Body clocks and light levels: some plants just aren’t morning people

Plants (and people) need to adapt to shorter daylight hours during the winter months in Northern European countries such as Scotland and Sweden. Image credit: Katie Pickup

Just like your lie-in-loving body clock might differ from your early-riser friend’s, the plants you’ve acquired during lockdown may have similar variations in the way their body clocks work. Researchers at the Earlham Institute in Norwich have identified a gene that determines whether a plant can be classed as a night owl or an early bird. This knowledge could help improve crop growth, especially in areas with extreme seasons and varying daylight.

The gene in question, COR28, is involved in regulating circadian rhythms in Arabidopsis plants, a commonly studied relative of mustards and cabbages. Circadian rhythms regulate the biological clock so that it can follow 24 hour light and dark cycles. In humans, this helps control when we sleep and eat, and in plants it affects flowering and photosynthesis, among other processes. In either case, the closer circadian rhythms follow the light cycles in our natural environment, the healthier this is.

A mutation in the COR28 gene was found in plants that were late to flower and had a longer circadian cycle, i.e. their circadian clock was running a little bit slower than those without the mutation. The length of the circadian cycle is thought to be what determines whether we as humans are ‘night owls’ or ‘larks’. Night owls who don’t like mornings, are happy staying up late, and feel more productive later in the day, are thought to have circadian clocks that run a bit longer than 24 hours, compared to early riser larks whose circadian clocks cycle slightly shorter than 24 hours. Outside of genetics, this can be influenced by lots of factors including daily routines, naps, travel, screen time and hormones. It’s similar for plants: while they don’t sleep or have smartphone addictions, they are still sensitive to cues like temperature changes and varying daylight hours. Even without a brain to keep active during the daytime, plants need to be busy photosynthesising when there is light available in order to grow and get energy. 

Light-sensitive circadian rhythms also help plants sense the seasons due to variation in daylight hours between summer and winter months. This study was performed in plants from Sweden where days are around 12 hours longer in summer than in winter, meaning the plants need to be able to adjust their circadian clock accordingly since they can’t just get up and walk off to a sunnier place. Sensing the length of daylight allows plants to know when to flower, making sure they bloom in sync with each other to allow them to breed when conditions are right. Timing of flowering was affected by the COR28 mutation discovered in this study, with the ‘night owl’ plants flowering later than those without the mutation. The gene is also linked to frost tolerance and seems to help the plants sense and adjust to wide variations in daylight hours, both important characteristics for surviving chilly and dark Swedish winters. This finding could be really key for developing crops that are optimised to grow well under extreme climates – something we will inevitably be facing more of in the future alongside increased demands for food production. 

Adapting to shorter daylight hours is important to human health as well as plants. In the UK we face a similar situation to Sweden in having to deal with encroaching darkness at 4pm in the depths of winter. Our body clocks are influenced by other lifestyle factors such that we’re not solely dependent on daylight to set our sleeping patterns, but lack of light in the winter months can throw off our circadian clocks a bit. Poorly adapted circadian rhythms are thought to contribute to Seasonal Affective Disorder, or SAD, a common depressive disorder associated with lower light levels in the winter months. There are suggestions that those with SAD have circadian rhythms that are less responsive to light, and therefore need stronger cues to get their body clocks functioning more effectively. Some studies have suggested that night owls with longer circadian cycles may be more prone to SAD, but several others claim no association, and overall evidence is inconclusive. Although the mechanisms are different across plants and animals, there are some similarities in the way circadian rhythms are controlled and it remains a fascinating field that could help us understand more about how the environment can influence health in different species. 

Regardless of whether your circadian rhythms run slow or fast, there are several suggestions that caring for plants – or even just having them around – may be good for your mental health, and certainly many of us have enjoyed becoming plant parents during the pandemic. I think there is also a strange comfort in thinking that some of our plants may be a bit slow off the mark in the mornings just like us. 

Written by Katie Pickup and edited by Ailie McWhinnie.

Katie is a PhD Student in Genetics and Molecular Medicine. Find her on Twitter @_KatiePickup and   LinkedIn @Katie Pickup.

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