Investigation of light inputs into plant circadian clocks
Dixon, Laura Evelyn
Circadian clocks are biological signalling networks which have a period of ~24 hours under constant environmental conditions. They have been identified in a wide range of organisms, from cyanobacteria to mammals and through the temporal co-ordination of biological processes are believed to increase individual fitness. The mechanisms which generate these self-sustained rhythms, the pathways of entrainment and the target outputs of the clock are all areas of great interest to circadian biologists. The plant circadian clock is believed to comprise of interlocking feedback loops of transcription and translation. The morning MYB-transcription factors CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY) bind to the promoter of TIMING OF CAB2 1 (TOC1) and repress its expression, as well as their own. As levels of CCA1 and LHY fall, TOC1 is expressed and activates the expression of its repressors. This is a simplified version of the known clock components and the current model contains this core loop as well as an interlocked morning and evening loop, which also incorporates some post-translational modification (Chapter 1). Understanding the plant circadian network and its entrainment are the topics of this thesis. The study has focused on two plant species, the land plant Arabidopsis thaliana and the picoeukaryotic marine algae Ostreococcus tauri. In both of these species light-mediated entrainment of the clock has been investigated (Chapter 8), as well as the core circadian mechanism. In A. thaliana the role of a circadian associated gene, EARLY FLOWERING 3 has been a particular focus for investigation, through both experimentation and mathematical models (Chapters 4 and 5). In O. tauri the responses to light signals have been tested, as have the circadian responses to pharmacological manipulation (Chapters 6, 7 and 8). The work presented identifies a role for ELF3 in the repression of circadian genes and also links it with the regulation of protein stability. Likewise, in O. tauri the regulation of protein stability is identified to be a key mechanism for sustaining circadian rhythms. As well as investigating the clock in plants, certain photoreceptors have been characterised in S. cerevisiae with the aim of linking them to a synthetic oscillator. Together the work presented in this thesis provides evidence for the circadian community to aid with the understanding of circadian rhythms in plants, and possibly other organisms.