As if making food from light were not impressive enough, it may be time to add another advanced skill to the botanical repertoire: the ability to perform — at least at the molecular level — arithmetic division. Computer-generated models published in the journal eLife illustrate how plants might use molecular mathematics to regulate the rate at which they devour starch reserves to provide energy throughout the night, when energy from the Sun is off the menu1. If so, the authors say, it would be the first example of arithmetic division in biology. But it may not be the only one: many animals go through periods of fasting — during hibernations or migrations, for example — and must carefully ration internal energy stores in order to survive. Understanding how arithmetic division could occur at the molecular level might also be useful for the young field of synthetic biology, in which genetic engineers seek standardized methods of tinkering with molecular pathways to create new biological devices. Plants make the starch reserves they produce during the day last almost precisely until dawn. Researchers once thought that plants break down starch at a fixed rate during the night. But then they observed that the diminutive weed Arabidopsis thaliana, a plant favoured for laboratory work, could recalculate that rate on the fly when subjected to an unusually early or late night2.
To Alison Smith and Martin Howard of the John Innes Centre in Norwich, UK, and their colleagues, this suggested that a more sophisticated molecular calculation was at work. The team hypothesized the existence of two molecules: one, S, that tells the plant how much starch remains, and another, T, that informs it about the time left until dawn. The researchers built mathematical models to show that, in principle, the interactions of such molecules could indeed drive the rate of starch breakdown such that it reflected a continuous computation of the division of the amount of remaining starch by the amount of time until dawn. For example, the models predicted that plants would adjust the rate of starch breakdown if the night were interrupted by a period of light. During that period of light, the plants could again produce starch. When the lights went out again, the rate of starch breakdown should adjust to that increase in stored starch, the models predicted — a result that the researchers confirmed in Arabidopsis plants. The team then trawled the literature looking for Arabidopsis mutants with known handicaps at different steps along the starch-degradation pathway. These showed that the models were compatible with the behaviour of these mutants, which result in a higher than usual amount of starch remaining at the end of the night.