Plant Responses to Signals IV

1
Plant Responses to Signals IV

• Photomorphogenesis
• Circadian RhythmsGravitropism

http//sunflower.bio.indiana.edu/rhangart/plantsi
nmotion.html
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Signal Transductiongeneral
Second messengers, - kinase cascades, -
calcium concentration, - etc.
Receptors, - G-protein linked, - enzyme
linked, - protease linked, - etc.
Responses, - gene expression (/-), -
membrane dynamics, - metabolism -
cytoskeleton - etc.
Signals, - hormone, - light, -
temperature, - gravity, - etc.
Fig. 39.2
3
Photomorphogenesis

• Light is used by plants as a signal, as well as
  an energy source,
• quantity,
• quality (wavelength),
• direction,
• duration.

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

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Action Spectra

• Action Spectrum,
• graph of the magnitude of a biological response
  to light,
• as a function of wavelength.

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Molecular Switchlooking for a photoreceptor,
look for a photo-reversible pigment.
Fig. 39.18
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Absorption vs. Action Spectralooking for a
photoreceptor,
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Phytochromephotoreceptor molecule
dimer
Fig. 39.19
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Phytochromephotoreceptor molecule
Quantity, Time, Quality.
Fig. 39.20
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Phytochrome Location
Phtyochrome is a cytosolic protein.
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Phytochromehas multiple functions,

• Seed Germination,
• Flowering time (photoperiodism),
• Entraining (setting) the biological clock,
• End of day,
• Stem elongation,
• Leaf Expansion,
• Pigment synthesis.

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Photoperiodismflowering times,
Long-day plants, night breaks induce flowering.
Fig 39.22
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Phytochromephotoperiodism,
Fig 39.23
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Phytochrome
absorbtion spectra, the wavelengths absorbed by
specific pigments.
not phytochrome
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Cryptochromes

• blue light photoreceptors (I),
• evolved from a light dependent DNA repair
  enzyme,
• ...across phylogeny, these proteins have been
  used for many functions,
• ranging from blue-light-dependent development in
  plants,
• blue-light-mediated phase shifting of the
  circadian clock in insects,
• to a core circadian clock component in mammals.

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Phototropinsmediate phototropism,
blue light photoreceptors II

• contribute to stem, root and leaf movements in
  response to directional information,
• to maximize light gathering capacity,
• and to minimize light damage at high
  irradiances.

Phototropism action spectrum
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Concept Map
Functions
Photoreversible
Phytochrome
Cryptochrome
Signal transduction
Photoperiodism
Phototropin
Action Spectra, Absorption spectra
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Circadian Rhythms

• Relating to, or exhibiting approximately 24-hour
  periodicity,
• circa around dies day.

• Internal Biochemical Oscillators,
• found in all eukaryotes,
• eubacteria as well.

sleep movements
Fig 39.21
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Circadian Rhythms
of response,
Circadian processes continue even if light (or
dark) is continuous...
Entrainment
amplitude and period entrainment is
continuous, - allows fine control.
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• 480 of 8,000 (tested) genes are under circadian
  control,
• 1,500 (estimated) Arabidopsis genes, or 6
  follow circadian cycles of expression.

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Photosynthesis genes...
Secondary metabolism (wood, defense).
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Gravitropism

• the gravity directed growth processes that
  direct root and shoot orientation during a plants
  life-cycle,

• about 1.7, or roughly 500 genes, are transcribed
  in Arabidopsis when it is re-oriented 90o.

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Gravitropic Set Point
180o
90o
0o
Plant organs orient themselves to the gravity
vector.
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Starch Statolith Hypothesis
Moss
http//www.biosci.ohio-state.edu/plantbio/Sacklab
/timelapse.html

• Re-orientation of heavy starch grains signals
  gravity vector.

Fig 39.25