17
Photomorphogenesis: responding to
light
Fig.
9
18
Tab.
1 2 3 4 5 6 7 8
10 11 12 13 14 15 16 17
1
2
3
4
Light perception in plants
• Because plants do not enjoy the luxury of
being able to change their environment or
seek shelter from adverse conditions by
changing their location, they must be more
sensitive to changes in their surrounding so
they can adapt accordingly.
• Plants can sense light gradients and detect
subtle differences in spectral composition.
Photomorphogenesis
• Photomorphogenesis is referring to the
response of plant to light, which is the
central theme in plant development.
Photoreceptors
• Most photomorphogenic responses in higher
plants appear to be under control of one (or more)
of four classes of photoreceptors:
1. Phytochromes (red and far-red)
2. Cryptochrome (blue and UV-A): seedling
development and flowering
3. Phototropin (blue and UV-A): differential
growth in a light gradient
4. UV-B receptors: unknown
Chapter outline
• Red and far-red responses
• Blue and UV-A responses
• Interactions between photoreceptorsUV-B
responses
Phytochromes
• Phytochromes are plants photoreceptors.
• Phytochromes are photochromic. They can
absorb red (665nm) and far-red (730nm)
light and they have two forms, redabsorbing form (Pr) and far red-absorbing
form (Pfr).
Figure 17.1
Phytochrome is photoreversible
• Pr and Pfr forms of phytochrome can change to
the other form when expose to red or far-red light,
respectively.
Phytochrome is photoreversible
The photoreversibility of
phytochrome comes from its
chromophore, phytochromobilin
(PΦB)
PΦB is covalently linked with the
N-terminal part of phytochrome
Conformational change of PΦB
results in Pr  Pfr change
Pfr form is the active from of
phytochrome
Phytochrome is down regulated
after activation
• The down regulation of phytochrome involved
mRNA and protein degradation.
• Also, the expression of phytochrome will be down
regulated at transcriptional level after activation.
Figure 17.5
declines because Pfr is declining.
is relatively unstable, with a half life (t1/2) of 1~1.5hr
Figure 17.7
Five seconds of red light causes
mRNA level declines
15 minutes of lag period
follows
mRNA drops 50%
within the first hour
mRNA drops 95%
within first two hours
Pr and Pfr forms of phytochrome is
always in a dynamic equilibrium
Phytochrome responses can be
grouped into three groups
1~1000 mmol/m2
10-6~10-3 mmol/m2
Very Low Fluence Responses (VLFRs)
0.1nmol/m2 ~ 50 nmol/m2
only converts less than 0.01% of total
phytochrome to Pfr form
Because far-red light can only convert 97% Pfr
to Pr, which is more than what needed to induce
VLFRs, so VLFRs are not reversible
Very Low Fluence Responses (VLFRs)
the principle evidence that VLFRs is mediated
by phytochrome is the similarity of its action spectrum
to the absorption spectrum of Pr.
Most VLFRs are related to germination.
It obeys the law of reciprocity.
It peaks at red and blue.
Low Fluence Responses (LFRs)
1~1000 mmol/m2
Seed germination
Seedling development
Bioelectric potentials and ion distribution
Photoreversible
Exhibit reciprocity between duration of
irradiation and fluence rate
Peaks at red and far-red
LFR is induced by poising the system with a
maximum level of Pfr for a very brief period of time.
LFRs in seed germination
positively photoblastic – germination
stimulated by light
negatively photoblastic – germination
inhibited by light
A one mm thickness of fine soil will block
more than 99% of light. Only light with wavelength
longer than 700nm will be able to pass.
Very little Pfr is required to stimulate
germination.
LFRs in seedling
development
de-etiolation of seedlings
Figure 18.10
Table 17.3
LFRs in bioelectric potentials and ion
distribution
phytochrome-induced changes in the surface
potential of the dark-grown barley roots (T. Tanada)
red light  root tip become positively
charged
far-red light  root tip restore its
negative charge
Red light induces a
depolarization of the
membrane within 5-10s
following a red light
treatment.
