Photomorphogenesis

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Photomorphogenesis

• Environmental signals (light, temperature and
  gravity) are important signals for plant
  development.
• Light affects many aspects of plant development.
  
• Examples
• required for proper leaf development
• inhibits stem elongation in the emerging seedling
• promotes flowering in some species (daylength or
  photoperiodism)
• promotes (or inhibits) seed germination in some
  species
• Photomorphogenesis - control of growth devel.
  by light
• Phototropism - growth toward or away from light

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MOLECULAR BIOLOGY OF LEAF DEVELOPMENT

• Leaf development is light-dependent in
  angiosperms
• Chloroplast development is the signature feature
  of leaf development
• Proplastids gt (Etioplasts) gt Chloroplasts
• plastid number per cell also increases
• Light controls expression of important
  chloroplast proteins

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Arabidopsis
Dark-grown
Light-grown
cotyledon
hypocotyl
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Skotomorphogenesis seedling development in
darkness (i.e., elongated shoot (long
hypocotyl), little or no cotyledons or true
leaves)
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Barley (Hordeum vulgare) 7-10 days old
Older cells (etioplasts)
Young (meristematic) cells w/proplastids
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• Etioplasts lack
• Chlorophyll
• Photosynthetic capacity
• Major thylakoid membrane proteins

light
Etioplast
Chloroplast
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CF1- ?,? subunits of ATP synthetase (coupling
factor 1) PSI - photosystem I Chl-apoproteins PSI
I photosystem II Chl-apoproteins Pchlrd
protochlorophyllide reductase LHCII-
light-harvesting Chl-apoproteins of PSII LS -
large subunit of RuBPCase SS small subunit of
RuBPCase
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Angiosperms require light to make chlorophyll.
several steps
more steps Aminolevulinic acid
--------------gt Mg-Protoporphryin IX
--------------gt light NADPH
Protochlorophyllide --------------------gt
Chlorophyllide -----gt Chlorophyll Pchlrd
Step in chlorophyll synthesis that requires light
Pchlrd (Protochlorophyllide reductase) enzyme
that catalyzes the reduction of
protochlorophyllide it over-accumulates in
dark-grown plants, and is down-regulated by light.
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Protein synthesis by plastids isolated from
dark-grown barley and after periods of
illumination.
D psbA gene product other proteins are
described in preceding slide
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Levels of mRNAs for some chloroplast-encoded
proteins in dark-grown barley seedlings and after
1 hour of light.
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Regulation of Chloroplast-Encoded Proteins

• Result 1 - Based on pulse-labeling of isolated
  plastids, synthesis of the major thylakoid
  proteins is rapidly induced by light.
• Result 2 - Levels of mRNAs for several of these
  proteins are already abundant in dark-grown
  plants (on a per plastid basis), and do not
  change during the early hours of light treatment.
• Conclusion light induction of these
  chloroplast-encoded proteins is mainly at the
  translational or post-translational (i.e.,
  protein stability) level.

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How to distinguish between translational
repression (in the dark) versus rapid degradation
of new apoproteins?

• Experimental data indicates that
• - mRNA for PSI and PSII proteins is on polysomes
  in the dark (suggest at least some mRNA being
  translated in the dark)
• If chlorophyllide is added to plastids from
  dark-grown plants, then see greater
  pulse-labeling of the PSI and PSII proteins
• Conclusion induction is due, at least in part,
  to stabilization of newly synthesized
  Chl-apoproteins by chlorophyll

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Nuclear-encoded CabII/lhcII mRNAs are not present
in dark-grown plants. They are induced by white
light or pulses of red light, inhibited by
pulses of far-red light.
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Transcription run-off in isolated nuclei of
selected genes from dark-grown barley, and after
the indicated light treatments.
rbcS small subunit of RuBPCase cab/lhc
light-harvesting Chl- apoproteins of PSII pcr-
protochlorophyliide reductase
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Regulation of Nuclear-Encoded Chloroplast Protein
Genes by Light (in Dark-Grown Plants)

• rbcS, cab/lhc, and pcr genes regulated mainly by
  transcription
• No cab/lhc mRNA in dark-grown plants
• Light can down-regulate genes (pcr) as well as
  turn them on
• Transcriptional control of these genes mediated
  by the photoreceptor Phytochrome

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How does light control leaf plastid
development?

• 3 parts to this regulatory system
• photoreceptor(s)
• signal transduction chain (2nd messengers?)
• molecular responses (gene activation or
  de-repression)

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PHOTORECEPTORS

• Phytochrome (red/far-red)
• Blue/UV receptor (cryptochrome phototropin)
• PHYTOCHROME (PHY)
• Some major phytochrome-controlled processes
• Surface seed germination
• Inhibition of stem elongation in young seedlings
• Promotes leaf development
• Promotes stomatal opening
• Involved in circadian rhythms

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Phy exists in two interconvertible forms.
Pr - inactive, absorbs mainly red light (660
nm) Pfr - active, absorbs far-red light (730 nm)
Pfr ? Pr slowly in dark
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• Phytochrome properties
• Protein subunit of 125,000 Daltons (1100 a.a.)
• Chromophore is a linear tetrapyrrole, attached
  covalently to a cysteine
• Native Phy is a dimer
• Kinase activity

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• Many Phy-controlled processes are promoted by red
  light, inhibited by far-red light
• Red-Far Red Test
• Pulse of red light ? response
• Pulse of far-red light ? no response
• Pulse of red light ?pulse of far-red ???

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Phytochrome

• Multiple Phy genes (5 in Arabidopsis)
• Have overlapping functions, based on mutant
  analysis
• Also vary with respect to the light intensity or
  light quality required for activation
• Low fluence (LF) responses
• Very low fluence (VLF) response
• Far-red responses (Phy A)
• Can form heterodimers

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Blue-UV receptors (for leaf development)

• Absorb in the 350-450 nm range
• Sometimes called Cryptochrome
• Blue light also promotes leaf development
• First Cryptochrome gene/protein (Cry) identified
  using genetic approach
• Hy4 mutant of Arabidopsis deficient in blue-light
  stimulated leaf development
• Chromophore flavin (FAD)

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Receptors for Blue-light Specific Responses

• Phototropism
• photoreceptor is Phototropin
• NPH gene (similar to Cry)
• chromophore is FMN
• Chloroplast migration
• chloroplasts relocate based on intensity of blue
  light (bright move to wall dim move to front
  of cell toward light source)

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Cav (chloroplast avoidance) mutants of Arabidopsis
Chloroplasts dont relocate in strong blue light
to outer wall in the cav1-2 and cav1-5 mutants
Wada et al., 2001
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Defective gene in cav mutants similar to
Phototropin, Called NPL (for NPH-like) gene.
Comparison of NPL, NPH and Phy genes

• LOV
• - domain found in genes that mediate responses to
  either light, oxygen, or voltage
• absorbs blue light, probable location of
  chromophore