Wheels within wheels: the plant circadian system

Abstract

Circadian clocks integrate environmental signals with internal cues to coordinate diverse physiological outputs so that they occur at the most appropriate season or time of day. Recent studies using systems approaches, primarily in Arabidopsis, have expanded our understanding of the molecular regulation of the central circadian oscillator and its connections to input and output pathways. Similar approaches have also begun to reveal the importance of the clock for key agricultural traits in crop species. In this review, we discuss recent developments in the field, including a new understanding of the molecular architecture underlying the plant clock; mechanistic links between clock components and input and output pathways; and our growing understanding of the importance of clock genes for agronomically important traits.

Keywords: Arabidopsis; agricultural traits; circadian clock; input; output; systems biology.

Figure 1

Transcriptional regulation of the clock in Arabidopsis.(A) A simplified model for transcriptional regulation among classes of clock components. CCA1, LHY and RVE components are shown in yellow; pseudo response regulator (PRR) components are shown in blue; evening complex (EC) components are shown in green. Members of each group that contain the evening element(s) in their promoter regions are marked with red boxes. Arrows indicate activation; perpendicular bars indicate repression. In the morning, CCA1 and LHY repress most EE-regulated clock genes; whereas in the afternoon, RVE4, RVE6, and RVE8 activate EE-regulated clock genes. These EE-controlled components also feed back to regulate expression of CCA1, LHY, RVE4, RVE6, RVE8 and each other. (B) A more detailed model for transcriptional regulation among the clock components. The relative timing of action for each component during a day–night cycle is shown from left to right. White area indicates subjective day; gray area indicates subjective night. Yellow, blue and green colors indicate members of each family described above. Genes with EE(s) in their promoter regions are marked with red boxes. Solid red arrows indicate activation; broken red arrow indicates conditional activation; perpendicular bars indicate repression.

Figure 2

Post-transcriptional regulation of the clock in Arabidopsis. CCA1, LHY and RVE components are shown in yellow; pseudo response regulator components are shown in blue; evening complex components are shown in green. F-box proteins that are a part of E3 ubiquitin ligase complex are shown in orange. Other interacting proteins are shown in magenta. Abbreviations: AS, alternative splicing; P, protein phosphorylation; Ub, ubiquitination.