Chapter 39: Plant Responses to Internal

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Chapter 39 Plant Responses to Internal
External Signals
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Plant Reactions

• Animals respond to stimuli by changing their
  behavior ? called taxis
• Move to a positive stimulus
• Move away from a negative stimulus
• Plants respond to stimuli by adjusting their
  growth development
• Called tropism

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What causes the response seen below in a plant?
Grown in dark
1 week of light exposure
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Signal-Transduction Pathways

• Signal triggers a receptor
• Receptor activates internal cellular messengers
• Cellular response occurs
• Aka, cell communication

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Plant Hormones

• Chemical signals that coordinate interactions
  between the different parts of an organism
• Produced by 1 part of the body
• Transported to another part of the body
• Only minute amounts are required
• Binds to specific receptors
• Causes a response in target cells/tissues
• Environmental stimuli can change hormone
  concentrations or their speed of movement

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Examples of Plant Hormones

• Auxin
• Cytokinins
• Gibberllins
• Abscisic Acid
• Ethylene Gas
• Brassinosteroids

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Phototropism Research

• Darwin His Son, Francis Experiment
• Grass seedling only bends toward light with the
  presence of the coleoptile
• Tip of coleoptile must sense light
• Actual growth response (curving of the tip)
  occurs below the tip
• Boysen Jensen Experiment
• Determined the signal was mobile by placing
  blocks of gelatin silica b/w tip rest of the
  coleoptile

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Darwin Went Research
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Phototropism Research Continued

• Went Experiment
• Extracted chemical messenger of phototropism
• Removed the coleoptile tip placed it on a block
  of agar
• Chemical diffused into the agar
• Named the chemical auxin

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Went Experiment
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How does Auxin work?

• Causes the tip of the plant to grow towards the
  light
• Cells on the darker side (away from the light)
  elongate faster than those on the light side
• Phototropism!!!

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Plant Hormones

• Read about the various hormones in your text and
  complete the Hormone Chart

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Plant Movements/Responses

• Tropisms
• Gravitropism (positive negative based on
  gravity)
• Thigmotropism (movement based on contact)
• Phototropism (growth toward light)
• Turgor Movements caused by changes in the
  turgor pressure of specialized cells in response
  to a stimuli
• Rapid Leaf Movements
• Lose K ? H20 leaves cells ? cells b/c flaccid or
  limp
• Mimosa Plant, Venus Flytrap
• Sleep Movements
• Leaves may move up down due to daily turgor
  pressure changes

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Rapid Leaf Movement in Mimosa Plant
Mimosa Closed
Mimosa Opened
Tickle Me Plant
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Sleep Movements
NOON
MIDNIGHT
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Reponses to Light

• Photomorphogenesis
• Effects of light on plant growth
• Light Detection
• Direction
• Intensity
• Wavelength
• Blue-light receptors
• Phytochromes (red-light receptors)

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Circadian Rhythms in Plants

• Cycle of about 24 hours
• In complete darkness, plants cycle at about every
  26 hours
• Light keeps the biological clock set to about 24
  hours
• jet lag may occur in all eukaryotes

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Plant Defenses

• Defenses against herbivores
• Physical
• Thorns
• Chemical
• Distasteful or toxic compounds released
• Recruit predatory animals
• See picture ?

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Plant Defenses

• Defenses against pathogens
• 1st Line of Defense
• Epidermis periderm
• Pathogens can enter through stomata, injuries,
  etc
• 2nd Line of Defense
• Chemical Attack
• Kills pathogens prevents spread of the infection