Plant Responses

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Chapter 39

• Plant Responses

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Problem

• Plants cant and dont move, so how do they
  respond to stimuli in their environment?

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Solution

• Changes within a cell(s) cause morphological
  changes

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Review

• Signal transduction pathways
• Hormone or stimuli binds to receptor
• Receptor starts a chain reaction
• This elicits a cellular response

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Overview of plant hormones

• Produced in small amounts
• Many have multiple effects in the plant
• Many are produced in more than one part of the
  plant

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Auxin

• Released by
• Meristem of apical buds (end of the shoot)
• Also
• Embryo of seed
• Young leaves

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Auxin

• Main effect cell elongation
• Cells on dark side elongate more than the bright
  side
• Growth of shoot
• Secondary effects
• Fruit growth
• Branching of roots

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Auxin

• Commercial uses-
• Herbicide (synthetic form)
• Monocots can break it down, dicots cant and die

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Cytokinins

• Released by
• Actively growing tissues (roots, embryos, fruits)

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Cytokinins

• Major effect
• Stimulate cell growth and cell differentiation
• Secondary effect
• Retards aging

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Cytokinins

• Commercial uses
• Florists spray flowers to keep them fresher

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Apical Dominance

• Keeps plants growing from the terminal bud (and
  inhibiting the axillary bud)
• Depends on the ratio between Auxins and Cytokinins

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Gibberellins

• Released by
• Roots and young leaves

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Gibberellins

• Major Effects
• Stimulates growth of stems and leaves, but not
  roots
• Secondary effects
• Larger fruit
• Seed germination

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Giberellins

• Commercial use
• Larger fruit (grapes)

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Brassinosteroids

• Released by
• Mature cells

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Brassinosteroids

• Major Effects
• Retards leaf abscission (dropping of leaves)
• Secondary Effects
• Elongation of stem cells

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Abscisic Acid (ABA)

• Released by
• Most plant cells

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Abscisic Acid (ABA)

• Major Effects
• Slows growth
• Keeps seeds dormant
• Drought tolerance
• Closes stomates

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Ethylene

• Released by
• Ageing leaves and flowers
• Ripening fruit

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Ethylene

• Major Effects
• Triple Response
• Apoptosis
• Leaf Abscission
• Fruit ripening

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Ethylene

• Triple Response
• Slow stem elongation
• Thicken stem
• Horizontal growth of stem
• Response to an obstacle

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Ethylene

• Apoptosis
• Programmed cell death

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Ethylene

• Leaf Abscission
• Dropping of leaves in response to temperature and
  photoperiod changes

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Ethylene

• Fruit Ripening
• One bad fruit does spoil the bunch

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Ethylene

• Commercial Uses
• Ethylene inhibition keeps fruit fresh

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Responses to gravity

• Gravitotropism
• Roots positive
• Shoots negative
• Plants always grow up

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Responses to touch

• Thigmotropism
• Leaves may close (mimosa)

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Reponses to light

• Phototropism
• Plants grow towards light
• Gravity overrides light

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Responses to Light

• Many plant responses are based on the amount of
  lightPhotoperiod
• Two different groups of photoreceptors are
  responsible for changes based on sunlight

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Blue-Light Photoreceptors

• Responsible for phototropism and closing of
  stomates

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Phytochromes

• Chiefly responsible for seed germination and
  flowering

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Circadian Rhythms

• Response based on 24 hour cycle regardless of
  external cues
• Not just in plants- almost all eukaryotic cells

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Circadian Rhythms

• Photoperiod helps to synchronize the clock to
  environmental cues

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