Plants can harness light energy to produce their own sugars from carbon dioxide and water in a process known as photosynthesis. Much of the sugar produced during the day is stored as starch to be used as an energy source overnight when photosynthesis is not possible. To enable the plant to maximise photosynthesis during the day and regulate the use of its starch energy stores at night, plants need to be able to “tell the time”. Plants have an internal “circadian” clock, which maintains 24-hour rhythms that modulate many plant processes, including photosynthesis.
At the core of the plant circadian clock are three proteins called CCA1, LHY and TOC1. These proteins (known as transcription factors) can regulate the production of other proteins. In the morning, the CCA1 and LHY proteins repress the production of TOC1 protein (see figure below). Over the course of the day, the levels of CCA1 and LHY proteins decline as their production is repressed by other transcription factors, including PRR7. With fewer CCA1 and LHY proteins to prevent its production, TOC1 levels rise at dusk and this further represses CCA1/LHY production. During the night TOC1 production falls and CCA1/LHY levels start to rise again.
The CCA1/LHY and TOC1 cycle (plus their regulators) creates fairly robust 24-hour rhythms of protein production that can regulate other processes. However, circadian clocks are only useful if they stay in sync with the environment. The 24-hour cycles of environmental day-night (a result of the Earth’s rotation on its axis) and circadian clocks are not exactly the same, so they would tend to drift apart over time. To keep them in sync with the external environment, circadian clocks are modulated by external factors like light and temperature.
Circadian regulation of photosynthesis and starch formation/breakdown leads to the amount of sugar in the plant rising and falling in 24 hour cycles, with sugar content peaking 4-8 hours after dawn. In a paper published last year in Nature, the authors found that sugar can modulate the circadian clock. The researchers lowered the sugar content of the plants by inhibiting photosynthesis (by growing the plants in the absence of carbon dioxide, or with a chemical inhibitor). In these conditions, the period of the circadian clock was lengthened by 2.5-3 hours, production of PRR7 protein was increased and CCA1 production decreased. A normal circadian clock period was restored when the plants were treated with sucrose (a sugar).
The effects of sucrose treatment on the circadian clock vary depending on time the day. When a dose of sucrose was given to plants growing in continuous low light at the time dawn would have been, it shifted the clock forward by 2 hours. However, when a dose of sucrose was given 12 hours later at the equivalent of dusk the clock was shifted back by 2 hours. The PRR7 protein was found to be required for this effect because sugars did not alter the length of the circadian period in plants that were lacking a functional PRR7 protein. Sucrose treatment at any time in the cycle led to the down-regulation of PRR7, but this was most pronounced in the morning. So at dawn, increases in plant sugar content can modulate the circadian clock, via the PRR7 protein, to move it into the day-phase but if sugar levels are still high later in the day, the onset of the night-phase can be delayed.
Although PRR7 production is decreased by sugars, it is increased by light. The opposite regulation of PRR7 by light and sugars might seem contradictory, but the authors propose a model for how it could work. Following the light-activation of PRR7 at dawn, which decreases CCA1/LHY production, the increase of sugars due to photosynthesis decreases PRR7 levels, leading to the increased production of CCA1/LHY proteins. Later in the day as sugar levels decline, production of CCA1/LHY are decreased by PRR7 regulation, leading to the increase in TOC1 at dusk.
1) Haydon et. al (2013) Photosynthetic entrainment of the Arabidopsis circadian clock. Nature. Free access via PubMed Central.