Plant Physiol. 2022 Sep 28;190(2):921-923. doi: 10.1093/plphys/kiac353.
Plant Physiology, Volume 190, Issue 2, October 2022, Pages 921–923,
In the 24 years since the first plant circadian oscillator genes were cloned (Schaffer et al., 1998; Wang and Tobin, 1998), there has been a concerted effort to identify the mechanistic basis of circadian rhythms and to understand how the circadian system impacts the biology of plants. Progress has been so great that there is now a good understanding of the circadian biology of Arabidopsis and insight into the circadian biology of the major crops is developing. For this Focus Issue on Circadian Rhythms, we encouraged submission of articles that emphasize less understood aspects, such as interactions between the circadian system and the natural environment, cell-specific function, setting of the circadian clock by environmental signals, and evolution of plant circadian oscillators.
The Arabidopsis circadian oscillator was first modeled as a simple negative feedback loop, with two Myb-like transcription factors, CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY), directly inhibiting expression of TIMING OF CAB EXPRESSION 1 (TOC1). TOC1, a member of the PSEUDO-RESPONSE REGULATOR family, was first proposed to promote CCA1 and LHY expression (Alabadi et al., 2001). However, later studies revealed that TOC1 directly represses CCA1 and LHY expression (Gendron et al., 2012; Huang et al., 2012; Pokhilko et al., 2012), with these three genes forming a double negative feedback loop. Subsequent work by many laboratories has revealed that the circadian oscillator mechanism in Arabidopsis is instead actually composed of dozens of genes that regulate each other’s expression, forming a very complex interconnected network (Nakamichi, 2020). Post-translational regulation of circadian proteins is also pervasive, enhancing the robustness of the circadian system and fine-tuning circadian period (Yan et al., 2021)….
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