Chapter VII Plant
Photomorphogenensis
（植物光形态建成）
In the dark
In light
Comparison of dark-grown (etiolated) and
light-grown (de-etiolated) seedlings
Etiolated characteristics
De-etiolated characteristics
Distinct "apical hook" (dicot)
Apical hook opens or
or coleoptile (monocot)
coleoptile splits open
No leaf growth
Leaf growth promoted
No chlorophyll
Chlorophyll produced
Rapid stem elongation
Stem elongation
Limited radial expansion of
suppressed
stem
Radial expansion of stem
Limited root elongation
Root elongation promoted
Limited production of lateral Lateral root development
roots
accelerated
Bean (Phaseolus vulgaris)
seedlings grown under
different light conditions for 6
days. Five minutes of dim red
light per day is sufficient to
prevent some of the symptoms
of etiolation that appear under
conditions of total darkness,
such as reduced leaf size and
maintenance of the apical
hook. (Photo courtesy of H.
Smith.)
Photomorphogenensis and
Skotomorphogenensis
• SEEDLINGS GROWN IN DARKNESS have a
pale, almost ethereal appearance. This phenomenon
is caused by skotomorphogenensis(暗形态建成）.
• SEEDLINGS GROWN IN LIGHT have a stockier,
green appearance. The regulation of plant growth
and development by light is called
photomorphogenensis（光形态建成）.
• The driving force for the transitions of the two
distinct appearance is LIGHT as a signal.
I. Photoreceptors involved in
photomorphogenensis
• Phytochrome（光敏色素）: Red/Far-red
(660 and 735 nm)
• Cryptochrome（隐花色素）: Blue (400-450
nm)
• Phototropin （向光素）: UV-A (320-400nm)
• UV-B receptor: (280-320 nm) has not been
characterized.
A: Phytochrome
(Red/Far-Red receptor)
1. Discovery:
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1936 Lewis Flint: the germination of lettuce seeds is
stimulated by red light and inhibited by far-red light
1952，H Borthwick(Botanist), S Hendricks(Physical
chemist): the effects of red light （660nm) and farred light (735nm) is reversible. Supposing there was a
single pigment that could exist in two interconvertible
forms, a red light–absorbing form and a far-red
light–absorbing form.
1959, WL Butler extracted this pigment and proved
Borthwick’s prediction.
1960, Borthwick et al., named “phytochrome”.
Red light
Far-red light
Red light
Far-red light
Phytochrome has two forms: Pr and Pfr
2. Structure, synthesis & distribution
1) Structure
Phytochrome is a dimer, each consists of two
dormain (photosensory and regulatory dormain)
Phytochrome consists of chromophore
and apoprotein
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Pigment (chromophore: 生色团).
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blue-green
open chain tetrapyrolle; called phytochromobilin(植物胆色素）
made in the plastids.
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Transported into cytosol and combined with apoprotein to phytochrome
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Protein (apoprotein：脱辅基蛋白).
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glycoprotein
soluble
dimer (MW 240,000 D = 240 kD); each of the two peptides are
identical with a MW ca. 125,000 D and comprised of ca. 1128 amino
acids.
gene(s) have been cloned and the amino acid sequence is known; large
proportion of hydrophobic amino acids; suggests phytochrome is
associated with membranes.
tetrapyrolle is covalently-bonded to the protein via a thioether linkage
involving a cysteine.
Synthesized in
plastids
Both the Chromophore and the Protein Undergo
Conformational Changes
• Since the chromophore is what absorbs the light,
conformational changes in the protein are initiated
by changes in the chromophore.
• Upon absorption of light, the Pr chromophore
undergoes a cis–trans isomerization by rotation
around the double bond between carbons 15 and
16.
• This change results in a more extended
conformation of the tetrapyrrole.
Both the Chromophore and the Protein Undergo
Conformational Changes
• During the conversion of Pr to Pfr, the
protein moiety of phytochrome (the
apoprotein) also undergoes a subtle
conformational change.
• Pr and Pfr differ in their susceptibilities to
proteases and in their phosphorylation by
exogenous protein kinases.
Distribution of phytochrome
1）Phytochrome is photoreversible
Degradation
Responses
Precursor
Dark-return
• Pr and Pfr forms of phytochrome can change to
the other form when expose to red or far-red light,
respectively.
Figure 17.5
Pfr is relatively unstable, with a half life (t1/2) of 1~1.5hr
declines because Pfr is declining.
Note:
• The absorbance spectra of Pr and Pfr overlap significantly in
the red region of the spectrum (<700nm), and the Pr form of
phytochrome absorbs a small amount of light in the far-red
region
• As a consequence, a dynamic equilibrium exists between the
two forms. The proportion of phytochrome in the Pfr form
after saturating irradiation by red light is only about 85%.
