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Circadian rhythms and photperiodism. Eva Farre. 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. - PowerPoint PPT Presentation
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Circadian rhythms and photperiodism
Eva Farre
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
What processes are regulated by day length?
Examples to discuss today:*Flowering time*Growth cessation and bud set
Circadian vs. diel/diurnal oscillations
Case 1: Flowering time regulation in Arabidopsis
Short days Long days
Long Days Short Dayswild type early lateco late lateft late lateco ft late lateco-ox early earlyco-ox ft late lateco ft-ox early early
FT and CO are necessary for flowering under long day conditions
CO FT Flowering+ long days
OX= overexpressorco = constants mutantft = flowering locus T mutant
MODEL
FT and CO are necessary for flowering in LD and function in the same pathway
FT acts downstream of CO
How does CO know it is a long day?
Hour glass model
External coincidence model(Bunning's 1936)
Internal coincidence model(Pittendrigh 1960)
Yanovsky & Kay 2003
Previous knowledge:
CO activates FT under long daysFT induces floweringLight is necessary for FT inductionCircadian clock mutants display flowering phenotypes
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?
CO
FT
Figures 1 and 2: CO effect on FT under 24 h days
toc1-1
Wild-type
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?
Figure 1
toc1-1
Wild-type
Figure 2
toc1-1
Wild-type
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?
Figure 4
Questions:
10. Does the data presented in this paper support the “external coincidence model” or the “internal coincidence model” of flowering time?
The external coincidence model
Imaizumi and Kay, 2006
Could this model explain the day length dependent growth arrest phenotypes of trees?
Low FT levels correlate with faster growth arrest and bud formation
mutant lines with decreased FT levels
Different aspen clones display differences in the timing of growth arrest 19 light: 5 h night
arrest
arrest
growth
growth
North South
Different aspen clones display differences in the timing of growth arrest
earlier (longer days)
later (shorter days)
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?
NORTH
SOUTH
19 light: 5 h night
arrest
arrest
growth
growth
The circadian clock regulates the phase/timing of gene expression
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.
Entrainability of circadian clocks
Thain et al., Curr Biol 2000
CAB2:LUC
http://millar.bio.ed.ac.uk/video.html