Multiple light inputs to a simple clock circuit allow complex biological rhythms

School of Biological Sciences, University of Edinburgh and Centre for Systems Biology at Edinburgh, Edinburgh EH93JD, UK.
The Plant Journal (Impact Factor: 5.97). 04/2011; 66(2):375-85. DOI: 10.1111/j.1365-313X.2011.04489.x
Source: PubMed


Circadian clocks are biological timekeepers that allow living cells to time their activity in anticipation of predictable environmental changes. Detailed understanding of the circadian network of higher plants, such as Arabidopsis thaliana, is hampered by the high number of partially redundant genes. However, the picoeukaryotic alga Ostreococcus tauri, which was recently shown to possess a small number of non-redundant clock genes, presents an attractive alternative target for detailed modelling of circadian clocks in the green lineage. Based on extensive time-series data from in vivo reporter gene assays, we developed a model of the Ostreococcus clock as a feedback loop between the genes TOC1 and CCA1. The model reproduces the dynamics of the transcriptional and translational reporters over a range of photoperiods. Surprisingly, the model is also able to predict the transient behaviour of the clock when the light conditions are altered. Despite the apparent simplicity of the clock circuit, it displays considerable complexity in its response to changing light conditions. Systematic screening of the effects of altered day length revealed a complex relationship between phase and photoperiod, which is also captured by the model. The complex light response is shown to stem from circadian gating of light-dependent mechanisms. This study provides insights into the contributions of light inputs to the Ostreococcus clock. The model suggests that a high number of light-dependent reactions are important for flexible timing in a circadian clock with only one feedback loop.

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    • "Diverse models have been created to simulate the circadian clock in different species (Fig. 2). Mathematical differential-equation models have been developed for mammals (Mus musculus) [14] [41] [42], insects (D. melanogaster) [43] [44] [45] [46], plants (Arabidopsis thaliana) [47] [48] [49], Ostreococcus tauri [50] [51], fungi (Neurospora crassa) [52] [53] [54], and cyanobacteria [55] [56]. So far, focus is set on modelling of the core-clock in an attempt to elucidate its characteristic properties, such as self-sustained oscillations, robustness, synchronization among cells and entrainment to external inputs. "
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    • "This finding was supported further by mathematical modeling studies, which could reproduce experimental expression time-courses with great accuracy (Morant et al., 2010; Thommen et al., 2010, 2012; Troein et al., 2011; Dixon et al., 2014). Remarkably, some of these analyzes showed that the light sensing mechanisms in O. tauri is carefully designed so that the clock is robust to daylight intensity fluctuations (Thommen et al., 2010) but flexibly adapts to photoperiod changes (Thommen et al., 2012), allowing it to cope with weather and seasonal variations (Troein et al., 2009; Pfeuty et al., 2012). However, the evidence was indirect and did not specify the light input pathway components. "
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