Light regulation of plant
development
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
Light wavelengths detected by plant light receptors
Percent of light absorbed
100
chlorophyll b
80
60
chlorophyll a
40
20
0
400
500
Fig. 10-5, p. 152
600
700
Wavelength (nm)
Cryptochromes and
Phototropins
Phytochromes
Red light detection:
Phytochromes
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
History of Phytochrome discovery
long day,
short night
short day,
long night
white light
short day,
interrupted night
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 lightresponse was found to be red
flowers light. Moreover, the effect of
the red light treatment could
be suppressed by treatment
with far red light.
red light
short day,
red interruption
short day,
red followed
by far-red
red
Suggests the existence of a
receptor protein that is
activated by red light and
inhibited by far red light.
far-red
Fig. 15-22, p. 253
flowers
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.
The predicted properties of the receptor
A protein linked to a chromophore.
The chromophore (a tetrapyrrole compound) allows phytochrome to
change in response to red or far-red light.
,
leading to a change in its activity.
Far Red
light
Red
light
Active version of
Phytochrome:
Inactive version of
Phytochrome
Promotes seed
germination, shade
avoidance,
and controls circadian
entrainment, flowering,
etc…
Absorption spectra of Chlorophyll a and b
100
Percent of light absorbed
chlorophyll b
80
60
chlorophyll a
40
20
0
400
500
Fig. 10-5, p. 152
600
700
Wavelength (nm)
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.
Phytochrome promotes de-etiolation
Seedlings grown in the dark display an etiolated growth
pattern:
1) yellow unexpanded cotyledons
2) apical hook
3) Long hypocotyl
Seedlings grown in red light (or white light) display a deetiolated 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.
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.
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 deetiolation) and less leaf expansion.
Shade avoidance and Red:Far Red ratio
Active phytochrome
Blue light detection:
Phototropins
Blue Light and Plant Development
• To maximize photosynthesis
Phototropins promote:
1) Phototropism
2) Chloroplast movement
3) Stomatal opening
See also lecture on auxin effects on plant development.
(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).
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]
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) .
Fig. 11-9b, p. 170
Blue light detection:
Cryptochromes
Blue Light and Plant Development
• To maximize photosynthesis
Cryptochromes :
1) promote de-etiolation
2) control circadian entrainment
3) control flowering
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.