Frontiers Of Biology

Sugar Sets Circadian Clocks in Plants

The rotation of the earth is widely known to cause daily changes in its environment. The species which inhabit the earth have developed circadian rhythms in order to be better adapted to the earths periodically changing environment. The mechanism which causes these circadian rhythms is called the circadian clock. The components of the circadian clock and how it functions are still largely unknown. Dr. Michael J. Haydon and the researchers at Dr. Alex A. R. Webb’s lab at the department of plant sciences at the University of Cambridge published a paper in Nature in which they set out to find out if a photosynthetic by-product is able to set the circadian clock and if so, by which molecular mechanisms it does so.

Previous studies on Arabidopsis thaliana, a small plant, have shown that sucrose has a great effect on circadian function. This research led Haydon and co-workers to test how photosynthetically derived sugars affect circadian function.The first step taken to find out how photosynthesis affects circadian rhythms was to see how circadian rhythms react when photosynthesis is inhibited.When photosynthesis was inhibited Haydon and co-workers found that the period of the circadian clock was lengthened and when sucrose was added to these samples the circadian period was restored. These results show that sugar made in photosynthesis regulate the circadian clock.

Sugar Sets Circadian Clocks in Plants

After showing that sugars produced by photosynthesis can set the circadian clock in Arabidopsis, Haydon and co-workers aimed to show how this occurs. Through their tests, the researchers proposed the concept of a “metabolic dawn.” The researchers suggested that once the photosynthetically derived sugars reach a certain threshold concentration, a signal is sent to the central oscillator, thus setting the circadian clock. The sugar concentration threshold can be considered a “metabolic dawn.” Additionally Haydon and co-workers discovered which gene in the circadian clock is acted on by photosynthetic sugars to set the clock. By investigating the behavior of plants with mutant genes involved in circadian function Haydon and co-workers found that only the prr7-11 mutants are unaffected by pulses of sucrose and therefore PRR7 is the gene which photosynthetic sugar acts on to set the circadian clock.

The researchers were then able to propose a novel metabolic feedback loop which contributes to circadian rhythms in plants, shown by the orange lines in the figure below, developed by Haydon and co-workers. They suggest that at dawn, light activates PRR7 and photosynthesis, then sugars from photosynthesis accumulate and repress PRR7, which causes the derepression of CCA1. In this feedback loop PRR7 responds to both light and photosynthesis, which allows light and photosynthesis to entrain the circadian clock in plants. In the figure below the relationships shown by the black lines are ones previously known. The red dotted line in the figure represents sugar levels through out the day and the shading represents whether or not the cells are exposed to light or dark. This figure illustrates the molecular components of the circadian clock in Arabidopsis thaliana to the fullest extent to which they are known today.
An interview was conducted with Dr. Haydon, the lead researcher in this paper, and with Dr. Webb, the head of the lab where this research took place. When asked about what implications this research may have, both said that the new information discovered in their research could be used in crop breeding and in the study of circadian function in other organisms. Dr. Webb also said, “I really believe in the pyramid of science. Everyone likes the headline, where you publish a paper and it changes the world. Actually that doesn’t happen very often. But what happens is that you change the way that people think.”

Additionally Haydon and Webb were asked about their experience as scientists. Dr. Webb noted the rewards and challeneges of being a biologist when he said, “The reason I think that it is rewarding is because it doesn’t stay still. Particularly in the biological sciences. One of the challenges of being a biologist is that the laws of biology are changing incredibly rapidly. Whole new areas of biology have been discovered since I graduated. I think that this can make studying biology both challenging and rewarding.” Dr. Haydon also expressed his opinions on studying science, stating, “I think the biggest attraction of being an academic scientist is the relative freedom. This brings both its rewards and challenges because sometimes it is not clear what should be done with that freedom.” Both Haydon and Webb are continuing their work in the circadian clock of Arabidopsis thaliana to gain a more complete view of how the clock functions.