Subsequent far-red
treatment causes a slow
return to normal polarity
or small
hyperpolarization.
Figure 18.11
Nyctinastic (sleep) movement
Pulvinus (bulbous zone) at the base of
leaf/leaflet will drive leaf movement by
altering its shape as a result of differential
changes in the volume of cells on the upper
and lower side of the organ.
Pulvinus is osmotically driven by rapid redistribution of
K+, Cl- and malate.
Pulvinus is osmotically driven by rapid redistribution of
K+, Cl- and malate.
H+ efflux
K+ channels open
High Irradiance Responses (HIRs)
-prolonged/continuous exposure to light (far-red or
direct sunlight) of relatively high irradiance
-Response is proportional to the irradiance within a
certain range (That’s why they are called HIRs, not
HFRs.)
-Not photoreversible
-Not obeying the law of reciprocity
-Many of them are also LFRs
Example 1: anthocyanin synthesis
Example 2: Inhibition of stem elongation
Example 1:
Anthocyanin
synthesis
The initiation of anthocyanin accumulation is
classical LFR, peaks at red region. However, when
the duration of irradiation lengthens, peak shifts from
R  FR.
Example 2:
Inhibition of
stem elongation
in white mustard
Only dark-grown tissue
respond to far-red.
Green tissue is more
responsive to red light.
During de-etiolation,
HIR peak shifts from
far-red to red.
Light-grown
Dark-grown
Phytochrome under natural
conditions
• Under natural conditions, phyA may just
detect the presence/absence of light since it
only accumulate under dark-grown
conditions.
• Other phytochrome response observed
under natural conditions is shade avoidance
syndrome.
Light under canopy is far-red
enriched
FR+R
Far-red
Shade avoidance is triggered by far-red light,
which can be shown in end-of-day treatment
Shade-avoidance syndrome
Figure 17.14
Phytochrome signal transduction
• Phytochrome is a protein kinase.
• When activated, it will phosphorylate other
proteins and begin signal pathways.
Proteins that are phosphorylated
by phytochrome
Phytochrome regulates gene
expression
• A lot of nuclear-encoded genes are regulated by
phytochromes, including the small subunit of
rubsico (RBCS) and the light-harvesting
chlorophyll a/b binding proteins (CAB).
• Some proteins are positively regulated, like RBCS
and CAB; others are negatively regulated, like
phyA and NADPH-protochlorophyllide
oxidoreductase.
Figure 17.17
Phytochrome also regulates other
transcription factor’s activities
• PIF3 (phytochrome interacting factor 3) is a
transcription activator.
• When phytochrome activates (Pr  Pfr),
the Pfr form binds to PIF3 and activates it.
Then activated PIF3 will activate
transcription of a large variety of proteins
containing G-box motifs.
Figure 17.18
Blue and UV-A light responses
Cryptochrome
Phototropin
Cryptochrome is a flavoprotein
5,10-methenyltetrahydrofolate
Cryptochromes
• Cryptochromes are blue/UV-A photoreceptors
mediating seedling development/flowering
responses in plants.
• In Arabidopsis, there are two cryptochromes, cry1
and cry2. The structure of cry2 is also similar to
cry1 with two chromophores.
• Cry2 has a role in determining flowering time.
Phototropin
• Phototropin was
orginally isolated as
nph1 (nonphototropic
hypocotyl 1).
Phototropin
• Phototropin is also a flavoprotein with two flavin
mononucleotide (FMN) chromophores.
• FMN chromophores binds to domain called LOV
(light, oxygen and voltage) domain.
Phototropin could be a blue-light
dependent protein kinase
Interactions between
Photoreceptors
100%
20%
68%
Hook straightening and cotyledon
unfolding are controlled by all
three photoreceptors
Cotyledon expansion
is controlled by phyB
and cry1
phyB controls
hypocotyl elongation
CAB genes can be induced by
either phyA (VLFR) or phyB
(LFR).