Plants count in their sleep

27 June 2013

Interview with

Alison Smith, Martin Howard, John Innes Centre

Plants use molecular mathematics through the night to stave off starvation! The coca plant - Erythroxylum coca.Alison Smith and Martin Howard from the John Innes Centre explain the problem posed to plants when the sun dips to Naked Scientist Martha Henriques.

Martin - During the night, the sun goes down. That's a pretty fundamental problem here, in that during the day, plants can of course get energy from sunlight through photosynthesis. But during the night, when the sun goes down, their source of energy disappears and they had to come up with some way to compensate for that so the plant metabolism and growth can go on during the night. So, the way in which plants solve this problem is that during the day, they store some of the energy which they get from sunlight in the form of starch and during the night, this starch is broken down. And the observation was that the rate at which starch was broken down during the night was more or less constant during the night. If you measure the amount of starch as a function of time during the night, it kind of goes down as a straight line.

Martha - Okay, so it seems that the plants are budgeting correctly for the amount of daylight that there is.

Martin - That's right. They somehow seem to know how much starch they have and they know how much time there is left until dawn, and then they budget appropriately for the starch degradation rate based on those two numbers. Interestingly, if you give the plants unexpectedly early night, so these plants which we have in the growth rooms have spent their entire lives in one light regime and suddenly you give them an earlier night than they've ever seen before, they're able to seamlessly re-adjust, rebudget so that they can adjust their starch degradation, such that they ran out of their food reserves exactly at the time of expected dawn - even when the sunlight has disappeared earlier than they were expecting.

Martha - And so, your new interpretation in this study is that the plants were actually dividing one quantity by another. How did you come up with this idea?

Martin - Well, that seemed to be the only hypothesis that was consistent with our data. It didn't seem to matter how we tried to mess up the plants and the sunlight regimes to which they're exposed, whiether we gave them early nights, late nights, whether we altered the intensity of light during the day. We tried lots of things to try to interfere with this process but whatever we did, the plants seem to be able to rebudget appropriately. They would always get the right rates of degradation during the night, so they'd run out of their food reserves at dawn. The only way this seems to be consistent with our data and to be able to do that in all circumstances, was that they were really doing this division calculation, that they knew, that plants knew how much starch they have. They'll have information about how much time there was to dawn and they would divide these two numbers together to work out the right rate to eat up their starch.

Martha - And what exactly does it mean for a plant to divide one number by another as plants don't have a brain to do calculations? What does it mean for a plant to do arithmetic division on a molecular level?

Martin - Right. I mean, it does sound rather an extraordinary thing. We were certainly struck by this interpretation. The way in which we think it doesn't work, thinking about it naively, could it be like a computer or a calculator? Is a plant doing a calculation like it's done on a computer? And we think that's extremely unlikely because the way in which computers do these calculations is very, very complicated. We think it's much more likely that it's implemented in a completely separate way. There are two molecules that are giving the plant information about the amount of starch and the time to dawn. Basically, one of these molecules is enhancing the rate of starch degradation and the other molecule which is measuring the time to dawn is basically inhibiting it. So, if you have one molecule that's trying to make something happen, another molecule that's trying to stop it, that's a little bit like doing a division calculation because the more you have the molecule that's trying to stop it, the slower the rates you have, and that's rather like doing a division calculation. So, it turns out that if you have these two molecules and you set up the right set of chemical reactions then you can create a degradation rate that's the ratio of these two quantities. That's how we think this is done. It's not really done digitally like it is in your computer. It's done in a much more analogue fashion through chemical reactions.

Martha - And part of this analogue reaction is regulated by the internal clock that the plants have.

Martin - Yeah, so the molecule that's giving the information about the time to dawn comes from the circadian clock. So, plants just like us have an internal circadian clock which tells them when dawn is going to come and when the sun is going to rise so they can switch on the appropriate genes to do the right things at that time. So, we think that the timing information comes from the circadian clock and indeed, I think we're fairly certain about that because we know that in certain plants where the circadian clock has been interfered with, the plants start to make decisions that are incorrect.

Martha - And Alison, this work happened through quite a novel collaboration and came up with a very novel result.

Alison - The idea of any organism doing arithmetic division is new. This may be something that needs to be looked at more widely in different types of organisms. We're very surprised and excited by this, the fact that plants can do maths and they sit there in the night and they have to do maths. So, this is perhaps something else that I'd like to emphasise. This is not some trivial thing. If the plant gets the sum wrong, so it degrades its starch too fast and runs out of starch before dawn, we know that the plant gets into serious problems. We have mutants which do this and those plants grow more slowly because they can't manage their carbon budgets properly. So, this has profound implications for plant productivity, obviously, with knock-on effects on the way that we might study how to increase plant productivity in the future.

Similarly, if the plant gets the sun wrong and doesn't use all of its starch up, then it's wasting energy that it stored. Again, it's less productive. So, this is really important for plant productivity. Just as a very rough back-of-an-envelope calculation, perhaps a third of the carbon that's in life on Earth went through this metabolic pathway. So understanding how the plant controls this is really quite fundamental to biology in general.

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