Plant Responses to Signals

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Plant Responses to Signals

• A weed is a plant that has mastered every
  survival skill except for learning how to grow in
  rows.
• - anonymous

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External Signals

• External signals are used by plant cells to alter
  their physiology, morphology and development,
• physical environment,
• chemical environment,
• biological environment,
• sometimes other plants,
• Plants receive signals at the cell level, and
  have no well defined sensory organs,
• Except for gravity, all other signals are
  constantly variable.

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Internal Signals

• Signals can be processed by growing and
  non-growing cells,
• Signals are transduced into biologically
  meaningful results through numerous and
  co-ordinated pathways,
• changes in ion flux,
• regulation of metabolic pathways,
• regulation of gene expression,
• changes in the cytoskeleton.

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Signal Transductionmodels
Signal
Signal
Signal
Signals
Signal
Response
Response
Response
Linear Model
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Reception

• Cell surface receptors,
• hydrophilic molecules such as peptides and
  carbohydrates dont readily cross the membrane,
• are perceived on the cell surface,
• Amphiphilic and hydrophobic molecules may pass
  through the membrane to receptors,
• steroid hormones for example,
• Light may be perceived at the cell surface, or in
  the cell.

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Plant Receptorsi.e. light responses

• Where does one look?
• no clearly differentiated organs (i.e., eyes,
  ears, etc.),
• sensitive tissues, however, no clearly
  differentiated cells,
• lots of responses.

• Germination (/-)
• Stem length (-)
• Leaf expansion ()
• Flowering (/-)
• Phototropism (/-)
• Stomatal opening ()
• Chloroplast development ()
• Pigment synthesis ()

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De-etiolationgreening
Etiolated
De-etiolated

• Etiolated growth habit,
• long stem,
• unexpanded closed leaves,
• etioplasts vs chloroplasts,
• lack of chlorophyll,
• apical hook,
• short root,
• Photomorphogenesis,
• Inhibited stem growth,
• Expanded leaves,
• Pigmentation.
• Root development.

Nature Reviews Molecular Cell Biology 3 85-93
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Phytochromeplant photoreceptor

• 1920s,
• researchers observed chlorophyll deficient
  mutants (albinos) that underwent de-etiolation
  when given physiologically active light,
• 1950s,
• phytochrome discovered,
• Molecular switch, signal transducer.

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Phytochrome
or
chromophore

• Proteins with tetra-pyrrole chromophores, and
  transmission kinase domains,
• Phytochrome gene family contains at least five
  members,
• Gene family members serve different functions.

kinase domains
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The Experiments
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Phytochrome Signal Transductionhypothesis
Signal red light
Response greening
See Fig. 39.2 for general signal transduction.
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Phytochrome Signal Transductionfor real
phytochrome (Pfr) enhances the expression of
MYB,

• Active phytochrome (Pfr) is transported to the
  nucleus,
• binds and inactivates transcriptional repressor
  (PIF3),

MYB transcription factor is expressed, in turn
activates CCA1 transcription, CCA1 enhances
the expression of CAB (chlorophyll A/B) proteins.
red light activates phytocrome, active
phytochrome (Pfr) is transported to the nucleus,
CCA1 expression, in turn, enhances greening
proteins.
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...response.
...thousands of genes,
phytochromes
hundreds of transcription factors,
Nature Reviews Molecular Cell Biology 3 85-93
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Plant Responses to Signals IIPhytohormones

• No one can look at the plants growing on a bank
  or on the borders of a thick wood, and doubt that
  the young stems and leaves place themselves so
  that the leaves may be well illuminated...they
  are extremely heliotropic and this probably
  serves...as a guide (for) the buried seeds
  through fissures in the ground or through
  overlying masses of vegetation, into the light
  and air.
• - Charles Darwin
• The Power of Movement in Plants (1880)

Charles Darwin was a Plant Physiologist, Phototro
pism, Introduction to the plant hormone Auxin.
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Phytohormones

• a plant product that is able to stimulate
  physiological responses at very low
  concentrations,
• either in the tissue in which it is synthesised,
• or in other regions of the plant to which it is
  transported,
• ...do not operate in isolation from one another,
  but often act in co-ordination to produce subtle
  responses,
• affect gene expression, enzyme activity and
  membrane function.

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Tropism a growth response in plants that results
in curvature toward, or away from a stimulus.
Phototropism

• Charles Darwin and his son Francis localized the
  location of perception for blue light
  phototropism.

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Positive Phototropism growth toward a light
stimulus

• Peter Boysen-Jensen demonstrated that a
  diffusable substance was involved .

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IAA

• Peter Went demonstrated that the diffusible
  substance resulted in cell expansion. Went
  isolated the active compound.

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Bioassays
auxin

• Bioassay identification (or quantitation) of a
  biologically active substance by measuring the
  effect the substance has on living material.

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Auxin Transportpolar

• Auxin moves basipetally (from apex to base).

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Auxininduces apical dominance,
SAM intact
SAM removed
basipetal movement of auxin inhibits axillary
buds,
axillary buds are released.
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Apical Dominance
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Auxins induce vascular differentiation
hydathode
xylem differentiates between hydathodes and leaf
vasculature ...following the path of basipetal
auxin transport.
xylem differentiation occurs around the
wound, ...following the path of auxin diffusion.
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Auxins induces lateral and adventitious root
formation,
Honeysuckle cut stems
Rooting compounds.
auxin
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Auxins promotes fruit development,
seeds removed
seeds removed auxin
seed
Normal embryo produces auxin that stimulates
fruit development.
Strawberry
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Auxinpromotes cell expansion,
See Fig. 39.11
Auxin activates the H-ATPase.
Acidification of the cell wall activates expansin
(cell wall proteins).
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Acid Growth Hypothesis
See Fig. 39.11a
ABP Auxin activates the pump.
AUXIN BINDING PROTEIN
Time course of action.
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Auxin Binding Proteinauxin receptor?
over-express ABP1 gene

• Auxin receptor has not conclusively been
  identified,
• a protein that is found to bind auxin, (Auxin
  Binding Protein, ABP) has been proposed as a
  candidate receptor,
• A common test for receptors, is to over-express
  (express more than the normal amount of protein),
  and test to see if the signal is amplified,
• ABP passes this test.

add auxin
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Auxin Binding Proteinauxin receptor?
dead
ABP controls cell division and elongation in
embryogenesis.
http//genesdev.cshlp.org/content/15/7/902
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To know TIR1 represents a new signal
transduction paradigm.
TIR1auxin receptor affects transcriptional
control
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Auxins promote gene expression,
5 - 50 minutes
Genes
Products
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Concept Map
Receptor
Discovery
Auxin
Function(s)
Transport