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 Days wild typeearlylate co late late ft late late co ft late late co-ox early early co-ox ftlate late co ft-oxearly early FT and CO are necessary for flowering under long day conditions CO FT Flowering + long days OX= overexpressor co = constants mutant ft = 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 days FT induces flowering Light is necessary for FT induction Circadian 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 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 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