Do plants have body clocks?

30 November 2008

Interview with

Steve Kay, UCSD

Steve - What our lab principally studies is biological rhythms in plants. People understand biological rhythms or circadian rhythms really from the jetlag they experience such as the BBC team might be feeling coming to San Diego. Plants also have these 24 hour rhythms but of course, they don't a sleep-wake cycle like humans have. What plants do is control their metabolism, their photosynthesis rates and what we've discovered recently is plants actually grow rhythmically.

The Forest LabChris - When does a plant do most of its growth? Most people presume plants need sunlight to grow. They're therefore going to grow during the day and go to sleep at night.

Steve - That's quite right. Plants do need sunlight for energy through photosynthesis. What we and others have discovered in recent years is that the vast majority of plants on this planet actually grow at night.

Chris - Why?

Steve - We're not exactly sure why. It could potentially be that DNA synthesis, which has to happen every time you divide plant cells and grow, perhaps DNA synthesis needs to occur in the dark to protect it from damage. It could be something like this.

Chris - Presumably if you understand how plants grow you can grow bigger ones?

Steve - Well absolutely. There are lots of numbers in science but the two numbers that worry me those most are 6.5 billion (the number of people there are now) and 9 billion (the likely number of people in 2050). We're going to be facing a serious food crisis so what we really want to understand is what makes plants tick? What are the processes that make plants grow better, compete in their environment better, use nutrients more efficiently? What we're discovering is that the plant's biological clock is at the centre of all these processes, regulating plant growth.

Chris - How do plants detect time? We know humans do it well. We know there are clusters of nerve cells in the brain that have this sort of genetic domino effect which keeps time. How do plants do it?

Some of the plants in the forest labSteve - Plants have a localised clock like we do in our brain. Plants have distributed their clock into every cell. For a human, our clock is reset by light entering our eyes. Although this may sound a little creepy there are eyes in every cell of the plant. There are proteins that transmit light to the plant and reset the plant every day.

Chris - But plants would be interesting because the ones you've got on your shelf here, they have flowers. This means certain bits of this plant know it's now time for me to make a flower. How does the plant tell one bit of the plant 'stop growing leaves and start growing modified leaves called flowers'?

Steve - It's a fascinating story. Plants use their leaves to tell daily time and to measure the amount of light that's around. They combine the measurement of dawn and dusk with an internal timekeeper to actually discriminate day length. These plants that are sitting next to us can actually tell when the days are getting shorter because they're short-day plants. That will induce flowering. Other plants are long-day plants as they measure the days getting longer they send a signal to the tip of the plant and they start making flowers.

Chris - Does this mean then that plants from one part of the world which are well-adapted to a certain day length, because they've evolved there - if you take them to another latitude - will grow less well?

Steve - Yes, absolutely. If you take plants from one clime or latitude and bring them to another then they're not going to be able to correctly interpret the day length signals and they're going to beless well adapted to that niche. What we really want to understand is, how can we begin to take advantage of what we're learning about plant growth mechanisms? Really to have an impact on agriculture.

Chris - What about climate change though? If we assume that the environment's going to change and therefore plants may have to grow in different environments and different latitudes in future but they're unable to reset their clocks. How are they going to compensate? We think climate change is going to happen more quickly than these plants have evolved in the first place.

Steve - Climate change is going to be a significant challenge both for natural ecosystems as well as for man-made agricultural biologies. What we need to do is understand the basic mechanisms by which plants adapt to stress, adapt to light levels, adapt to temperature, measure day length and essentially what we're going to be able to do from an agricultural perspective is to have tailor-made crops that we'll be able to adapt to the new types of environments produced by climate change.

Chris - How easy is that to do?

Steve - It's very hard. It's going to require, I think, a lot more effort in plant biology in general. Here in the United States, for example, we spend less than one per cent of the federal research budget on plant science versus biomedicine.

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