Plant Hormones

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Chapter 39

• Plant Hormones

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Signals in Plants

• All living organisms respond to stimuli
• Plants receive information from the environment,
  and react appropriately
• Much like removing your hand from a hot stove
• Plant example dark grown potato

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Signal Transduction Pathway
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Plant responses to hormones

• Hormones chemical signals that coordinate the
  parts of the organism
• It is produced by one part of the plant and
  transported to other parts where it binds to
  receptors and triggers responses in target cells
  and tissues.
• Examples of plant responses
• tropism- growth response that results in
  curvatures of whole plant organs toward or away
  from a stimulus
• Positive phototropism

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Discovery of Plant Hormones
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Classes of Plant Hormones

• Auxins
• Cytokinins
• Gibberellins
• Abscisic acid
• Ethylene
• Brassinosteroids

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Auxins

• Class of hormones responsible for stem elongation
• It moves from stem tip down
• It is produced in the apical meristem

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Cytokinins

• Produced in actively growing parts of the plant
• Work in concert with auxin to control apical
  dominance
• Anti-aging effect

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Gibberellins

• Responsible for stem elongation
• Fruit growth
• Seed germination

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Abscisic acid

• Slows down growth
• Responsible for seed dormancy
• Desert seeds wait for rain to germinate
• The water washes the ABA off of the seed
• Relieves drought stress
• Closes stomata

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Ethylene

• Gaseous hormone
• Plants produce ethylene in response to
• Stress
• Mechanical pressure
• Injury
• infection
• Triple response

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Ethylene cont.

• Leaf abscission
• Fruit ripening

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Flowering control

• Plant must flower on time (prevent seeds from
  maturing in winter)
• daylength is reliable cue
• day-neutral plant flowers when mature
• tomatoes, corn, snapdragons
• long-day plant flowers when daylength increases
• short-day plant flowers when daylength decreases

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Measuring Daylength

• Need two things
• clock to measure time (how long has it been
  light)
• light detecting system
• Accomplished with phytochromes
• Pr absorbs red light
• turns into Pfr
• Pfr absorbs far red light
• turns into Pr
• Amount of Pr determines when flowering occurs

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Phytochromes

• Pfr inhibits cell elongation
• Pfr turns into Pr in the dark so seeds
  germinating grow faster
• also allows shaded plants to grow faster

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Gravitropism

• Growth in response to gravity
• Plants can tell up and down by the settling of
  statoliths
• Located in certain cells of the root cap

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Mechanical Stimuli

• Thigmomorphogenesis changes in form that result
  from mechanical stimulation
• A plant that is touched twice a day will grow
  shorter than a plant that is untouched

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Plant Defenses

• Plants produce a wide variety of toxins to deter
  herbivory
• Some plants recruit parasitoid insects to aid in
  stopping insects from eating the plant