Plant responses to hormones

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Plant responses to hormones
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What the spec says.
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Types of plants hormone
Cell division
Cell elongation

• Auxins
• Gibberellins
• Cytokinins
• Abscisic acid
• Ethene

Promotors of growth
Initiation of organs
Differentiation
Inhibitors of growth / Antagonists to growth
stimulators
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Growth substances can be

• Synergistic enhance each others effects
• Antagonistic counteract each other

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Auxin

• Indoleacetic acid
• Causes elongation of cells
• Increases plasticity of cells
• Cell walls soften
• Cell becomes less turgid and take up more water
• Cell expands
• Responsible for phototropism

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Phototropism experiments
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Other activities of auxins

• Apical dominance apical bud inhibits lateral
  (axillary buds)
• E.g. pruning stimulates axillary buds
• Formation of lateral roots stimulates
  INITIATION of lateral roots and adventitious
  roots
• E.g. napthalene acetic acid (NAA)
• Indole butyric acid (IBA)

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More auxin functions

• Abscission of leaves and fruit
• Abscission falling of fruit and leaves
• IAA delays early stages, promotes later stages
• Stimulates ethene production
• Ensure fruit stays on trees until harvest

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Even more auxin functions

• Fruit development
• Produced in pollen developing seed
• Stimulates fruit development after fertilisation
• Used in production of seedless fruits
• Weed killers
• 2,4-D causes abnormal growth of Dicots (e.g.
  dandelions) monocots are unaffected

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Giberellins

• Stem elongation in dwarf plants
• E.g. dwarf peas
• Bolting of long day plants when the days are
  short i.e. cause them to flower
• Overcome lack of cold period required for
  flowering in long day plants

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• A wild type rice plant (left) compared to a
  semidwarf transgenic plant (right) containing 
  the gai mutant allele. 
• The gai mutant allele confers giberellin
  insensitivity upon the transgenic plant, and
  causes its reduced stature.

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More giberellins

• Fruit development
• Stimulates growth of fruit stalk
• Delays senescence (aging)
• Seed germination stimulates
• Stimulate a amylase production in seeds,
  improves malt yields from barley

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Cytokinins

• Produced by meristemic tissue
• Apices of roots and shoots
• Development of roots, shoots and lateral buds
• Cell enlargement
• Maturation of chloroplasts
• Dependant on auxin for function

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• Cytokinins are commonly used tostimulate shoot
  initiation in culture as is shown by
  shoot/bulblet formation on bulb scales of
  Bowieavolubilis. Benzyladenine was used here to
  stimulate shoot growth

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Abscissic acid

• Inhibit / alter growth
• Linked to environmental stress
• Seeds can become dormant
• Abscission of leaves, flowers and fruits
• Close stomata in times of water deficiency

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Ethene (ethylene)

• Promotes ripening of fruits
• Promotes abscission (auxin prevents it)
• Abscission layer
• Antagonistic to auxin