Abstract
In order to cope with night-time darkness, plants during the day allocate part of their photosynthate for storage, often as starch. This stored reserve is then degraded at night to sustain metabolism and growth. However, night-time starch degradation must be tightly controlled, as over-rapid turnover results in premature depletion of starch before dawn, leading to starvation. Recent experiments in Arabidopsis have shown that starch degradation proceeds at a constant rate during the night and is set such that starch reserves are exhausted almost precisely at dawn. Intriguingly, this pattern is robust with the degradation rate being adjusted to compensate for unexpected changes in the time of darkness onset. While a fundamental role for the circadian clock is well-established, the underlying mechanisms controlling starch degradation remain poorly characterized. Here, we discuss recent quantitative models that have been proposed to explain how plants can compute the appropriate starch degradation rate, a process that requires an effective arithmetic division calculation. We review experimental confirmation of the models, and describe aspects that require further investigation. Overall, the process of night-time starch degradation necessitates a fundamental metabolic role for the circadian clock and, more generally, highlights how cells process information in order to optimally manage their resources.
Highlights
Plants use photosynthesis to assimilate carbon and fuel their metabolism and growth
Given that the phosphorylation/dephosphorylation cycle is essential for starch degradation, as it regulates access of enzymes to the granule surface, this cycle is an attractive candidate both for flux modulation and for the storage of information related to starch dynamics
If the degradation rate is fixed at dusk, it would not be possible to adjust for fluctuations in reaction rates that could occur at night and cause too rapid or too slow a starch turnover
Summary
Plants use photosynthesis to assimilate carbon and fuel their metabolism and growth. Atmospheric carbon dioxide is converted into organic compounds, utilizing the energy provided by sunlight. It has been estimated that it would take days for a change in transcript levels to significantly affect the levels of most of these enzymes (Gibon et al, 2006; Piques et al, 2009; Stitt and Zeeman, 2012) These facts, along with the immediate adjustment of the degradation rate to unexpected perturbations, strongly suggest that the underlying mechanism operates at a post-translational level, possibly through protein modifications, rather than at the level of transcription. In support of this idea, many enzymes involved in starch metabolism show the potential to be modulated by phosphorylation, redox and allosteric regulation (see Kötting et al, 2010; Glaring et al, 2012 and references therein). In this review we briefly describe the models, illustrate what they have contributed, and clarify which aspects need further investigation
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