Transcript Plant Developmental Physiology IX

Lectures in Plant Developmental Physiology, 2 cr.
Department of Biological and Environmental Sciences
Plant Biology
Viikki Biocenter
Spring 2006
Developmental Responses
to Light
Lecture 8
Cercis siliquastrum
Development of Arabidopsis seedling is strongly
dependent on light
Light perception
• phytochromes, cryptochromes, phototropins.
• all photoreceptors consist of proteins bound to light
absorbing pigments i.e. chromophores.
• the spectral sensitivity of each photoreceptor
depends on its chromophore’s ability to absorb
different wavelengths i.e. on the chromophore´s
absorption spectrum.
• in response to light absorption, downstream
signaling is mediated by the photoreceptor protein.
Phytochromes
120 kDa protein
family

TKDs
transmitter
kinase domains
HKLD
prokaryotic
histidine kinase like
domain (Quail 2002)
Phytochrome response
modes- photoreversibility
Classical LFR reaction
LFR=Low fluence red/far
red response
Phytochrome is synthesized as Pr i.e. seeds and
seedlings grown in total darkness contain only this isomer.
Responses promoted by a few minutes of dim light are
prevented by subsequent brief exposure to dim far red light.
ARABIDOPSIS has 5
phytochrome genes: PHYA-E
• Phytochrome A is light labile and it is
the predominant phytochrome present
after prolonged dark periods in both
imbibed seeds and seedlings.
• Phytochromes B-E are light stable and
are the major type in light-grown plants.
Dark
•
Light
Developmental responses to light
• Mosses, liverworts, ferns and some gymnosperms
(e.g. conifers) show similar development in light
and darkness.
• In angiosperms, gametophytic and embryonic
development are largely insensitive to light.
• Light regulates development at all other stages of
the life cycle of angiosperms.
• photomorphogenesis
• phototropism
• photoperiodism
Schäfer & Bowler 2002.
EMBO reports 3: 1042-1048.
” in all cases regulation of
genes responsible for
photomorphogenesis is
predicted
to require chromatin
remodelling
mediated by DET1/DDB1
nucleosome-binding
complex”
DTT=de-etiolated
Light induced germination is largely
mediated by phytochromes
• In darkness: seedlings adopt an etiolated
morhology: Rapid elongation of the
hypocotyl or epicotyl, apical hook formation.
• In light: massive cell elongation in radicle
and shoot coordinated with the mobilization
of food reserves in the seed. Opening of the
apical hook and growth of the cotyledons.
Phytochrome reponses
• HIR, high irradiance reaction, is not
photoreversible. Also other UV-A and blue light
photoreceptors are involved.
– anthocyanin biosynthesis
– inhibition of hypocotyl elongation growth
• LFR, low fluence response, classical example of
reversible red / far-red reaction. PhyB .
• VLFR, very low fluence response, not reversible.
Schäfer & Bowler 2002.
EMBO reports 3: 10421048.
VLFR=Very Low
Fluence Response
LFR=Low Fluence
Response
R-HIR=Red High
Irradiance Response
FR-HIR= Far red High
Irradiance Response
Arabidopsis – seed germination
• some newly imbibed seeds germinate in darkness and this
is promoted by low amounts of red light (low fluence).
• after several days in darkness they come dramatically
more sensitive to light and will germinate in response to a
broad spectrum of radiation (from UV-B to far-red)
provided by VLFR.
• The extreme light sensitivity develops because of the
accumulation of high quantities of phyA in the Pr form
during prolonged darkness > germination is promoted
even if a tiny fraction of the accumulated PrA is converted
into PfrA.
• PrA displays some absorption over the whole spectrum >
very low fluence of any wavelength may induce
germination.
• more than one photoreceptor control germination which is
supported by mutant studies.
Darkness
germination in
darkness
requires phyB
(in its Pfr B
form).
LFR promoted
germination
requires phyB.
Low fluence red light
VLFR promoted
germination
requires phyA.
Very low fluence illumination
Arabidopsis – seed germinate in a
wide range of environments
• Germination in darkness i.e. some seeds always
germinate when temperature and soil moisture
allow.
• Promotion of germination by red light, i.e.
enhancement of germination on the soil surface in
sunlight, which has a high ratio red / far-red light.
• PhyA mediated germination:
– Buried seeds with light flashes
– Buried seeds just below soil surface
– Seeds on soil surface but beneath a heavy canopy
Gibberellins and seed
germination
• Phytochromes mediate the effects of
gibberellins on germination by
influencing gibberellin biosynthesis and
sensitivity.
• Red light induces and far red light
represses transcription of the GA4 and
GA4H genes of Arabidopsis.
