Findings are published in The Arabidopsis Circadian Clock Incorporates a cADPR-Based Feedback Loop on www.sciencemag.org Photo: Science Magazine

University of Cambridge researchers have shown that a signalling molecule that influences plant reactions to environmental stresses also has an effect on the plants’ circadian clock.

Scientists used Arabidopsis thaliana (thale cress) to study the circadian rhythm, which consists of feedback loops of gene expression that activate or repress one another in plants and animals in a 24 hour cycle.

The study examined cyclic adenosine diphosphate ribose (cADPR), a small cellular signalling molecule that is also important in environmental stress signalling. cADPR’s presence as a molecule rather than a protein or gene calls into question the belief that circadian clocks are formed solely from loops of genes, instead suggesting signalling networks may also play a role.

Dr Antony Dodd, Department of Plant Sciences at Cambridge University, worked on the research and highlighted its importance. “The biological clock is thought to be essential for plant life and so it is essential to discover how it works,” he explains. “In our study, we have identified a new type of circadian clock component that alters the concept of circadian clock architecture."

Researchers interfered with cADPR signalling by introducing a “reporter,” aequorin, into the plant. This produced light, the intensity of which depended on the amount of Ca2+ in cells, which is increased by cADPR. A luminescent protein, luciferase, was also introduced and controlled by the circadian clock, with an intensified photon counting camera measuring the amount of light produced by both elements in the plant cells.

Signalling was examined by introducing a chemical to block the activity of the enzyme creating cADPR. Gene encoding of an enzyme that produces cADPR was also expressed at a high level, roughly doubling the concentration of the molecule in plant cells. Both methods caused changes in the way the circadian clock functioned.

The research holds promise for crops. Output could potentially be increased with the manipulation of the circadian clock, holding the possibility of doubling plant productivity through an increased rate of photosynthesis. The regulation of seasonal flowering and seed production may also be altered.

“A couple of years ago, it was shown that correct regulation of the circadian clock could double photosynthesis and growth in Arabidopsis. If this is also the case for other plant species, including those grown commercially, then I think the circadian clock is incredibly important for optimal plant growth and agricultural productivity,” Dr Dodd says.

He also emphasised that although the study was not specifically tailored to provide information for agriculture, there could be future benefits for the industry. “Any research that identifies a signalling mechanism as being important has the potential to provide candidate targets for selective breeding, control agents or genetic manipulation to alter plant performance,” he notes.

“The circadian clock is also the internal timekeeper against which plants measure the day-length, and so regulates seasonal switches to reproduction, which is clearly important in terms of understanding ways to optimise agricultural productivity at different times of year.”

Findings were published in The Arabidopsis Circadian Clock Incorporates a cADPR-Based Feedback Loop (Antony N. Dodd, Michael J. Gardner, Carlos T. Hotta, Katharine E. Hubbard, Neil Dalchau, John Love, Jean-Maurice Assie, Fiona C. Robertson, Mia Kyed Jakobsen, Jorge Gonçalves, Dale Sanders, Alex A. R. Webb) on www.sciencemag.org.
By Rebecca Debens

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