Other members of the PRR family such as PRR5 and TIMING OF CAB EXPRESSION1 (TOC1/PRR1, herein referred as TOC1) are expressed later during the day or close to dusk, respectively ( Sanchez and Kay, 2016 McClung, 2019 Nakamichi, 2020). Clock repressors include the morning-expressed MYB transcription factors CIRCADIAN CLOCK ASSOCIATED1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY), as well as the members of the pseudo-response regulator (PRR) protein family PRR9 and PRR7 ( Sanchez and Kay, 2016 McClung, 2019 Nakamichi, 2020).
In broad outline, the current view of the circadian network includes a plethora of clock repressors that are expressed and function at specific times during the day and night. Increasing evidence also highlight a complex array of regulatory mechanisms that contribute to the rhythmic clock ( Seo and Mas, 2014 Mateos et al., 2018 Chen and Mas, 2019 Nakamichi, 2020). The number of clock components at the core of the oscillator has been considerably expanded over the last years. Many of the mechanistic insights of the circadian clock function in plants derive from studies in Arabidopsis thaliana ( Sanchez and Kay, 2016 McClung, 2019 Nakamichi, 2020).
The core of the clock generates circadian rhythms in output processes through the daily and seasonal synchronization, orchestrated largely by changes in light and temperature ( Young and Kay, 2001). The rhythms are sustained under constant environmental conditions indicating that are generated by a self-sustained timing mechanism or circadian clock. Multiple biological processes display a rhythmic oscillation with a period of approximately 24-h. The Arabidopsis Circadian Clock: A Brief Overview Evidence that metabolic signals can in turn feedback to the clock place the spotlight onto the molecular mechanisms and components linking the circadian function with metabolic homeostasis and energy. Here, we briefly describe the prevalent role for the circadian clock controlling the timing of mitochondrial activity and cellular energy in Arabidopsis thaliana. This endogenous timing mechanism works in resonance with the environment to control growth, development, responses to stress, and also metabolism. Virtually all photosensitive organisms have evolved a self-sustained timekeeping mechanism or circadian clock that anticipates and responds to the 24-h environmental changes that occur during the day and night cycle. Cellular metabolism is also able to adapt to the external time, and the mechanisms governing such an adaptation rely on the circadian clock. A highly precise spatiotemporal programming is required to couple metabolic capacity with energy allocation. A fundamental principle shared by all organisms is the metabolic conversion of nutrients into energy for cellular processes and structural building blocks.
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