but has effects in another part

  • Affects/alters activity of target cells/tissues

  • Has long-lasting effects

  • Wide spread affect