Plant Hormones

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Plant Hormones

• Controls of growth, development and movement

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Processes in growth

• Cell division.
• Cell enlargment.
• Cell differentiation.

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Primary growth
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Secondary growth
xylem
Phloem with bands of fibers
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Seed Germination

• Scarification
• mechanical
• chemical
• heat
• Mobilization of reserves

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Mobilization of reserves
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Movement in Plants

• Turgor movements (changes in turgor pressure in
  selected cells)
• Growth movements (elongation of selected cells in
  response to stimulus)
• phototropism
• geotropism
• thigmotropism

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Turgor movementMimosa pudica L. (sensitive plant)
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Pulvinus of Mimosa pudica
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Tropic responses

• Directional movements in response to a
  directional stimulus

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Growth movement
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Phototropism
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Geotropism
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Statoliths
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Thigmotropism
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Hormones

• Signal molecules produced at specific locations.
• Occur in low concentrations.
• Cause altered processes in target cells at other
  locations.

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Plant growth substances

• Artificially synthesized substances which produce
  hormone like responses

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General plant hormones

• Auxins
• Gibberellins
• Cytokinins
• Abscisic acid
• Ethylene

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Auxin
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Auxin associated with phototropism - early
experiments demonstrate tip as receptor.
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Demonstration of transported chemical
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Auxin

• Discovered as substance associated with
  phototropic response.
• Occurs in very low concentrations.
• Isolated from human urine, (40mg 33 gals-1)
• In coleoptiles (1g 20,000 tons-1)
• Differential response depending on dose.

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Loosening of cell wall
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Polar transport of Auxin
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Additional responses to auxin

• abscission - loss of leaves
• flower initiation
• sex determination
• fruit development

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Control of abscission by auxin
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Gibberellin
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Discovered in association with Foolish disease of
rice (Gibberella fujikuroi)
uninfected
infected
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Effects of Gibberellins

• General cell elongation.
• Breaking of dormancy.
• Promotion of flowering.
• Transport is non-polar, bidirectional producing
  general responses.

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Cytokinins
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Function of cytokinins

• Promotes cell division.
• Morphogenesis.
• Lateral bud development.
• Delay of senescence.
• Stomatal opening.
• Rapid transport in xylem stream.

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Abscisic acid
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Functions of abscisic acid

• General growth inhibitor.
• Causes stomatal closure.
• Readily translocated.
• Produced in response to stress.

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Ethylene
H H \ / C C / \ H H
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Functions of ethylene

• Gaseous in form.
• Rapid diffusion.
• Affects adjacent individuals.
• Fruit ripening.
• Senescence and abscission.
• Interference with auxin transport.
• Initiation of stem elongation and bud development.

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Action and Interaction of Hormones

• Constitutive enzymes - always present but level
  of hormone can affect level.
• Adaptive enzymes - formed or activated as a
  result of the presence of a hormone.

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Signal-transduction pathways in plants
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Interaction of cytokinin and auxin in tobacco
callus tissue