A bittersweet symphony: Metabolic signals in the circadian system

Plants must match their metabolism to daily and seasonal fluctuations in their environment to maximise performance in natural conditions. Circadian clocks enable organisms to anticipate and adapt to these predictable and unpredictable environmental challenges. Metabolism is increasingly recog-nised as an integrated feature of the plant circadian system. Metabolism is an important circadian-regulated output but also provides input to this dynamic timekeeping mechanism. The spatial organisation of metabolism within cells and between tissues, and the temporal features of metabolism across days, seasons and development, raise interesting questions about how metabolism influences circadian timekeeping. The various mechanisms by which metabolic signals influence the transcription-translation feedback loops of the circadian oscil-lator are emerging. These include roles for major metabolic signalling pathways, various retrograde signals, and direct metabolic modifications of clock genes or proteins. Such metabolic feedback loops enable intra-and intercellular coor-dination of rhythmic metabolism, and recent discoveries indi-cate these contribute to diverse aspects of daily, developmental and seasonal timekeeping.