Circadian rhythms: how do plants keep time?

 
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Circadian rhythms: how do plants keep time?

The rotation of the earth around the sun brings with it changes to the environment: cycles of light and dark, warm and cold - not only across a daily period but also seasonally. Circadian rhythms are biological cycles with a period of around 24 hours, which can be thought of as an internal clock. They are seen throughout a wide range of life; humans, animals, plants, fungi and even types of bacteria. The prolificacy of this biological process highlights its importance to successful survival.

Plants are sessile organisms (fixed in one place) and thus unable to escape when environmental conditions become unfavourable. Circadian rhythms allow plants to cope with adverse surroundings, as well as to synchronise themselves with predictable changes, such as the change from day to night. The circadian rhythm of an organism depends on their surrounding environment.

A circadian network has 3 properties:

  1. Input: Signals from the environment  (e.g light and temperature) that help to keep the internal clock in sync with the environment. This synchronisation is known as entrainment.

  2. Oscillator: A “clock” that generates a rhythm cycling approximately 24 hours and can self sustain in the absence of external stimuli. This is achieved via the action of genes and proteins.

  3. Output: Communication of the clock cycles to parts of the plants which require rhythmicity.

The basis of the circadian oscillator is a genetic feedback loop. A simple example of this is outlined below.

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  • Gene A produces Protein A and Gene B produces protein B.

  • Protein A switches on Gene B and Protein B Switches off Gene A.

  • At the start of a cycle, protein A will accumulate, eventually switching gene B on. Then protein B will also start to accumulate.

  • Once protein B reaches a sufficient amount, it will switch off gene A.

  • Protein A will be broken down now without being replenished, gene B will be switched off due to the lack of protein A.

  • This will cause protein B to decrease to a point where repression of Gene A is removed.

  • Gene A is switched on and the cycle starts again.

To Summarise, the accumulation of proteins switch certain genes on or off. As a result, different genes are expressed at different times of day within the oscillator system. Essentially, the accumulation of proteins switch certain genes on or off. As a result, different genes are expressed at different times of day within the oscillator system. This feedback loop allows the oscillator to be sustained, thus if moved into constant conditions plants still show rhythmicity. In humans, we see this in the form of jetlag, as our circadian clock is adjusting to the new day and night cycles.


So what are the advantages of having a circadian clock?

Below are several examples outlining these advantages:

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  • Runner bean seedlings are seen to change the position of their leaves between night and day. By positioning the leaves facing upwards during the day, the plant can capture light more efficiently (Bünning, 1973).

  • When the clock period of a plant is matched to the environment, they are seen to contain more chlorophyll, fix more carbon and subsequently grow faster. Experimentally, this approximately doubled the productivity of the plant when compared to plants with circadian clocks that differed from their environment (Dodd et al., 2005).

  • The circadian clock allows a plant to match the utilisation of carbohydrate reserves to the duration of the night. In short, this means is that the plant is ensuring that it has enough food to survive the night. This control is necessary for maintaining plant productivity. Plants which degrade their resource supplies before the night is over undergo “carbon starvation”, which impacts growth (Graf and Smith, 2011).

  • For a plant, the most beneficial time to grow is during the warmth and sun of spring and summer. Over winter, the extended periods of cold offer important signals to when these seasons begin. Some seeds will only germinate after long periods of chilling to ensure they start life during a time where plenty of light is available.

  • The length of daylight is a trigger for flowering in plants. The figure below outlines how this response comes about in  Arabidopsis thaliana. The black and white bars represent night and day, the line below represents the accumulation of the genes. When, and only when, CO is expressed at high levels during a long daylight phase, expression of the FT gene is induced (Eriksson and Millar, 2003).

  • As well as synchronising plant processes with the outside environment, circadian rhythms also allow for plants and animals to work on the same schedule. For many plants, pollination of flowers is reliant on the co-evolution with insects. Coordination is essential. Signals from the circadian clock provide cues dictating at what point in the year a plant should flower, the time in the day those flowers should open and when in the day a plant should emit chemicals to attract pollinators (Yakir et al. 2007).




How is this relevant in the context of vertical farming?

When growing in a vertical farm, like the one at LettUs Grow HQ, environmental conditions can be controlled precisely. The farm environment can be finely tuned to match the particular crop’s circadian clock, enhancing productivity and product quality of a crop. At LettUs Grow, we not only have the ability to match light and temperature cycles, but also irrigation.

Over the next year, Dr Antony Dodd, a world-renowned circadian plant biologist, will be working with LettUs Grow to help design optimal aeroponic cultivation recipes for a range of leafy green crops. We will be experimenting with a range of environmental inputs that should benefit plant performance.


References

  1. Webb, A.A., 2003. The physiology of circadian rhythms in plants. New Phytologist, 160(2), pp.281-303.

  2. McClung, C.R., 2006. Plant circadian rhythms. The Plant Cell, 18(4), pp.792-803.

  3. Dodd, A.N., Salathia, N., Hall, A., Kévei, E., Tóth, R., Nagy, F., Hibberd, J.M., Millar, A.J. and Webb, A.A., 2005. Plant circadian clocks increase photosynthesis, growth, survival, and competitive advantage. Science, 309(5734), pp.630-633.

  4. Webb, A.A., 2003. The physiology of circadian rhythms in plants. New Phytologist, 160(2), pp.281-303.

  5. Graf, A. and Smith, A.M., 2011. Starch and the clock: the dark side of plant productivity. Trends in plant science, 16(3), pp.169-175.

  6. Yakir, E., Hilman, D., Harir, Y. and Green, R.M., 2007. Regulation of output from the plant circadian clock. The FEBS journal, 274(2), pp.335-345.

  7. Eriksson, M.E. and Millar, A.J., 2003. The circadian clock. A plant's best friend in a spinning world. Plant Physiology, 132(2), pp.732-738.

  8. Bünning, E. (1973). The Physiological Clock. 3rd ed. (New York: Springer-Verlag).

 
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