Photomorphogenesis: responding to light

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17 Photomorphogenesis responding to light
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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.

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Photomorphogenesis

• Photomorphogenesis is referring to the response
  of plant to light, which is the central theme in
  plant development.

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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

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Chapter outline

• Red and far-red responses
• Blue and UV-A responses
• Interactions between photoreceptorsUV-B responses

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Phytochromes

• Phytochromes are plants photoreceptors.
• Phytochromes are photochromic. They can absorb
  red (665nm) and far-red (730nm) light and they
  have two forms, red-absorbing form (Pr) and far
  red-absorbing form (Pfr).

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Figure 17.1
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Phytochrome is photoreversible

• Pr and Pfr forms of phytochrome can change to the
  other form when expose to red or far-red light,
  respectively.

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Phytochrome is photoreversible
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The photoreversibility of phytochrome comes from
its chromophore, phytochromobilin (PFB)
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PFB is covalently linked with the N-terminal part
of phytochrome
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Conformational change of PFB results in Pr ?? Pfr
change
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Pfr form is the active from of phytochrome
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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.

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Figure 17.5
declines because Pfr is declining.
is relatively unstable, with a half life (t1/2)
of 11.5hr
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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
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Pr and Pfr forms of phytochrome is always in a
dynamic equilibrium
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Phytochrome responses can be grouped into three
groups
11000 mmol/m2
10-610-3 mmol/m2
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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
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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.
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Low Fluence Responses (LFRs) 11000
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.
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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.
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LFRs in seedling development de-etiolation of
seedlings
Figure 18.10
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Table 17.3
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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
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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.
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Nyctinastic (sleep) movement
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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.
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Pulvinus is osmotically driven by rapid
redistribution of K, Cl- and malate.
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Pulvinus is osmotically driven by rapid
redistribution of K, Cl- and malate.
H efflux
K channels open
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• 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 (Thats 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

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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.
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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
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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.

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Light under canopy is far-red enriched
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Shade avoidance is triggered by far-red light,
which can be shown in end-of-day treatment
Far-red
FRR
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Shade-avoidance syndrome
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Figure 17.14
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Phytochrome signal transduction

• Phytochrome is a protein kinase.
• When activated, it will phosphorylate other
  proteins and begin signal pathways.

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Proteins that are phosphorylated by phytochrome
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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.

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Figure 17.17
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Phytochrome also regulates other transcription
factors 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.

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Figure 17.18
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Blue and UV-A light responses

• Cryptochrome
• Phototropin

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Cryptochrome is a flavoprotein
5,10-methenyltetrahydrofolate
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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.

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Phototropin

• Phototropin was orginally isolated as nph1
  (nonphototropic hypocotyl 1).

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Phototropin

• Phototropin is also a flavoprotein with two
  flavin mononucleotide (FMN) chromophores.
• FMN chromophores binds to domain called LOV
  (light, oxygen and voltage) domain.

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Phototropin could be a blue-light dependent
protein kinase
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Interactions between Photoreceptors
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Hook straightening and cotyledon unfolding are
controlled by all three photoreceptors
Cotyledon expansion is controlled by phyB and cry1
phyB controls hypocotyl elongation
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CAB genes can be induced by either phyA (VLFR) or
phyB (LFR).