Plant Responses

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Plant Responses

• AP Biology Chapter 39

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Plants respond by signal transduction pathways
just like we do!

• Plants have cellular receptors that detect
  changes in their environment
• For a stimulus to elicit a response, certain
  cells must have an appropriate receptor
• Stimulation of the receptor initiates a specific
  signal transduction pathway

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A potatos response to light is an example of
cell-signal processing
Fig. 39-2
(a) Before exposure to light
(b) After a weeks exposure to natural
daylight
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Fig. 39-4-3
Transduction
Reception
Response
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2
3
Transcription factor 1
CYTOPLASM
NUCLEUS
NUCLEUS
Specific protein kinase 1 activated
Plasma membrane
cGMP
P
Second messenger produced
Transcription factor 2
Phytochrome activated by light
P
Cell wall
Specific protein kinase 2 activated
Transcription
Light
Translation
De-etiolation (greening) response proteins
Ca2 channel opened
Ca2
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Signaling pathways due to Auxin
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• A signal transduction pathway leads to regulation
  of one or more cellular activities
• In most cases, these responses to stimulation
  involve increased activity of enzymes involved in
  photosynthesis and chlorophyll production
• They may also lead to changes in gene expression.

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The Discovery of Plant Hormones

• Any response resulting in curvature of organs
  toward or away from a stimulus is called a
  tropism
• Tropisms are often caused by hormones

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• In the late 1800s, Charles Darwin and his son
  Francis conducted experiments on phototropism, a
  plants response to light
• They observed that a grass seedling could bend
  toward light only if the tip of the coleoptile
  was present
• They postulated that a signal was transmitted
  from the tip to the elongating region

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.

• F. Went concluded that the chemical was auxin
  and that it migrated to the shady side of the
  plant and caused cell growth in that area.

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• Boysen-Jensen demonstrated that the substance
  was mobile and could move through a block of
  gelatin.

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• But, maybe the light stimulates a GROWTH
  INHIBITOR on the lighted side!

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Fig. 39-5a
RESULTS
Shaded side of coleoptile
Control
Light
Illuminated side of coleoptile
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A Survey of Plant Hormones

• In general, hormones control plant growth and
  development by affecting the division,
  elongation, and differentiation of cells
• Plant hormones are produced in very low
  concentration, but a minute amount can greatly
  affect growth and development of a plant organ

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How does auxin work in stimulating cell
elongation in phototropism?
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AUXIN

• The term auxin refers to any chemical that
  promotes elongation of coleoptiles.
• The Role of Auxin in Cell Elongation
• According to the acid growth hypothesis, auxin
  stimulates proton pumps in the plasma membrane
• The proton pumps lower the pH in the cell wall,
  activating expansins, enzymes that loosen the
  walls fabric
• With the cellulose loosened, the cell can
  elongate

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Fig. 39-8
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Expansins separate microfibrils from
cross- linking polysaccharides.
Cell wallloosening enzymes
Cross-linking polysaccharides
Expansin
CELL WALL
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Cleaving allows microfibrils to slide.
Cellulose microfibril
H2O
Cell wall
Cell wall becomes more acidic.
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Plasma membrane
1
Auxin increases proton pump
activity.
Nucleus
Cytoplasm
Plasma membrane
Vacuole
CYTOPLASM
5
Cell can elongate.
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Uses of auxin

• Cell elongation in phototropism and gravitropism
• root formation and branching
• affects secondary growth by stimulating cambium
  growth.
• An overdose of synthetic auxins can kill eudicots
  ?! weedkillers

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Plant growth involves interaction between
metabolites such as sugars, phytohormones and
their action on gene expression. Auxin as a
signaling molecule has various effects depending
upon the tissue where it acts.
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CYTOKININS

• Cytokinins are so named because they stimulate
  cytokinesis (cell division).
• Cytokinins retard the aging of some plant organs

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• Control of Apical Dominance
• Cytokinins, auxin, and other factors interact in
  the control of apical dominance, a terminal buds
  ability to suppress development of axillary buds
• If the terminal bud is removed, plants become
  bushier

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Fig. 39-9
Lateral branches
Stump after removal of apical bud
(b) Apical bud removed
Axillary buds
(a) Apical bud intact (not shown in photo)
(c) Auxin added to decapitated stem
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Gibberellins

• Gibberellins or gibberellic acid (GA) have a
  variety of effects, such as stem elongation,
  fruit growth, and seed germination

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Seed Germination
Fig. 39-11
Gibberellins (GA) send signal to aleurone.
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Sugars and other nutrients are consumed.
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3
Aleurone secretes ?-amylase and other
enzymes.
Aleurone
Endosperm
?-amylase
Sugar
GA
GA
Water
Radicle
Scutellum (cotyledon)
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Abscisic Acid

• Abscisic acid (ABA) slows growth
• Two of the many effects of ABA
• Seed dormancy
• In some seeds, dormancy is broken when ABA is
  removed by heavy rain, light, or prolonged cold
• Drought tolerance
• ABA is the primary internal signal that enables
  plants to withstand drought

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Ethylene

• Plants produce ethylene in response to stresses
  such as drought, flooding, mechanical pressure,
  injury, and infection
• Also induces leaf fall (abscision) and fruit
  ripening.

