Light regulation of plant development

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Light regulation of plant development
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Light and Plant Development

• Plants detect parts of the light spectrum that
  are relevant for photosynthesis.
• Classes of major plant photoreceptors
• 1) Phytochromes detect red light
• 2) Cryptochromes detect blue light
• 3) Phototropins detect blue light

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Light wavelengths detected by plant light
receptors
100
chlorophyll b
80
60
Percent of light absorbed
chlorophyll a
40
20
0
400
500
600
700
Fig. 10-5, p. 152
Wavelength (nm)
Cryptochromes and Phototropins
Phytochromes
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Red light detectionPhytochromes
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Red Light and Plant Development

• To maximize photosynthesis
• Phytochromes
• 1) promote seed germination
• 2) promote de-etiolation
• 3) control shade avoidance
• 4) control circadian entrainment
• 5) control flowering

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History of Phytochrome discovery
Short-day plants flower only when nights are
sufficiently long. When long nights are
interrupted by a short dose of white light,
flowering is again delayed. The active wavelength
for this light-response was found to be red
light. Moreover, the effect of the red light
treatment could be suppressed by treatment with
far red light. Suggests the existence of a
receptor protein that is activated by red light
and inhibited by far red light.
long day, short night
short day, long night
flowers
white light
short day, interrupted night
red light
short day, red interruption
red
far-red
short day, red followed by far-red
flowers
Fig. 15-22, p. 253
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History of Phytochrome discovery
Phytochrome was also shown to control the
germination of seeds. Red light (activates the
receptor) promotes seed germination and far red
light suppresses the red light effect.
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The predicted properties of the receptor
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A protein linked to a chromophore.
The chromophore (a tetrapyrrole compound) allows
phytochrome to change in response to red or
far-red light.
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,
leading to a change in its activity.
Far Red light
Red light
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Active version of Phytochrome Promotes seed
germination, shade avoidance, and controls
circadian entrainment, flowering, etc
Inactive version of Phytochrome
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Absorption spectra of Chlorophyll a and b
660
730
The ratio of Red (660 nm) to Far Red (730 nm)
light will be low underneath green leaves that
absorb light between 640 and 700 nm.
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Phytochrome promotes de-etiolation

• Seedlings grown in the dark display an etiolated
  growth pattern
• yellow unexpanded cotyledons
• apical hook
• Long hypocotyl
• Seedlings grown in red light (or white light)
  display a de-etiolated growth pattern (opposite
  to etiolated)
• Green expanded cotyledons
• No apical hook
• Short hypocotyl
• Red light promotes chloroplast development and
  leaf expansion. Leaves (cotyledons) are also
  growing in upright position, allowing optimal
  light impact. Active phytochrome promotes
  seedling development that is optimal for
  photosynthesis.

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Phytochrome controls shade avoidance
Seedlings that are shaded by larger (taller)
plants that grow above them will show a shade
avoidance response. A shade avoidance response
involves increased elongation growth (stems and
petioles) and inhibition of leaf expansion. As
a result, the seedling will grow above of what
causes the shade and will now be able to perform
more efficient photosynthesis. As soon as the
seedling is not anymore shaded, shade-avoidance
growth stops.
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Phytochrome controls shade avoidance
The shade avoidance response is controlled by
Phytochromes and results from changes in the
ratio of red to far-red light. Chlorophyl from
plants that grow above the shaded seedling absorb
blue and red light (but not far red light). The
result is a lower ratio of red to far-red light
received by the shaded plant. Lower levels of red
light compared to far-red light means a lower
level of active Phytochrome (Pfr) compared to
inactive Phytochrome (Pr). Lower level of active
Phytochrome will lead to more elongation growth
(see etiolation versus de-etiolation) and less
leaf expansion.
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Shade avoidance and RedFar Red ratio
Active phytochrome
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Blue light detectionPhototropins
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Blue Light and Plant Development

• To maximize photosynthesis
• Phototropins promote
• 1) Phototropism
• 2) Chloroplast movement
• 3) Stomatal opening

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See also lecture on auxin effects on plant
development.
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(more energy reaches the leaf)
(too much light)
Chloroplasts move towards the source of light
(too maximalize light harvest)
Chloroplasts move away from the source of light
(to minimize damage by the excess light energy).
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High-light avoidance
The Chinese character for "light" on an
Arabidopsis leaf. This image was created by
exploiting the plant chloroplasts' protective
response to strong light. Upon selective
irradiation of the area within the character,
chloroplasts in this region move from the cell
surface to the side walls when light is detected
by the blue light receptor NPL1. The leaf surface
then appears paler in color in the irradiated
area. Image M. Wada
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Phototropins and stomatal opening
Light affects the opening of stomata. In dim or
no light, the stomata are closed as the light
intensity increases, the stomata open up to some
maximum value. The blue part of the light
spectrum is responsible for this response. Blue
light is perceived by phototropins that then
promote the increase in solute concentration of
guard cells starting with the conversion of
starch into malic acid (see lectures on
absorption and transportation) .
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Blue light detectionCryptochromes
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Blue Light and Plant Development

• To maximize photosynthesis
• Cryptochromes
• 1) promote de-etiolation
• 2) control circadian entrainment
• 3) control flowering

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Cryptochromes promote de-etiolation
Similar to Phytochromes, Cryptochromes promote
the de-etiolation of seedlings and control the
timing of flowering. However, in this case the
response depends on blue light (not red). The
combined effects of red and blue light in
promoting de-etiolation is stronger than
treatments with only red or only blue light.
Cryptochromes and Phytochromes enhance each
others effects in promoting seedling
de-etiolation.
When plants are exposed to both red and blue
light, their growth responses become optimal for
light harvesting. Light harvesting is done from
the red and blue parts of the light spectrum.