Can plants keep time?
We humans have body clocks or circadian rhythms that control when we wake up, when we go to sleep, and even how hungry we feel. It's why we succumb to jetlag when we go overseas. But plants also have daily rhythms that affect the way they grow and also how they transport nutrients. To find out how these work, Kate Lamble spoke to Alex Webb from the Department of Plant Sciences at Cambridge University and she started by asking exactly what a circadian rhythm is
Alex - It's any biological rhythm that has a period of 24 hours and which can persist in constant conditions. So, the one we're most familiar with is the sleep-wake cycle.
Kate - So, in me, it changes how awake I am and when I'm awake. What does it change in plants?
Alex - Well, it regulates nearly every aspect of plant biology. The one that I quite like to tell people about is in fact that plants grow over 24-hour cycle. So, just like you and me actually, plants do their maximal growth at the end of the night. It regulates photosynthesis, it regulates water flux through the plants, it regulates their ability to sense the environment, so plants are more sensitive to cold signals during the day than they are at night, and all aspects of plant biology.
Kate - When I'm aware of sort of how much light is around, I've got this environmental stimuli that I can see, how do plants recognise the environmental time signals that tell them when it's day and when it's night?
Alex - Now, you ask a difficult question. We know that they measure both temperature and light. And we know that they measure the rhythms of temperature and light and plants measure light using a series of proteins we call photoreceptors. There's a subset for red, and a subset for blue, and we know that those are involved in setting the clock. So, for example, the clock is re-set every morning by blue and red light signals. But actually, how that happens, we don't know. And then when it comes to temperature perception by plants, we know that they can measure temperature. They can exquisitely measure temperature, but we have very little idea how that works. And we know that they can in fact incorporate that temperature measurement into adjusting the clock.
Kate - Now, my circadian rhythm is controlled by my brain, but what is a plant's circadian rhythm controlled by?
Alex - Well, what an interesting idea. Is your circadian rhythm controlled by your brain or is there a circadian rhythm in your brain that controls all of you?
Kate - That's a very interesting question.
Alex - Which I think is probably the second of the two. How it works in mammals like ourselves is that there's a group of nerves, about 20,000 nerves in the brain which form a timing mechanism and they have this 24-hour rhythmic activity which they then transmit through to the rest of the organism using hormones. Plants are much more decentralised system and so in fact, every cell has a clock and that clock is made up of a suit of genes switching each other on and off with a rhythm, a bit like the clock work of an old fashioned watch. And so, all of these cells have a clock and one of the things we're interested in at the moment, I think a lot of plant biologists are interested in this, whether these clocks actually communicate between each other within the cells and is there a coordination between them or is each cell doing its own thing, but it just all occurs at the same time.
Kate - I was going to ask that because in order to be able to reset the clock, is each one of those cells communicating with the sensor that is sensing the light and the temperature that we were talking about earlier.
Alex - At least in leaf cells, each cell has its own light sensor. So, these proteins are present in all the leaf cells. The root is more interesting and there's a lot of interest at the moment because the roots grow in the dark and there's some nice work from laboratory in Glasgow, Hugh Nimmo's Laboratory and it does seem that there's some signal that comes from the leaves down to the roots and affects the clock. And that signal is probably sugar.
Kate - Why do we think that it's sugar that's communicating these signals between the different cells?
Alex - Well, if you stop photosynthesis using a drug then the root circadian clock functions differently and so, the most likely substance is sugar. And in fact, my group works on this problem as well.
Kate - I was going to say, you're particularly interested in how photosynthesis is linked to this circadian rhythm. Why would that be affected by the difference between day and night?
Alex - Plants harvest light energy and they capture it using chlorophyll and the electrons which get excited by the light energy are then used to make sugars from carbon dioxide in the air. This is an incredible amount of energy that moves through the plant and if the plant isn't ready in the morning to make all the sugars and use all that energy that it harvests, that energy will go off and do other things. And most of those things that it will do will be fairly destructive to the cells. So the plant needs to be ready at dawn to harvest all this light energy. So, the clock seems to modulate photosynthesis. So, it's more active during the day.
Kate - If plants aren't photosynthesising at night, how do they have enough food to keep their cells going during that period?
Alex - The spare sugars they make during the day are converted to starch and then the starch is broken down all through the night to keep the plant alive. And in fact, some wonderful experiments from the John Innes Laboratory, Allison Smith's lab in Norwich has shown that they actually are able to anticipate when dawn is, using a circadian clock and consume all available starch right at dawn. And if you artificially extend the night by 2 hours, the plant begins to starve and actually cannot grow because they've consumed all of their stored energy. And so, they actually anticipate when dawn is, use up all the energy that's stored at night, and then get ready to photosynthesise in the morning.
Kate - If we're beginning to understand the genes that control this circadian clock, can we exploit that in some way for farming?
Alex - Yeah, I think definitely we should be able to. Again, this is early days. Really intensive work on the circadian clock on plants has only been going on for about 15 years. But one of the things that we found out is that some of the genes which are very important for the domestication of crops are actually involved in the circadian clock. So, that tells us that early breeders were actually unknowingly selecting for altered behaviour of genes that form part of the clock. Now, what we don't understand mostly is why, but there are other areas where there's really quite a large potential for agricultural exploitation of the circadian system.
For example, the circadian clock of plants provides adaptation to help deter insect feeding. And so, if you take plants which don't have a functional circadian clock, they are much more damaged by insects than plants that do have a functional circadian clock. Now of course, that's a big step from that observation and then working out how we can take that observation and apply it. How can we get benefit? So mostly, what we seem to be able to do at the moment is make plants worse by interfering with the circadian clock. The next step is to find ways to make them better.
Kate - If we're thinking about trying to change the circadian clock of certain plants, how can we go about doing that? Is it a matter of genetically modifying and putting in the genes that we know are good circadian rhythm controllers?
Alex - That would be one approach. There is some evidence, Monsanto have I think it's a soya bean which has about 5% increase in yield which is an enormous increase in terms of agriculture and that is through genetic manipulation of a gene which is closely related to the circadian clock.
Another approach which our lab has used is we've looked for chemicals which affect the circadian clock and we've identified one chemical which can change the speed of the circadian clock and we're interested in finding out how that works. Actually, we have a few hypothesis but we don't know why the chemical is changing the speed of the clock. Interesting thing is, this chemical that we found surprisingly has exactly the same effects in circadian clocks across the kingdoms. It actually makes the clock run more slowly and it does that in plants, and then a year later, it was found after we discovered that it also has exactly the same effect in mice. And there's a lot of hypotheses about how it works, but as yet, we don't know.