Photomorphogenesis Developmental response of
plants to light
•Photoreceptors:
Chromophore binding sites
Phytochromes
Red, far-red
light
600-750 nm
Phototropins
blue, UVA light
320-500 nm
Cryptochromes
blue, UVA light
320-500 nm
UVB - unidentified
Sullivan and Deng, 2003
Figure 7.1
Phytochrome exists in two isomeric forms
Pr - absorbs strongly in the red (peak absorbance = 660 nm)
Pfr - absorbs strongly in the far-red (peak absorbance = 730 nm)
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•If exposure of plants to red light is followed by exposure to
far-red light, no red light response occurs
•Far-red cancels red exposure
•Photoreversibility
Photoconversion
•Phytochrome is synthesized in the Pr form
•Dark grown plants contain Pr
•Isomeric forms of phytochrome interconvert:
•when Pr absorbs light, it is converted into Pfr
•When Pfr absorbs light, it is converted into Pr
•Pfr also reverts to Pr independent of light = dark
reversion
•Pr = inactive form, Pfr = active form
•Both Pfr and Pr exist as dimers
inactive
active
Figure 7.1
phyB protein structure
input
output
phytochromobilin
Pr chromophore
Arabidopsis has 5 phytochrome genes: PHYA, PHYB, PHYC,
PHYD, PHYE
Pfr chromophore
Type 1 = PHYA = light-labile phytochrome - present in dark
grown plants, degrades rapidly upon exposure to red or white
light
Type 2 = PHYB-E = light stable phytochromes - present in light
grown plants
2
Characteristics of different phytochromes:
TYPE 1: PHYA
In darkness - transcription is enhanced
- makes the Pr form (PrA)
In light
-PrA is converted to PfrA
-PfrA rapidly degraded (short half life, 1 hour)
-transcription is inhibited
In darkness,
phyA (dimeric
form)
predominates,
in light, phyB
predominates
TYPE 2: PHYB-E
In darkness -transcription makes the Pr form
-no difference in transcription relative to light
In light
-Pr is converted to Pfr
-Pfr is as stable as Pr (7-8 hours)
Figure 7.2
Characteristics of Pr vs. Pfr forms:
Pfr
•kinase - autophosphorylates and phosphorylates
other proteins
•Phosphorylation modulates interactions and activities
•nuclear
Pr
•Cytosolic (exposure of Pr to red light, conversion to
Pfr causes movement to nucleus)
•For phyB, if subsequently expose to far-red light,
conversion to PrB causes it to be cytosolic again
•For phyA, subsequent exposure to far-red light does
not stop nuclear import
•Suggests that PrA can be nuclear if it has cycled
through PfrA, but not if its newly synthesized
Chen et al.,
2004
•Red light is required for nuclear import, high red light for
compartmentalization into nuclear bodies
•Function debated - I) localize to NBs to desensitize phyB signaling
under high red light; ii) localize to NB where it functions in subset of
high fluence responses
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In nucleus, phytochromes interact with
• crptochromes
•Aux/IAA proteins
•Transcription factors - PfrB binds to PIF3
Crytochromes:
•Blue and UV-A receptors
•CRY1 and CRY2
•Homology to bacterial photolyases that catalze
blue/UVA-dependent repair of pyrimidine dimers
cry2 = light labile - degraded rapidly upon dark
grown seedling exposure to blue light
cry1 - light stable
Light-induced germination:
NPH1 (NONPHOTOTROPIC HYPOCOTYL) encodes a
blue/UV-A light receptor
•NPH1 = plasma membrane associated protein + 2
flavin chromophores
•In E.coli, undergoes blue-light-dependent
autophosphorylation
Arabidopsis newly imbibed seeds require light to germinate
•Promoted by low amounts of red
•Inhibited by low amounts of far red
•Suggests involvement of phytochromes
•Sunlight has high ratio of red to far red, therefore
promotes germination
•After several days of dark imbibition, germinate in
response to broad spectrum of very low light
•Suggests involvement of multiple photoreceptors
4
Fig 7.3
•Only phyB mutants show reduced germination in darkness,
suggesting phyB but not phyA acts to induce germination in
darkness
•Following dark imbibition, wild type seeds germinate in response to
extremely low light
•phyB mutants retain this response, phyA mutants do not
•Suggests that PhyA acts in this reponse
•In darkness, high levels of PrA accumulate
•Exposure to low levels of any sort of light will convert a small
proportion to PfrA
•Small proportion is sufficient to induce germination
•Germination of dark grown wild type is enhanced by red light
•This response still occurs in phyA mutants, but not in phyB
•Suggests that PhyB is responsible for red-light induced
germination
•Suggests that PhyB must be converted to Pfr form (PfrB) in
order to induce germination
•Dark grown germination may be due to low levels of PfrB
produced during embryogenesis
Figure 7.4
•Gibberellins are
required for germination
•ga1 mutants are unable
to germinate even in
light, suggesting light is
upstream of gibberellins
•Rescued by exposure to
exogenous GAs
•Red light induces,
while far red light
inhibits GA4 and GA4H
transcription
• suggests that Pfr is
involved in
transcriptional activation
•Requirement of ga1 for
GAs is reduced in red
light, suggesting that Pfr
increases GA sensitivity
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