Abstract
Plants synthesize sucrose in source tissues (mainly mature leafs) and supply it for growth of sink tissues (young leafs). Sucrose is derived from photosynthesis during daytime and from starch at night. Because the diurnal regulation of sucrose fluxes is not completely understood, we built a mathematical model designed to reproduce all key experimental observations. For this, assumptions were made about the molecular mechanisms underlying the regulations, which are all motivated by experimental facts. The key regulators in our model are two kinases (SnRK1 and osmo-sensitive kinase OsmK) under the control of the circadian clock. SnRK1 is activated in the night to prepare for regularly occurring carbon-limiting conditions, whereas OsmK is activated during the day to prepare for water deficit, which often occurs in the afternoon. Decrease of SnRK1 and increase of OsmK result in partitioning of carbon towards sucrose to supply growing sink tissues. Concomitantly, increasing levels of the growth regulator trehalose-6-phosphate stimulates the development of new sink tissues and thus sink demand, which further activates sucrose supply in a positive feedback loop. We propose that OsmK acts as a timer to measure the length of the photoperiod and suggest experiments how this hypothesis can be validated.
Highlights
The afternoon increase in sucrose levels is explained by activation of sucrose synthesis by the osmo-sensitive kinases (OsmK) kinase, which in turn is upregulated by the expected water deficit in the afternoon by CaK kinase
No consistent explanation has been provided for this strong correlation of sucrose and trehalose 6-phosphate (T6P) concentrations, which is persistently observed under various conditions [45]
In the context of our model, it can be explained by the cross-regulations between these two metabolites, in which sucrose levels positively influence T6P levels and T6P levels positively affect sucrose levels by stimulating carbon partitioning towards sucrose synthesis in source tissues
Summary
Photosynthesis-derived carbon is partly used for sucrose production and partly for starch accumulation. These transiently stored starch reserves are used to facilitate a continued sucrose production during the night [1,2]. Carbon fluxes determining which fraction of the assimilated carbon is used for starch accumulation and how fast the stored starch is degraded during the night, must be regulated in order to ensure a continuous supply of sucrose and to avoid starvation near the end of the night. Due to the lower amount of total available carbon, in short days less sucrose can be consumed by sink tissues and plant growth is slower [5].
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