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Abstract
Photosynthetic starch reserves that accumulate in Arabidopsis leaves during the day decrease approximately linearly with time at night to support metabolism and growth. We find that the rate of decrease is adjusted to accommodate variation in the time of onset of darkness and starch content, such that reserves last almost precisely until dawn. Generation of these dynamics therefore requires an arithmetic division computation between the starch content and expected time to dawn. We introduce two novel chemical kinetic models capable of implementing analog arithmetic division. Predictions from the models are successfully tested in plants perturbed by a night-time light period or by mutations in starch degradation pathways. Our experiments indicate which components of the starch degradation apparatus may be important for appropriate arithmetic division. Our results are potentially relevant for any biological system dependent on a food reserve for survival over a predictable time period.DOI: http://dx.doi.org/10.7554/eLife.00669.001
Highlights
Organisms must control the rate of consumption of their stored food reserves to prevent starvation during periods when food acquisition is not possible
We showed that the rate of starch degradation is appropriately adjusted in response to an unexpectedly early night
In 12-hr light/12-hr dark cycles, this mutant degrades its starch by approximately 21 hr after dawn, rather than the normal 24 hr (Graf et al, 2010; Figure 1B).When subjected to an unexpected early night, the starch degradation rate in the mutant was adjusted, such that starch reserves were again exhausted at around 21 hr after dawn (Figure 1B)
Summary
Organisms must control the rate of consumption of their stored food reserves to prevent starvation during periods when food acquisition is not possible. Starch content decreases approximately linearly with time such that 95% of starch is utilized by dawn (Gibon et al, 2004; Graf et al, 2010). One intriguing possibility is the existence of a mechanism that dynamically measures the starch content and the expected time to dawn, arithmetically divides these two quantities to compute the appropriate starch degradation rate. Such a mechanism could ensure complete utilization of available starch reserves at dawn despite variation in both the starch content at the onset of darkness and the subsequent duration of darkness.
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