How plants implement arithmetic division to optimise resource
Antonio Scialdone, John Innes Center, Norwich, UK
Organisms must control the rate of consumption of their stored food reserves to prevent starvation when food acquisition is not possible. An example is the response to the light-dark cycle in plants. During the day, plants utilize solar energy for carbon assimilation through photosynthesis. During the night, when solar energy is unavailable, plants degrade stored carbohydrate to allow continued metabolism and growth. Arabidopsis thaliana accumulates starch during the day, then the starch is linearly degraded at night at such a rate that reserves last almost precisely until dawn. Remarkably, this pattern is extremely robust, and can accommodate changes in the initial starch content and time of onset of darkness. We showed that generation of these dynamics requires an arithmetic division computation between the starch content and expected time to dawn, and used mathematical modelling to generate chemical kinetic models capable of implementing analog arithmetic division. Predictions from the arithmetic division hypothesis were successfully tested in plants perturbed by a night-time light period or by mutations in starch degradation pathways. Components of the starch degradation apparatus that may be important for arithmetic division were identified. Our results are potentially relevant for any biological system dependent on a food reserve for survival over a predictable time period. Analog chemical kinetic approaches may also be useful for calculations in synthetic biology applications, where they are likely to prove much simpler to implement than alternative schemes based on much more complex digital circuitry.