Photomorphogenesis
(control of growth & development by light)
•
Environmental signals (light, temperature and gravity)
are important signals for plant development
•
Light affects many aspects of plant development, for
example:
1.
2.
3.
4.
required for proper leaf development
inhibits stem elongation in the emerging seedling
promotes flowering (photoperiodism)
promotes (or inhibits) seed germination
Molecular Biol. of Leaf Development
• Leaf development is light-dependent in
angiosperms
Arabidopsis
Dark-grown Light-grown
cotyledon
• Chloroplast development is the
signature feature:
hypocotyl
proplastids > (etioplasts) >chloroplasts
(plastid number per cell increases)
• Light controls expression of important
chloroplast proteins
Skotomorphogenesis – seedling development in darkness
Barley (Hordeum vulgare) 7-10 days old
Older cells
(etioplasts)
Young cells
w/proplastids
light
CF1- α, β subunits of ATP synthetase
PSI - photosystem I Chl-apoproteins
PSII – photosystem II Chl-apoproteins
Pchlrd – protochlorophyllide reductase
LHCII- light-harvesting Chl-apoproteins of PSII
LS - large subunit of RuBPCase
SS – small subunit of RuBPCase
From J. Mullet & colleagues
several steps
light + NADPH
aminolevulinic acid -------------> protochlorophyllide  Chlorophyllide  Chl
Pchlrd
Step in chlorophyll synthesis that requires light
Pchlrd (Protochlorophyllide reductase) – enzyme that catalyzes
the reduction of protochlorophyllide; it over-accumulates in darkgrown plants, and is down-regulated by light.
Protochlorophyllide
Chlorophyllide
Protein synthesis and select mRNA
levels in plastids from dark-grown
barley and after illumination.
D – psbA gene product, other
proteins were described in a
preceding slide
From J. Mullet & colleagues
Regulation of Plastid Proteins by Light
1. Light induction of the chloroplast-encoded proteins is
mainly at the translational and post-translational (i.e.,
protein stability) levels
John Mullet
Nuclear-encoded Cab/lhc 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.
N-H. Chua and
colleagues
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 Chlapoproteins of PSII
pcr- protochlorophyliide reductase
Klaus Apel
Regulation of Plastid Proteins by Light
1. Light induction of the chloroplast-encoded proteins is
mainly at the translational and post-translational (i.e.,
protein stability) levels
2. Regulation of the nuclear-encoded genes (e.g., rbcS,
cab/lhc, and pcr ) is mainly at transcription
- light can down-regulate (pcr) as well as up-regulate
- transcriptional control also mediated by Phytochrome
How does light control gene transcription
and plastid development ?: The
photoreceptor(s)
Plants See:
- Light Intensity
- Light Direction
- Colors
PHYTOCHROME (PHY)
Some major phytochrome-controlled processes:
1.
Surface seed germination
2.
Inhibition of stem elongation in young seedlings
3.
Promoting leaf development in young seedlings
4.
promotes stomatal opening
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
More Phytochrome properties :
1. Protein subunit of 125,000 Daltons
(~1100 amino acids).
2. Chromophore is a linear
tetrapyrrole, attached covalently to
a cysteine.
3. Native Phy is a dimer.
4. Has His-kinase activity.
The “Red Far-Red” test for Phy control:
Pulse of red light  response
Pulse of far-red light  no response
Pulse of red light pulse of far-red no
response
- 5 Phytochrome (PhyA-PhyE)
genes
- Have overlapping functions, based
on mutant analysis
- Vary with respect to the light
intensity or light quality
required for activation:
- e.g., far-red responses are
mediated by Phy A
- Can form heterodimers
•
•
•
•
absorb in the 350-450 nm range
a.k.a. Cryptochrome
Cryptochrome gene (Cry) identified using
genetic approach (Cashmore &
colleagues):
- hy4 mutant of Arabidopsis
chromophore = flavin (FAD)
Tony
Cashmore