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The dosage effect of ethylene on impatiens.
Plants not exposed to ethylene (A). Plants
exposed to 2 ppm ethylene for one day (B), two
days (C), and three days (D). Initially only
open flowers abscised, then buds began to
abscise. After three days of exposure, all
flowers and buds had been shed
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Light Cues in Plants

• Effects of light on plant morphology are called
  photomorphogenesis

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Fig. 39-16b
Light
Time 0 min
Effects of light on plant morphology are called
photomorphogenesis
Time 90 min
(b) Coleoptile response to light colors
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Phytochromes as Photoreceptors

• Phytochromes are pigments that regulate many of a
  plants responses to light throughout its life
• These responses include seed germination and
  shade avoidance
• Phytochromes exist in two photoreversible states,
  with conversion of Pr to Pfr triggering many
  developmental responses

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Fig. 39-19
Pfr
Pr
Red light
Responses seed germination, control
of flowering, etc.
Synthesis
Far-red light
Slow conversion in darkness (some plants)
Enzymatic destruction
Absorption of red light causes the Pr to change
to Pfr. Far-red light reverses the conversion.
Mostly, it is the Pfr that switches on
physiological and developmental responses.
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Fig. 39-17
How does the order of red and far-red
illumination affect seed germination?
RESULTS

• red-light ?
• Far-red ?
• Determing factor?
• Are the effects reversible?

1. Simulates
2. Inhibits
3. Final-light exposure
4. yes

Dark (control)
Dark
Red
Far-red
Dark
Red
Red
Far-red
Far-red
Red
Dark
Red
Red
Far-red
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Biological Clocks and Circadian Rhythms

• Many plant processes oscillate during the day
• Many legumes lower their leaves in the evening
  and raise them in the morning, even when kept
  under constant light or dark conditions

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Fig. 39-20
Midnight
Noon
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Photoperiodism and Responses to Seasons

• Photoperiod, the relative lengths of night and
  day, is the environmental stimulus plants use
  most often to detect the time of year
• Photoperiodism is a physiological response to
  photoperiod
• Some processes, including flowering in many
  species, require a certain photoperiod

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• Critical Night Length
• In the 1940s, researchers discovered that
  flowering and other responses to photoperiod are
  actually controlled by night length, not day
  length

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Fig. 39-21
24 hours
(a) Short-day (long-night) plant
What does this experiment indicate?
Light
Flash of light
Darkness
Critical dark period
(b) Long-day (short-night) plant
Red light (received by phytochromes) can
interrupt the nighttime portion of the
photoperiod
Flash of light
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Fig. 39-22
24 hours
A flash of far-red can reverse the effect though.
R
RFR
RFRR
RFRRFR
Short-day (long-night) plant
Long-day (short-night) plant
Critical dark period
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Other ResponsesGravity

• Response to gravity is known as gravitropism
• Roots show positive gravitropism shoots show
  negative gravitropism
• Plants may detect gravity by the settling of
  statoliths, specialized plastids containing dense
  starch grains

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Fig. 39-24
Statoliths
20 µm
(b) Statoliths settling
(a) Root gravitropic bending
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Mechanical Stimuli

• The term thigmomorphogenesis refers to changes in
  form that result from mechanical disturbance
• Rubbing stems of young plants a couple of times
  daily results in plants that are shorter than
  controls

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Fig. 39-25
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• Thigmotropism is growth in response to touch
• It occurs in vines and other climbing plants
• Rapid leaf movements in response to mechanical
  stimulation are examples of transmission of
  electrical impulses called action potentials

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Fig. 39-26
(a) Unstimulated state
(b) Stimulated state
Side of pulvinus with flaccid cells
Leaflets after stimulation
Side of pulvinus with turgid cells
Vein
Pulvinus (motor organ)
0.5 µm
(c) Cross section of a leaflet pair in the
stimulated state (LM)
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How plants react to environmental stresses

• Drought close stomata, slow leaf growth, reduce
  exposed surface, deep roots
• Heat stress heat shock proteins protect them
• Cold alter lipids in cell membrane
• Salt increased solute conc in cells
• Flooding make air spaces in root cortex

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How plants resist herbivores and pathogens

• Physical and chemical defenses
• Recruit predatory animals
• Immune system gene for gene recognition,
  hypersensitive response, system acquired
  response, salicylic acid

In addition to being a compound that is
chemically similar to but not identical to the
active component of aspirin (acetylsalicylic
acid), it is probably best known for its use in
anti-acne treatments.
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Beware!Chemical Defenses
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• Physical Defenses

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Recruiting predatory animals

• Ants and acacia tree

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Fig. 39-28
Recruitment of parasitoid wasps that lay their
eggs within caterpillars
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Synthesis and release of volatile attractants
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Chemical in saliva
Wounding
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1
Signal transduction pathway
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Recognizing plant pathogens
Fig. 39-29
Signal
Signal transduction pathway
Hypersensitive response
Signal transduction pathway
Acquired resistance
Avirulent pathogen
R-Avr recognition and hypersensitive response
Systemic acquired resistance