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