Plant-environment interaction 1

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• Plant-environment interaction 1
• How do plants respond to light?
• Lecture 1 The phytochrome pathways
• Photomorphogenesis -- The light-induced
  developmental and morphological changes. So light
  not only serves as energy source for
  photosynthesis but also as a signal for growth
  and development. How does plants respond to
  light signal?

Light dark
Light dark
Monocot dicot
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• 2. Molecular changes during photomorphogenesis
• From signal (light) to responses at the molecular
  level
• Light-induced gene expressionthe greening
  process as a model.
• Etiolated plants---exposed to light---genes that
  are activated (related to the greening
  process---building the chloroplasts for
  photosynthesis).
• 3. Plants have different receptors to detect
  various spectra of light
• Red light receptors and blue light receptors are
  best defined. The receptors are the beginning
  step in light signaling pathways.
• 4. The phytochrome pathway
• The function of red light in development seed
  germination, greening (chloroplast development),
  regulation of internode elongation, flowering
  time, bud dormancy etc
• The red light receptorsphytochrome familyhave
  two reversible light absorbing forms---Pr for red
  light-absorbing form and Pfr for far red
  absorbing form. The Pfr is the biologically
  active form.

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666
Pr
Pfr
730nm
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Red/far red switch in seed germination
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3) In arabidopsis there are at least 5 different
isoforms of phytochrome PhyA,B,C,D,E. Each of
them is a peptide of about 120 kDa. The native
form is a dimer of about 250 kDa with chromophore
group called phytochromobilin. PhyA-E function
differently but with some overlap. PhyA and PhyB
are better studied Upon red/far red switch, the
protein and the chromophore both change
conformations
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4) The phytochrome domains The
chromophore-binding domain and the signaling
domain
PEST is for photo-degradation
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5) The mechanism of signaling from phytochrome
to gene expression in the nucleus a) Where is
phytochrome? From cytosol to nucleus!
Much phyA/B protein Are found in the cytosol in
the dark but light induce nuclear localization of
the Phy proteins, suggesting that downstream
events may occur in the nucleus But things are
more complicatedstudies show that both cytosolic
and nuclear pathways exist
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• b) The pathways
• PhyB directly interacts with the transcriptional
  factor
• The yeast two hybrid system
• A powerful tool to find the partner proteinsas
  most proteins function by interacting with other
  proteins, finding partner proteins is critical
  for identifying functions of a protein in the
  cell.
• Your protein of interest is used as a bait to
  fish out the prey in a cDNA library.

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A partner protein is identified for PhyB called
PIF3 (PhyB-Interacting Factor 3).
Red light causes PhyB to move into nucleus and
interact with PIF3, a transcription factor that
activates the genes encoding other transcription
factors for activation of photosynthetic genes.
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ii) The phyA serves as a protein kinase and
phosphorylates substrates in the cytoplasm The
bacterial phytochrome is a histidine kinase
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iii) The G protein pathway? phyA mutant rescue
by microinjection of G protein pathway components
(GTP) and calcium-CaM, and cGMP. iv) What is
going on in the nucleus? Protein degradation is a
key regulatory process. Identification of a E3
ligase (called COP1) as a negative regulator in
the light signaling pathwaymutant causes
constitutive photomorphogenesis---the gene
encodes a ubiquitin pathway ligase for degrading
transcription factors required for light-induced
genes expression. v) Other photoreceptors
modulate phytochrome pathway
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