Rhythm and Blue: Plant Circadian Rhythms

Anyone who's suffered from jet lag will know that circadian rhythms are an important process. But perhaps what you didn't know is that plants are also regulated by a circadian clock - and - that the same receptor is used in both plants and animals.

Circadian rhythm?

The circadian rhythm is a biological process that completes one cycle in around a 24 hour period. It is probably the most important rhythm in chronobiology - the field of biology that studies such rhythms, including tidal and seasonal rhythms. The name derives from Latin circa (around) and diem (day), so literally means around a day.

Circadian rhythms are a built-in process that are self-sustained, but are constantly adjusted by environmental factors, such as light. Interestingly both plants and animals are regulated by blue-light receptors known as cryptochromes. These chytochromes play an important role in generating, maintenance, and resetting of our circadian clocks by light - which is why if you're suffering from jet lag, you should spend time in the sun rather than cooped up in a dark hotel room!

In plants, the circadian rhythm modulates many processes in plants, such as photosynthesis and leaf movements. Like in humans, if a plant's light/dark cycle is changed, it will take a few days to adjust to the new cycle. This can be exhibited in a plants leaves opening and closing at the wrong time until the circadian clock fully adjusted.

The majority of the research in plant circadian rhythms has been performed using (surprise, surprise) Arabidopsis thaliana. This species displays many circadian rhythms inluding:

• Rhythmic cotyledon (seed leaf) and leaf movement
• Rate of hypocotyl (stem below the seed leaf) elongation
• Rate of inflorescence (flower) stem
• Rate of elongation in abaxial (lower side) and adaxial (upper side) cells of petiole.

So, it's clear to see that the circadian rhythm is at least as important in plants as it is in animals. Furthermore, as both plants and animals use cryptochrome for the same function, it is likely that it evolved in single-celled organisms and has been so useful that it has been conserved ever since. It's easy to see how this would be useful, especially in plants that can't move (in human timescales) from the spot their rooted in. Some early studies in tomata plants, showed that light/dark cycles of 24 hours, rather than short 6 hour light / 6 hour dark cycles or long cycles of 24 hour light / 24 hour dark - or indeed continuous light.

It's important to remember that the circadian clock is constantly being maintained in response to environmental factors - such as hours of day light. But that didn't stop Linnaeus, of Taxonomy fame, creating a flower clock based on plant circadian rhythms! More of which can be read about here.


Resources:
McClung, C. (2006). Plant Circadian Rhythms THE PLANT CELL ONLINE, 18 (4), 792-803 DOI: 10.1105/tpc.106.040980