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7th-8th Ed Ch 39 Plant Responses

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					Plant Responses

Chapter 39
Signal Transduction
   Plants have cellular receptors that detect
    important changes in their environment
Grown in Dark   1 week exposure to Light



Etiolation                       Greening
                                 Response
Signal Transduction Pathway Model
       signal triggers receptor
       receptor triggers internal cellular
        messengers & then cellular response


   receptor
   signal
    pathway
      (2° messengers)
   response


 What kinds of
 molecules are
 the receptors?
Signal Transduction Pathway
Plant Hormones

   Chemical signals that coordinate
    different parts of an organism
     only minute amounts are required
     produced by 1 part of body
     transported to another part
     binds to specific receptor
     triggers response in target cells & tissues
Plant Hormones
 auxins
 cytokinins
 gibberellins
 brassinosteroids
 abscisic acid
 ethylene
The Discovery of Plant Hormones

 Tropisms : Phototropism
Signal is a a mobile chemical
The Went Experiments

Went named the chemical
“auxin”

Later purified as
indolacetic acid
(IAA)
  How Auxins Cause Cell Elongation
         Increase in H+ activates expansins
         Weakened cell wall allows more water to enter




H+ pump
Auxins
   Herbicides2-4-D
   Affects secondary growth Synthetic auxins
    sprayed on green house plants induce fruit
    development
Cytokinins

 Stimulate Cytokinesis
 Are produced in actively growing tissues
Gibberellins
 Stimulate cell elongation and division
 Bolting
 Fruit Growth
 Germination
   Effects
     stem  elongation
     fruit growth
     seed germination
Figure 39.11 The effect of gibberellin treatment on seedless grapes
Abscisic Acid (ABA)
 Slows growth
 Maintains seed dormancy
 Drought tolerance
Ethylene A magic gas?
   Response to stress
   Leaf abscission
   Fruit ripening
   Slowing, thickening,
    curving
    Ethylene induces the triple response in pea seedlings
    Slowing, thickening,curving
ETHYLENE

A magic gas?

   Response to stress
   Leaf abscission
   Fruit ripening
Abscission of a maple leaf




     Apoptosis
     Caused by
      ethylene/auxin
      shift
Responses to gravity
   How does a sprouting shoot “know” to grow
    towards the surface from underground?
    environmental
      cues?
        roots = positive
         gravitropism
        shoots = negative
         gravitropism
        settling of statoliths
         (dense starch
         grains) may
         detect gravity
Molecular Switch Mechanism of
Phytochromes
    Phytochrome photoreceptors
          Molecular switch reaction to red light
             conversion of Pr  Pfr in sunlight stimulates
              germination, flowering, branching…
             conversion of Pfr  Pr in dark inhibits response, &
              stimulates other responses: growth in height

Chromophore                                          Light induced
Photorecptor



Kinase activity

                  Phytochrome         Phytochrome

Response:
Vertical growth
Circadian Rhythms
   Internal (endogenous) 24-hour cycles




                                4 O’clock

                      Noon          Midnight

    Morning glory
Figure 39.21 Sleep movements of a bean plant
Figure 39.x1 Biological clocks
Figure 39.22 Photoperiodic control of
flowering
Figure 39.23 Reversible effects of red and far-red
light on photoperiodic response
Is there a flowering hormone?
   Plant on left is
    induced to flower &
    then grafted onto
    plant on right
     plant on right is
      triggered to flower


     What can you conclude?
 Thigmomorphogenesis
    Change in form
     resulting from
     mechanical
     perturbation




Altering gene expression by
touch in Arabidopsis
Figure 39.27 Rapid turgor movements by the sensitive
plant (Mimosa pudica)
Plant Defenses
 Defense against herbivores
 Defense against pathogens
 Systematic acquired resistance
Figure 39.29 A corn leaf recruits a parasitoid wasp as a
defensive response to an herbivore, an army-worm
caterpillar
Figure 39.31 Defense responses against an avirulent
pathogen

				
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