Circadian clocks are 24-h timing devices that phase cellular responses; coordinate growth, physiology, and metabolism; and anticipate the day-night cycle. Here we report sensitivity of the Arabidopsis thaliana circadian oscillator to sucrose, providing evidence that plant metabolism can regulate circadian function. We found that the Arabidopsis circadian system is particularly sensitive to sucrose in the dark. These data suggest that there is a feedback between the molecular components that comprise the circadian oscillator and plant metabolism, with the circadian clock both regulating and being regulated by metabolism. We used also simulations within a three-loop mathematical model of the Arabidopsis circadian oscillator to identify components of the circadian clock sensitive to sucrose. The mathematical studies identified GIGANTEA (GI) as being associated with sucrose sensing. Experimental validation of this prediction demonstrated that GI is required for the full response of the circadian clock to sucrose. We demonstrate that GI acts as part of the sucrose-signaling network and propose this role permits metabolic input into circadian timing in Arabidopsis.he Arabidopsis thaliana circadian clock confers growth and competitive advantage (1). The phase of circadian rhythms in plants is adjusted by light signals to entrain the clock to dawn and dusk (2). Additionally, it has been proposed that the Arabidopsis circadian clock is sensitive to nitrogen acting as a nutritional cue (3) and phytohormones, possibly as an input from stress signaling and growth pathways (4). In Arabidopsis, circadian oscillations are generated and maintained by interlocking transcriptionaltranslational feedback loops and posttranslational regulation (1,(5)(6)(7)(8). In the morning, light activates the expression of two Myblike transcription factors, CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY), leading to expression of PSEUDO RESPONSE REGULATORS (PRR) 7 and 9, which in turn repress CCA1 and LHY expression. This "morning" loop of the oscillator is connected to "evening" expressed genes through direct repression of the expression of TIMING OF CAB EXPRESSION 1 (TOC1/PRR1) by binding of LHY/CCA1 to the TOC1 promoter. TOC1 is expressed in the evening and feeds back to activate LHY/CCA1 through an unknown pathway. TOC1 physically interacts with and antagonizes CCA1 HIKING EXPEDITION (CHE), a transcriptional repressor of CCA1 (9). CCA1 and LHY are also coupled with TOC1 through GIGANTEA (GI). LHY/CCA1 repress GI and GI forms a loop with TOC1, GI activating TOC1 and TOC1 repressing GI. GI encodes a protein of unknown biochemical function but in blue light, GI physically interacts with and stabilizes ZEITLUPE (10), an F-box protein that targets TOC1 for degradation (11). To entrain the circadian oscillator, signals are incorporated from light, second messengers, and metabolites to alter circadian clock gene expression and modulate circadian function (7,(12)(13)(14)(15).Carbohydrates are a major energy store in most forms ...

Circadian clocks are signalling networks that enhance an organism's relationship with the rhythmic environment. The plant circadian clock modulates a wide range of physiological and biochemical events, such as stomatal and organ movements, photosynthesis and induction of flowering. Environmental signals regulate the phase and period of the plant circadian clock, which results in an approximate synchronization of clock outputs with external events. One of the consequences of circadian control is that stimuli of the same strength applied at different times of the day can result in responses of different intensities. This is known as 'gating'. Gating of a signal may allow plants to better process and react to the wide range and intensities of environmental signals to which they are constantly subjected. Light signalling, stomatal movements and low-temperature responses are examples of signalling pathways that are gated by the circadian clock. In this review, we describe the many levels at which the circadian clock interacts with responses to the environment. We discuss how environmental rhythms of temperature and light intensity entrain the circadian clock, how photoperiodism may be regulated by the relationship between environmental rhythms and the phasing of clock outputs, and how gating modulates the sensitivity of the clock and other responses to environmental and physiological signals. Finally, we describe evidence that the circadian clock can increase plant fitness.