Chapter 39: Plant responses to External and Internal Signals

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Chapter 39 Plant responses to External and
Internal Signals
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Summary of key points

• Plant responses tropism and phototropism
• Experiments in phototropism and The Went
  experiment.
• Role of auxins in cell elongation in young
  developing shoots
• Role of cytokinins in stimulating cell division
  and differentiation
• Role of gibberellins in cell elongation and cell
  division in stems and leaves, role in fruit
  growth and seed germination.
• Role of Abscisic acid seed dormancy and drought
  stress
• Role of ethelyne gas stimulates triple response
  to mechanical damage, and stimulates apoptosis
  (programmed cell death), leaf abscission and
  fruit ripening
• Photoperiodism
• Response to gravity and Gravitropism
• Response to mechanical stimuli and Thigmotropism
• Response to stress such as drought, flooding,
  salt and heat.

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Why do plants bend towards the light? Phototropism

• Study done in grasses.
• Grasses grown in the dark or uniform light? the
  coleoptile (outer sheath) grown straight.
• When exposed to light from one side, the
  coleoptile grows towards it
• Cells on the dark side elongate faster than the
  cells on the illuminated side.
• If the tip of the coleoptile is removed or
  covered you do not get bending.

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Boysen-Jensen demonstrated that a chemical made
in the tip caused the bending effect
(phototropism)
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F.W.Went discovered the chemical ? AuxinThe
hormone is asymmetrically distributed which
promotes cell growth towards the light.
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Just like the endocrine system, we need to know
these plant hormones and what they do
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Plant Hormones

• All control plant growth and development by
  altering cell division, elongation and
  differentiation.
• All have multiple effects depending upon site of
  action, concentration and developmental stage of
  plant.

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Auxin Indoleacetic Acid (IAA)

• Major function Affects cell elongation in
  developing shoots.
• Transported by a polar mechanism.

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Polar transport of Auxin
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Auxin and Cell Elongation

• Works at concentrations of 10-8 to 10-4 M.
• Anything higher, ethylene gas produced which
  inhibits cell elongation.
• Acid growth hypothesis.

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Acid Growth Hypothesis
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Cytokinins

• Stimulates cytokinesis and cell division.
• Produced in actively growing regions (roots,
  shoots and embryos)
• Cell culture in vitro
• cytokinins, -auxin ? cells grow large, but no
  division
• cytokinins, auxin ? cell division
• Ratio of cytokinin and auxin causes plant cells
  to differentiate.

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Terminal Bud removed
Terminal Bud Intact

• Terminal bud intact? inhibits auxiliary buds
  because auxin transport down.
• Favors shoot to lengthen. Cytokinins moving
  upward from roots counteract and cause auxiliary
  bud development.
• If balanced, shoot growth upward at the expense
  of lateral growth of auxiliary buds.
• Explains why auxiliary buds closer to the root
  are more likely to grow.

• Terminal bud removed, you remove repression of
  auxiliary bud growth by auxin.
• Cytokinins still produced which causes auxiliary
  buds to develop.

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Gibberellins Effect growth in stem and leaves
but no effect in roots

• Gibberillins stimulate Stem Elongation and
  Division
• Fruit growth
• Germination

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Abscisic Acid (ABA)

• Generally slows down plant growth
• Antagonist of growth hormones (Auxin, cytokines
  and gibberillins
• Seed dormancy ABA levels are high seed is
  dormant and the seed matures. Remember that
  Gibberillins induce germination. So its really
  the ratio of ABAGibberillins that controls when
  seeds break dormancy.
• Also, controls Drought Stress. Causes the stomata
  to close rapidly. Do you remember how? (hint K)

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Ethylene Gas has 4 major effects on plants.

• Triple Response to mechanical stress.
• Apoptosis programmed cell death
• Leaf Abscission (falling off)
• Fruit Ripening

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Triple response to Mechanical Stress

• Consider a pea plant pushing up from the soil
  then encountering a rock. What happens?
  Production of ethylene gas.
• 1.) Slows Stem elongation ?2.) Thicken stem to
  make stronger? 3.) Curves stem to start growing
  horizontally.

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Triple response
Slow elongation of stem
Thicken stem
Horizontal Growth
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Ethylene Gas controls apoptosis

• What are some reasons why a plant would want to
  intentionally destroy cells?
• Leaf in the Fall
• Annual flowering plant
• Xylem vessel elements when its living contents
  are destroyed.

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Leaf Abscission

• Essential nutrients are salvaged before falling
  off.

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Fruit Ripening

• Make the fruit sweet to be attractive to animals
• Coordinated with seed maturation
• Ethylene gas triggers starch and acids to become
  sugars, making the fruit attractive.
• Positive feedback in plants. Even spreads from
  fruit to fruit.

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Brassinosteroids

• Similar to cholesterol
• Induce cell elongation
• Retards leaf abscission and promotes xylem
  differentiation.

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Plant Responses to Light

• Light has many roles other than photosynthesis.
• Light has effects upon growth and development ?
  Photomorphogenesis.
• Light allows for a plant to tell time
  seasonally and daily.
• Two types of photoreceptors Blue light
  receptors and phytochromes (red light receptors)

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Action Spectrum in the blue range induced
phototropism ? Receptor Phototropin
Time 0 Time 90 min.
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The Phytochrome Switch and Seed Germinaton

• Red light (660nm) promotes germination Far Red
  (730 nm) inhibits germination It turns out that
  the last flash is the one that counts.

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How does this happen?
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What is a tree is growing under the canopy of
others (in the shade)?

• Less red light is getting to the plant
  (chlorophyll of the canopy is absorbing it), more
  far red is getting through.
• Thus, there will be more of the phytochrome Pr
  form than Pfr. This stimulates the tree to grow
  taller.
• In the sun, it is the reverse more Pfr from and
  this stimulates branching.

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The equilibrium between Pr Pfr allows the
plants to have a circadian rhythm.

• Phytochrome is synthesized in its Pr form. Any
  Pfr will be converted back to Pfr for at night.
• The sun rises and there is a rapid conversion to
  the Pfr form.
• In this sense plants can tell how long the day is
  and how long the night is. ? Photoperiod
• Through this they can track the seasons.

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Photoperiodism

• How do plants know when to start making leaves in
  the spring/ How do they know when to flower when
  insects will be around?
• Answer They can tell by sensing the amount of
  light in a given day.

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Control of Flowering

• Short day plants? Need daylight to be shorter
  than a certain amount of light (generally 14
  hours). Really long night plants. Need
  continuous darkness for a set period.
• Long day plants? Need daylight to be longer than
  a certain amount of light. short night plants
• Day neutral plants? Flower independent of
  daylength. When they reach maturity, they flower.
• In reality, its night length that controls
  flowering.

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Reversible effects of red and far red light on
photoperiodic response. The far red flash cancels
the red flash.
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Other responses to stimuli

• Gravitopism Response to gravity, causes a root
  from a new germinating seed to grow down and the
  shoot to grow up, regardless of initial position
  in the soil.
• Thigmotroptism Response to touch or mechanical
  stress.

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