Plants are mathematicians

Here’s something new about plants: they can perform mathematical division.


During the daylight hours as plants are photosynthesizing, they are storing up all that glucose in the form of starch. Plants use the starch overnight to fuel other useful biochemistry. Every night plants use up nearly all of this starch by the next morning. Plants can tell time biochemically and have an internal circadian clock that tells them how long to expect day and night to be. So somehow plants know how to coordinate the breakdown of starch stores with how long they ‘think’ night will be.1

If scientists trick their plants in the lab by turning the lights on later to give a longer night than their normal cycle, plant growth slows down because the plants were expecting dawn sooner and ate their starch accordingly. Over time plants can adjust to the new light/dark cycle. If scientists trick their plants in a different way by turning the lights off early, plants immediately slow down their consumption to accommodate a longer night. If plants have a shorter night because scientists leave the lights on for a little longer, the plants burn through their reserve starch faster so that it is mostly depleted by morning. Somehow, plants translate the time information from their internal clock to their starch metabolism machinery. Scientists have identified mutant plants with broken internal clocks, and these plants have a difficult time regulating their night-time starch usage (i.e. they use up their starch too quickly or have more leftover in the morning).2

How do plants manage to do this? Well, Scialdone and co-authors3 report in a publication this week in eLife that plants can perform simple division. If a plant ‘knows’ what time it is and has a mechanism for ‘measuring’ starch content, then it can simply divide the amount of starch by the time. The answer gives the plant the optimal rate for digesting its starch to just last the night.

Plants don’t use a calculator or pencil and paper to do these calculations; they have to use biochemistry. How does that work? This is where biologists have to use mathematical models to get some ideas. These models were used to simulate results (starch breakdown under various cycles or tricked cycles) and then compared to real plant results. Two possible models yielded results that match the experimental plant results. Both models assume that there is (1) a molecule S whose concentration is proportional to the amount of starch and (2) a molecule T whose concentration tells the plant the expected time to dawn and whose value is reset sometime after dawn. Here is one model. The S molecules associate with the starch and allow for the breakdown of both the starch and themselves. This process is inhibited by T molecules that hold S molecules and prevent them from interacting with starch. So, the longer the time until expected dawn, more T molecules are available to slow down starch breakdown. The shorter the time until expected dawn, the fewer T molecules there are and starch is broken down more rapidly.

These scientists still don’t know what plant molecules (protein, chemical, etc) correspond to T and S. However, they were able to test how well their models predicted starch consumption in different mutant plants with defects in starch degrading enzymes.  Their models hold up surprisingly well and they were able to identify one enzyme, called PWD, that may be the calculator crunching the S and T values.

Here’s how the science breaks down in method form:

Observations: Plants accumulate starch (glucose storage molecule) during the day when they perform photosynthesis. At night, plants use that stored starch for other biochemistry in such a way that there is very little leftover at dawn. How do plants work this out?
Hypothesis: We know plants have an internal clock to measure their days and nights. They also have a way of monitoring how much starch they have. Plants perform math biochemically to appropriately divide their starch level by the expected length of night to find a consumption rate that allows them to nearly completely use all of their starch reserves over a daily cycle.
Experiment: Scialdone et. al. used mathematical models to predict starch usage, then compared that to real data. These models predicted that defects in starch consumption would result in a certain degradation pattern for given cycle conditions.
Results: The authors were able to confirm the predictions of their models using mutant plants with characteristic starch consumption defects. They identified one enzyme called PWD that may be responsible for the biochemistry behind the information needed for the division operation.
Conclusion: Plants know how to divide. They only need two molecular components to do this; one to measure the amount of starch present (S) and one to tell the time until the expected dawn (T). The PWD enzyme may be what is crunching these values.
Think Ahead: The next steps would involve figuring out how PWD does this and what the molecular components are defining starch content (S) and time until dawn (T). How broad is this biological model? Does it only describe plant starch use or does it explain other situations where reserves must be rationed over an estimated time (like migrating birds or hibernating bears)?

Sometimes I wish my body knew how to degrade all of its excess reserves overnight while I’m sleeping, but I guess I’m just not as smart as plants*.


*I realize that plants can do this because their next ‘meal’ is as sure as the rising sun. For humans this is not the case, so it’s probably a great thing that I keep all of that extra fuel around.


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