Interaction of phytochromes and gibberellin
Seedling etiolation and photomorphogenesis:
Etiolation is an adaptation to germination below the
soil surface
Etiolation i.e. skotomorphogenesis
Photomorphogenesis i.e. de-etiolation
Light perception by the
seedling
• Photomorphogenesis can be induced by a broad
spectrum of illumination and full de-etiolation requires
continuous illumination.
• Some aspects such as changes in gene expression or
the inhibition of hypocotyl elongation may be induced by
brief pulses of light.
• Signal transduction downstream of the photoreceptors
induces photomorphogenesis in two ways.
– Photosynthetic genes may be activated by direct positive
regulation downstream of light perception
– Chromatin remodelling
– Light inducing signalling inactivates negative regulators of
photomorphogenesis.
Photomorphogenesis is
promoted by continuous
red light, far red light and
UV-A/ blue light in wild
type seedlings.
For the response
to red light
phyB is needed,
for far red light phyA
is needed,
for UV-A / blue light
cry1 is needed
Cop/det/fus mutants show some degree
of photomorphogenesis in the dark
•
•
•
•
det = de-etiolated
cop = constitutively photomorphogenic
Fus = fusca
At least 11 mutants display a whole suite of
photomorphogenic characteristics i.e. the mutants are
pleiotropic
• Most of the Cop/det/fus mutants were identified by more
than one mutant screen which means that the genes
have more than one name.
Negative regulators of photomorphogenesis
Blue
light
Wild type
Red
light
DET = de-etiolated
COP = constitutively
photomorphogenic
det/cop mutant
COP9 complex or signalosome
(CSN)
• Some of the COP/DET/FUS genes, including
at least COP8, COP9, FUS5 and FUS6
encode components of a multisubunit protein
complex.
• COP9 was the first member of the complex
to be identified.
• Biochemically COP9 is a multifunctional
regulator of protein turnover.
In darkness COP1 is in
the nucleus, where it
accelerates the
proteolysis of the HY5
transcription factor.
In the light COP1 is in
the cytoplasm.
COP1 is constitutively
cytoplasmic in det1
mutants and in mutants
that lack COP9
complex.
Schäfer & Bowler 2002.
EMBO reports 3: 1042-1048.
” in all cases regulation of
genes responsible for
photomorphogenesis is
predicted
to require chromatin
remodelling
mediated by DET1/DDB1
nucleosome-binding complex”
COP1
the movement of COP1 may be a mechanism for
the maintenance rather than initiation of
photomorphogenesis
• COP1 is a ubiquitin/protein ligase that acts by
attaching ubiquitin.
• Important target is HY5 =transcription factor
ELONGATED HYPOCOTYLS
• The activity of HY5 and the extent of
photomorphogenesis is regulated by:
– Gene is transcribed at a greater rate in the light than in the
dark
– HY5 protein is phosphorylated in darkness causing a
reduction in activity
– HY5 protein has a shorter half-life in darkness due to more
rapid ubiquitination of HY5 by COP1.
Phototropism due to
directional illumination
• Shoots are positively and roots negatively
phototropic.
• Leaves have more complex phototropic responses
that affect both the position and orientation of lamina.
This allows the formation of leaf mosaics in which
mutual shading between leaves in canopy is
minimized.
• Phototropism normally occurs through differential cell
expansion. Cell expansion on illuminated side of the
stem decreases and on the shaded side often
increases.
Chloroplasts move to maximize or
minimize the absorption of light
dim light
Cross-section through the leaf of Arabidopsis thaliana
bright light
Chloroplast movement in green algal genus Mougeotia
Perception of light and signal
transduction in phototropism
• Primarily mediated by blue/UV-A
photoreceptors.
• Requirement for auxin signalling:
– Auxin redistribution (Cholodny-Went
hypothesis) and or changes in auxin
sensitivity.
Cholodny-Went theory
Photoperiodic control of flowering
•
•
•
•
•
control of flowering by daylength.
Long-day plants: flower when days are long & nights
short and photoperiods above a critical length
(facultative LDPArabidopsis).
Short-day plants: flower when days are short & nights
long and photoperiods below a critical length
(obligate SDP soybean).
Day-neutral plants: photoperiod not a factor in
flowering (tobacco).
The photoperiodic control of flowering ensures
that flowers are produced in a favourable season
and allows floral synchronization in local
populations leading to more efficient crosspollination.
Measuring the photoperiod, importance of night length
Measuring the photoperiod
Exposing only particular sections of the shoot to
inductive photoperiods suggests that perception of
night length occurs in mature leaves and inductive
photoperiods stimulate mature leaves to produce
positive or negative flower promoting signal (florigen).
A function of Circadian clock in
the photoperiodic response
Photoreceptors and circadian clock in the regulation of CO expression
The external
coincidence model
of photoperiodism
Light sets an inducible endogenous rhythm in relation to a
treshold for response. When this inducible phase is above the
treshold and coincides with light detection a photoperiodic
response is induced or repressed.