CIRCADIAN RHYTHMS IN PLANTS

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CIRCADIAN RHYTHMS IN PLANTS
WHAT IS A CIRCADIAN CLOCK? CLOCK OUTPUTS
CLOCK INPUTS THE CENTRAL OSCILLATOR OTHER
COMPONENTS SEASONAL RHYTHMS EVOLUTIONARY
RELATIONSHIPS
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WHAT IS A CIRCADIAN CLOCK?
Rhythmicity in behavior, physiology and
biochemistry of organisms
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c) plants leaf movements, cell elongation rates,
stomatal aperture, CO2 assimilation, Calvin
cycle, ethylene production,
hypocotyl elongation
very conserved motif, evening element, present
46 times in the promoters of 31 genes
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CLOCK RESETTING CLOCK INPUTS
circadian clocks must be

• reset so that internal time matches local time
  (entrainment)gt signals such as light,
  temperature,
• and nutrient availability

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THE CENTRAL OSCILLATOR
1) CCA1 and LHY
MYB-like DNA-binding domains of high similarity
- TFs
2) TOC1
atypical response regulator (of His-kinase)
C-terminus similarity to the CONSTANS family of
TF

• mediate pt-pt interactions and nuclear
  localization

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THE CENTRAL OSCILLATOR
1) CCA1 and LHY
when overexpressed gt arrhythmicity under
constant light or dark
gt reduction in mRNA levels negative feedback
loop
loss of either gene affects periodicity but
doesnt abolish rhythmicity overlapping functions
both mRNA and PT levels oscillate, peaking at
dawn
2) TOC1
mutations period shortening independently of
light, but still rhythmicity gt other factors
model for a feedback loop involving LHY, CCA1
and TOC1 based on
a) Toc1 expression oscillates peaking during
early evening, opposite to CCA1 and LHY
b) TOC1 expression low in LHY or CCA1
overexpressors gt transcriptional repression by
CCA1/LHY?
c) TOC1 expression high in lhy/cca1 double mutants
d) In TOC1 mutants CCA1/LHY expression very low gt
TOC1 positive regulator of LHY/CCA1?
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MODEL FOR A FEEDBACK LOOP OF LHY, CCA1 AND TOC1
EXPRESSION
morning
evening
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OTHER COMPONENTS
a) PIF3
PAS-domain-containing bHLH transcription factor
interacts with phyB and binds to a number of
phy-regulated promoters (e.g. CCA1 and LHY)
TOC1 binds PIF3 gt interaction necessary for LHY
and CCA1 activation?
clock hypersensitive to light in the night gt
causing arrhythmia
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c) ZEITLUPE (ZTL)
long period for cab/other CCGs, which dependent
on fluence rate gt light input to the clock
interacts with PhyB and CRY1
PAS domain kelch domain for pt-pt
interactions F-box gt targeting of pts to the
proteasome?
Transcript levels dont oscillate
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LIGHT INPUT STUDIES
organisms held in constant darkness or dim
light are treated with a brief pulse of light

• change in phase of the oscillator

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MODEL FOR A FEEDBACK LOOP INVOLVING LHY, CCA1,
AND TOC1
1) PHY and CRY as photoreceptors
2) LHY, CCA1 and TOC1 negative feedback loop
3) LHY, CCA1 repress expression of TOC1, their
positive regulator
4) Generation of circadian rhythms, including
that of CO for flowering time
6) ZLP and GI also act on light input
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SEASONAL RHYTHMS
In plants formation of flowers at the most
appropriate times of the year to ensure
reproductive success

Controlled by changes in day length
(photoperiodism) monitored by the circadian
clock.
Normally, Arabidopsis flowers more rapidly in
LD (summer) than SD conditions (winter)

• misregulation or mutation of genes implicated in
  clock function disrupts this response

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EVOLUTIONARY ASPECTS
No conservation of clock components among
organisms gt clocks have arisen multiple times
implicated in clock function in many organisms
e.g. cryptochromes
arose independently from the DNA repair enzyme
photolyase in an example of repeated evolution
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MOLECULAR BASIS OF SEASONAL TIME MEASUREMENT IN
ARABIDOPSIS
(Yanovsky and Kay, Nature 2002)
Many signaling and clock components identified,
but unknown how information integrated
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How does CO promote flowering through FT?
How to distinguish between the circadian and
light effect?
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1. CO-EXPRESSION IN TOC1 VS. WT PLANTS
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Co accelerates flowering through direct
activation of FT expression same mechanism here?
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If effects on CO and FT expression due only to
the period length defect of the mutant

• similar changes in WT under SD of 30 h (10L and
  20D)?

(in SD24 FT and TOC peak in WT at 10 h after the
onset of light)
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FT mRNA levels high but still rhythmic in LD gt
not only due to CO amounts
CO overexpressors
Light regulation of CO function and/or
independent clock regulation of FT?
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5. KINETICS OF LIGHT EFFECT ON FT mRNA LEVELS
Plants grown in SD transferred at dusk (when CO
starts rising) to constant light or darkness

• Direct effect of light rather than indirect
  effect
• in clock entrainment

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CONCLUSIONS
Light (PHYA/CRY2) high CO expression
high FT expression
flowering
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The Arabidopsis SRR1 gene mediates phyB signaling
and is required for normal circadian clock
function
(Staiger et al. 2003, Gen. Dev.)
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1. SRR1 IS IMPAIRED IN PHYB SIGNALING
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PHYB TYPICAL RESPONSES
a) Reduced chlorophyll in red-light grown
seedlings
b) Increased petiole length
srr1
wt
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2. SRR1 ALSO ACTS INDEPENDENTLY OF PHYB
dark
Red-light
White-light
Double mutants srr1/phyB
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3. SRR1 REQUIRED FOR MULTIPLE OUTPUTS OF THE
CIRCADIAN CLOCK
Flowering time regulated also by the circadian
clock gt Clock affected in srr1?
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4. SRR1 A CONSERVED NUCLEAR/CYTOPLASMIC PROTEIN
no recognizable domains
Putative nuclear localization sequence

• SRR1-GFP fusion SRR1 found both in cytoplasm and
  nucleus

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6. CONCLUSIONS
srr1 role in phyB signaling and in regulation
of circadian clock (like ELF3 and GI)
elf3 arrhythmia in light but remains rhythmic
in darkness gt light input to clock
srr1 circadian phenotype both in light and
darknes gt required for normal oscillator function
elf3 interacts with phyB in vitro gt Interaction
between srr1 and phyB?
srr1 homolog even in human
Lack of conservation among clock components,
but some common features

• srr1 involved in some of these?

(regulated nuclear translocation, phosphorylation
and negative feedback loops)