Chapter 39 Plant Responses

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Chapter 39 Plant Responses Powered By Docstoc
					Chapter 39: Plant responses to
External and Internal Signals
           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
•Response to gravity and Gravitropism
•Response to mechanical stimuli and Thigmotropism
•Response to stress such as drought, flooding, salt and heat.
   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.
Boysen-Jensen demonstrated that a chemical
 made in the tip caused the bending effect
 discovered the
  chemical 
     The hormone is
asymmetrically distributed
   which promotes cell
 growth towards the light.
 Just like the
 system, we
need to know
  these plant
hormones and
   what they
            Plant Hormones
• All control plant growth and development
  by altering cell division, elongation and

• All have multiple effects depending upon
  site of action, concentration and
  developmental stage of plant.
Auxin = Indoleacetic Acid (IAA)
• Major function: Affects cell elongation in
  developing shoots.

• Transported by a polar mechanism.
transport of
    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.
Acid Growth Hypothesis
• 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.
Terminal Bud                                                Terminal Bud
Intact                                                      removed

 •   Terminal bud intact
     inhibits auxiliary buds
     because auxin transport       •   Terminal bud removed, you
     down.                             remove repression of auxiliary
 •   Favors shoot to lengthen.         bud growth by auxin.
     Cytokinins moving upward
     from roots counteract and     •   Cytokinins still produced which
     cause auxiliary bud               causes auxiliary buds to develop.
 •   If balanced, shoot growth
     upward at the expense of
     lateral growth of auxiliary
 •   Explains why auxiliary buds
     closer to the root are more
     likely to grow.
 Gibberellins: Effect growth in stem
  and leaves but no effect in roots
• Gibberillins stimulate Stem Elongation and
• Fruit growth
• Germination
         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 ABA:Gibberillins that controls when seeds
  break dormancy.
• Also, controls Drought Stress. Causes the stomata
  to close rapidly. Do you remember how? (hint:
Ethylene Gas has 4 major effects on
•   Triple Response to mechanical stress.
•   Apoptosis: programmed cell death
•   Leaf Abscission (falling off)
•   Fruit Ripening
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.
             Triple response

elongation                     Thicken stem
of stem

 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.
             Leaf Abscission
• Essential nutrients are salvaged before
  falling off.
             Fruit Ripening
• Make the fruit sweet to be attractive to
• 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.
• Similar to cholesterol
• Induce cell elongation
• Retards leaf abscission and promotes xylem
      Plant Responses to Light
• Light has many roles other than photosynthesis.
• Light has effects upon growth and development 
• Light allows for a plant to “tell time” seasonally
  and daily.
• Two types of photoreceptors; Blue –light receptors
  and phytochromes (red light receptors)
 Action Spectrum in the blue range
induced phototropism  Receptor =

                             Time 0

                             Time 90 min.
  The Phytochrome Switch and Seed

• Red light (660nm) promotes germination; Far Red
  (730 nm) inhibits germination; It turns out that the
  last flash is the one that counts.
How does this happen?
 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.
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
• How do plants know when to start making
  leaves in the spring/ How do they know
  when to flower when insects will be
• Answer: They can tell by sensing the
  amount of light in a given day.
          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.
 Reversible effects of red and far red
light on photoperiodic response. The
  far red flash cancels the red flash.
     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.