The Importance of the Circadian Clock in Regulating Plant Metabolism

Abstract

Carbohydrates are the primary energy source for plant development. Plants synthesize sucrose in source organs and transport them to sink organs during plant growth. This metabolism is sensitive to environmental changes in light quantity, quality, and photoperiod. In the daytime, the synthesis of sucrose and starch accumulates, and starch is degraded at nighttime. The circadian clock genes provide plants with information on the daily environmental changes and directly control many developmental processes, which are related to the path of primary metabolites throughout the life cycle. The circadian clock mechanism and processes of metabolism controlled by the circadian rhythm were studied in the model plant Arabidopsis and in the crops potato and rice. However, the translation of molecular mechanisms obtained from studies of model plants to crop plants is still difficult. Crop plants have specific organs such as edible seed and tuber that increase the size or accumulate valuable metabolites by harvestable metabolic components. Human consumers are interested in the regulation and promotion of these agriculturally significant crops. Circadian clock manipulation may suggest various strategies for the increased productivity of food crops through using environmental signal or overcoming environmental stress.

Keywords: carbohydrate; circadian clock gene; circadian rhythms; crop productivity; diurnal regulation; metabolism; photoperiodic control.

Figure 1

A schematic illustration of the relationship between the circadian clock and carbohydrate metabolism. Information from the circadian clock is transmitted to the chloroplast and mitochondria. Triose-phosphates (TP) fixed during the day by photosynthesis are partitioned to synthesize sucrose and starch. During the day, sucrose synthesis is inhibited by the SNF1-related kinase 1 (SnRK1) and is activated by the osmo-sensitive kinase OsmK [13]. SnRK1 and OsmK sense rhythmic changes by light and the clock protein late elongated hypocotyl (LHY). Sucrose is exported and consumed by sink tissues. Trehalose 6-phosphate (T6P) accelerates the development of sink tissues, thus increasing the sink demand for carbon during the day. Increased demand in turn activates OsmK, creating a positive feedback loop activating the source supply by sink demand. At night, OsmK accelerates starch degradation and thus up-regulates sucrose production. Therefore, activation of the sucrose supply during the day increases the sink demand, which in turn increases OsmK and up-regulates starch degradation and thus sucrose supply at night. Consumption of sugars by sink tissues is regulated to the clock via activation by LHY/CCA1 and inhibition by the evening complex (EC) (Early Flowering 3 (ELF3), ELF4, and lux arrhythmo (LUX)). The clock genes pseudo-response regulator 5 (PRR5), 7, and 9 regulate the tricarboxylic acid cycle (TCA) at night [43,44]. Black thick lines on plants and show carbon movement. Arrow end lines and blocked end lines indicate activate and inhibit the reactions and expressions, respectively. Night reactions are written in gray box. Clock cartoons emphasize the clock genes. Arrow end dotted line indicates feedback control of sugar by sink strength [13]. The character of cons means consumption of sugar.