Adjusting the Circadian Clock

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Published on: June 3, 2014

As highlighted in Lisa’s excellent weekly Arabidopsis Research Round-up two weeks ago, a paper on the feedback loop mechanisms that give the circadian clock flexibility was recently published in New Phytologist Early View (DOI: 10.1111/nph.12853Open Access) by GARNet 2014 speaker Andrew Millar. Here first author Laura Dixon, post-doctoral researcher in flowering regulation in the Department of Crop Genetics at the John Innes Centre, explains the research.

Dixon May2014
Arabidopsis thaliana (left) and single celled green alga Ostreococcus tauri

The circadian clock is an innate time-keeping mechanism found in most organisms, and has a period of about 24 hours. The circadian rhythm syncs to the environment as the clock mechanism adjusts to long or short photoperiods, or environmental summer and winter, and so co-ordinates many biological processes with respect to time of day and season. How quickly these adjustments can occur varies between species, and is believed to be a property of how many interlocking feedback loops the circadian clock mechanism is comprised of.

To empirically test the idea that clock flexibility is linked to the number of interlocking feedback loops within the circadian clock mechanism, we compared the fairly complex Arabidopsis thaliana clock to the very reduced clock of the smallest free-living eukaryote, unicellular green alga Ostreococcus tauri. We use A. thaliana as a plant model as it is a simple system relative to often very complex crop species. Many crop species are polyploid and so have very complicated signalling pathways; Arabidopsis is simpler but still contains complex regulation which can inform crop research. The Arabidopsis clock is a network of interlocking feedback loops. Groups of gene families encode clock components and at least 10 photoreceptor proteins.

We switched photoperiod conditions directly between short day and long day and observed what happened in the two systems. In combination with network analysis through mathematical modelling of the proposed possible clock structures, we showed that flexibility of entrainment to environmental conditions is a property of both the number of interlocking loops and the number of light inputs to the clock mechanism. Our research highlights one of the mechanisms through which circadian clock transcriptional and translational loops are flexible and adaptable in response to environmental conditions.

Images: A. thaliana from GARNet; TEM of Ostreococcus from Eikrem and Throndsen University of Oslo

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