Phytochromes

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Lecture 25

• Phytochromes

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Light regulates plant development
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Light regulates plant development
Skotomorphogenesis (dark form begins) plants
response to darkness etiolated growth
Photomorphogenesis (light form begins) plants
response to light
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Information-transducing photoreceptors
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The roles of light in the life of plants
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Lettuce seed germination is a typical
photoreversible response controlled by phytochrome
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Photoreversibility a distinctive feature of
phytochrome
Red light
Far-red light
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Absorption spectra of purified phytochrome
Far red
Red
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Phytochrome structure

• - soluble protein of 250 kDa
• Dimer composed of two polypeptides
• each monomer consists of two components
  chromophore apoprotein
• holoprotein

chromophore
I, II monomers A chromophore- binding
domain B non-chromophore domain
apoprotein
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Structure of the Pr and Pfr forms of the
chromophore (phytochromobilin)
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Phytochrome is encoded by a multigene family
Pre-dominant forms
PhyA ? responses to FR
PhyB ? responses to R or white light
Have unique roles in regulating responses to red
and far-red
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phyA mutant seedlings show reduced sensitivityto
far-red light
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phy B mutant seedlings show reduced sensitivity
to red light
Phytochrome B acts as a red light sensor
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Phytochrome deficiencies alter growth and
development in pea and tomato
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Ecological functions of phytochromes
Light-grown phyB mutants are elongated and early
flowering
Resembles phyB mutant phenotype
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Phytochrome enables plant adaptation to light
quality changes
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Shade Avoidance Response
Plants that increase stem extension in response
to shading ? shade avoidance response (sun
plants)

shade plant
sun plant
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Ecological functions of PHYA and PHYB
Effect of PHYA and PHYB on seedling development
in sunlight versus canopy shade
After Quail et al. 1995
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Reducing shade avoidance responses can improve
crop yield
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PHYA, B, C, D, E