Circadian rhythms and photperiodism

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Circadian rhythms and photperiodism
Eva Farre
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• Objectives for today
• Students will be able to
• Distinguish between circadian vs. diurnal rhythms
• Interpret the role of the circadian clock in
  photoperiodism
• Understand the current molecular model for
  daylength sensing

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(No Transcript)
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What processes are regulated by day length?
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Examples to discuss today Flowering
time Growth cessation and bud set
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Circadian vs. diel/diurnal oscillations
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Case 1 Flowering time regulation in Arabidopsis
Long days
Short days
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FT and CO are necessary for flowering under long
day conditions
Long Days Short Days wild
type early late co late late ft
late late co ft late
late co-ox early early co-ox
ft late late co ft-ox early early
FT and CO are necessary for flowering in LD and
function in the same pathway
FT acts downstream of CO
long days
MODEL
CO
Flowering
FT
OX overexpressor co constants mutant ft
flowering locus T mutant
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How does CO know it is a long day?
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Hour glass model
External coincidence model (Bunning's 1936)
Internal coincidence model (Pittendrigh 1960)
Yanovsky Kay 2003
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Previous knowledge CO activates FT under long
days FT induces flowering Light is necessary for
FT induction Circadian clock mutants display
flowering phenotypes
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Questions 1. When does the peak of CO RNA
expression occur in wild type Arabidopsis
plants? 2. Does the CO expression peak at the
same under long day and under short day
conditions in the wild type? 3. Does the
expression of FT change between short day and
long day conditions in the wild type?
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Figures 1 and 2 CO effect on FT under 24 h days
CO
FT
toc1-1
Wild-type
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Questions 4. What is the circadian period of
the toc1-1 mutant under constant light
conditions? 5. What is the flowering phenotype
of the toc1-1 mutant grown under short day
conditions (8 h light and 24 h total day
length)? 6. What is the flowering phenotype of
the toc1-1 mutant grown under days of only 21h
total length? 7. How does the toc1-1 mutation
influence the expression of CO?
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Figure 1
toc1-1
Wild-type
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Figure 2
toc1-1
Wild-type
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Questions 8. What happens to FT expression in
plants overexpressing CO, i.e. with constitutive
high levels of CO expression? 9. What does CO
need to induce the expression of FT?
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Figure 4
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Questions 10. Does the data presented in this
paper support the external coincidence model or
the internal coincidence model of flowering
time?
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The external coincidence model
Imaizumi and Kay, 2006
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Could this model explain the day length dependent
growth arrest phenotypes of trees?
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Low FT levels correlate with faster growth arrest
and bud formation
mutant lines with decreased FT levels
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Different aspen clones display differences in the
timing of growth arrest
19 light 5 h night
arrest
arrest
growth
growth
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Different aspen clones display differences in the
timing of growth arrest
North
South
earlier (longer days)
later (shorter days)
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Questions a. Based on what you have learned
from the work of Yanovsky Kay, establish a
hypothesis that could explain the observations of
Bohlenius et al. b. Why would this be of
evolutionary advantage to the trees?
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19 light 5 h night
NORTH
arrest
arrest
growth
SOUTH
growth
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The circadian clock regulates the phase/timing of
gene expression
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Entrainment experiment
Q3. Predict the growth pattern of the seedling in
frame 3. What is the rationale for your
prediction? Individuals write on carbonless
paper.
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Entrainability of circadian clocks
http//millar.bio.ed.ac.uk/video.html