Similarly, an equilibrium of 97% Pr and 3% Pfr is achieved
after saturating irradiation by far-red light . This equilibrium
is termed the photostationary state（光稳定态）and Pfr
percentage over total phytochrome(Pfr and Pr) is called
photostationary equilibrium（: 光稳定平衡） .
e.g. in red light, =0.8, while in far-red light, =0.03.
2) Pfr is the Physiologically Active Form of
Phytochrome
•
In general, the magnitude of the physiological response
to red light is proportional to the amount of Pfr produced.
•
In some cases the magnitude of the response is
proportional to the ration of Pfr to Pr, or of Pfr to Ptot.
•
Phytochrome deficient (hy) Arabidopsis mutants have
long hypocotyls in both darkness and white light. If the
red light response were due to a lack of Pr, we would
expect the opposite to be true, i.e. the hypocotyls would
be short in both darkness and white light.
4. Two Types of Phytochrome
Have Been identified
Type I (phyA)
a) About 9X more abundant in dark-grown tissues.
b) The Pfr form is rapidly degraded.
c) The Pfr form feed-back inhibits its own synthesis.
Type II (phy B-E)
a) Present in equal amounts with Type I phytochrome
in light-grown tissues.
b) The Pfr form is not degraded.
c) Synthesis of Type II phytochrome is not feed-back
inhibited by Pfr.
Phytochrome is Encoded By a
Multigene Family
• Arabidopsis has five structurally related
phytochrome genes: PHYA, PHYB, PHYC,
PHYD, and PHYE.
• PHYA is the only Type I phytochrome
• PHYB - PHYE are all Type II
phytochromes
5. Responses related to phytochrome
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Shade avoidance
De-etiolation
Seed germination
Circadian rhythms
Hook opening
Floral induction
Internode elongation
Plastid development
Leaf or stem succulence
Pigment formation such as anthocyanin
Enzyme activity (more than 60) such as glyceraldehyde-3phosphate dehydrogenase
6. PHYTOCHROME RESPONSES VARY IN
THE AMOUNT OF LIGHT(energy) REQURIED
A. FLUENCE - TOTAL NUMBER OF PHOTONS IMPINGING ON
A UNIT SURFACE AREA (micromoles/m2)
VLFR - VERY LOW FLUENCE RESPONSE (0.001~0.10μmol/m2)
LFR - LOW FLUENCE RESPONSE (1~1000μmol/m2)
B. IRRADIANCE - FLUENCE RATE; NUMBER OF PHOTONS
IMPINGING ON UNIT SURFACE AREA PER UNIT TIME
(micromoles/m2/s)
HIR - HIGH IRRADIANCE RESPONSE
Examples of VLFRs
• In dark-grown oat seedlings, red light can
stimulate the growth of the coleoptile and
inhibit the growth of the mesocotyl(中胚轴）.
• Arabidopsis seeds can be induced to
germinate with red light in the range of 1 to
100 nmol m–2.
All photoreversible responses are LFRs
7. Action model of phytochrome
• Phytochrome induced response falls into
rapid response and long-term response.
The rapid responses involve changes in
membrane permeability; the slower
responses require alterations in gene
expression.
(1): Phytochrome Regulates Membrane Potentials
and Ion Fluxes
• Phytochrome can rapidly alter the properties
of membranes.
• Studies have proven that phytochrome
regulate of K+ channels. (rapid leaflet
closure during nyctinasty)
(2): Phytochrome Regulates Gene
Expression
GFP trangenic plants showed
phytochrome also in nucleus
Blue and UV-A light responses
Cryptochrome
Phototropin
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.
Cryptochrome is a flavoprotein
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.
Blue light responses
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Phototropism
Chloroplast movement
Stomatal opening
Inhibition of stem and hypocotyl elongation
Synthesis of chlorophyll and carotenoids
Synthesis of anthocyanin.
PHOTOTROPINS
ARE
FLAVOPROTEINS
WITH SER/THR
PROTEIN
KINASES
phototropism
CHLOROPLAST MOVEMENTS -LEMNA
DARK
DARK
WEAK BLUE LIGHT
WEAK BLUE LIGHT
STRONG BLUE LIGHT
STRONG BLUE LIGHT
darkness
Blue light
C: UV-B receptor
still to be identified
UV-B responses:
Inhibition of growth, dwarf stem
Destruction of chloroplast and chlorophyll
Inhibition of electron transfer
Synthesis of anthocyanin and falvonoids.
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