Molecular mechanisms at the core of the plant circadian oscillator

Abstract

Circadian clocks are endogenous timekeeping networks that allow organisms to align their physiology with their changing environment and to perform biological processes at the most relevant times of the day and year. Initial feedback-loop models of the oscillator have been enriched by emerging evidence highlighting the increasing variety of factors and mechanisms that contribute to the generation of rhythms. In this Review, we consider the two major input pathways that connect the circadian clock of the model plant Arabidopsis thaliana to its environment and discuss recent advances in understanding of how transcriptional, post-translational and post-transcriptional mechanisms contribute to clock function.

Figure 1

Environmental signals are integrated by the central oscillator to coordinate multiple physiological processes. External signals such as light and temperature influence the pace of the clock (exemplified by the black wave in the oscillator) and entrain it by impinging on different molecular processes at the core of the oscillator. The clock then coordinates output rhythms (colored waves) accordingly. Proper clock function is required for the orchestration of multiple physiological pathways, including photoperiodic flowering, hormone signaling, growth, metabolism, and biotic and abiotic stress responses. Although the circadian system has traditionally been seen as a linear pathway, growing evidence supports the notion that it is a highly intricate network. Oscillator function is not unidirectionally regulated by external stimuli, but it also modulates its own sensitivity to them. In addition, multiple output pathways carry out feedback regulation of clock function, as is the case of hormones and metabolites. There is also extensive cross-talk among output pathways (not depicted), which can additionally be directly influenced by external conditions. Integration of these complex interconnections gives rise to a robust yet flexible network that plays an essential role in the coordination of plant physiology in natural environments.

Figure 2

Transcriptional feedback loops at the core of the circadian oscillator in Arabidopsis thaliana. The sequential expression of each component throughout the day is shown from left to right, and the time of activity is expressed in hours after dawn. The yellow and gray areas represent day and night, respectively. Black bars indicate repression, and green arrows indicate activation of transcription. Broken lines indicate relationships not proven to be direct or detected only under specific conditions. Ovals represent functional groups. The sun icon depicts light promotion of transcription. (a) At dawn, CCA1 and LHY repress the expression of the PRR-encoding genes, TOC1, GI and the EC members LUX, ELF3 and ELF4. PRR9, PRR7, PRR5 and TOC1 are sequentially expressed and repress the expression of CCA1 and LHY, as well as their own transcription. In the evening, TOC1 represses all of the previously expressed components in addition to GI, LUX and ELF4. Subsequently, the EC maintains the repression of GI and represses PRR9 and PRR7. (b) LWD1 and LWD2 promote expression of CCA1, PRR9, PRR7 and TOC1, and are probably repressed by PRR9. In the afternoon, transcriptional activation is mediated by RVE8 and the LNKs, which stimulate expression of PRR5, TOC1 and the EC component ELF4. RVE8 additionally induces expression of PRR9, GI and LUX. GI appears to be required for activation of CCA1 and LHY, as does an EC containing NOX.

Figure 3

Post-translational regulatory circuits within the clock of Arabidopsis thaliana. The purple oval depicts the nucleus; the brown area depicts the cytoplasm. (a) Protein-protein interactions among clock components. Activation of CCA1 by TCP20 and TCP22 requires LWD1 as a coactivator. CCA1 and LHY homo- and heterodimerize and repress evening-phased genes by binding to a specific cis-regulatory motif in their promoters, the EE. To repress gene targets, they require DET1 as a corepressor. Transcriptional repression of CCA1 and LHY is achieved through sequential expression of the PRRs (denoted PRR9/7/5), which bind to the CCA1 and LHY promoters and recruit TPL and HDA6, thereby inhibiting transcription. TOC1 is thought to be recruited to the CCA1 and LHY promoters through interaction with CHE. Additionally, LNKs interact with RVE8 and act as coactivators inducing expression of PRR5 and TOC1. ELF4 promotes nuclear translocation of ELF3, which then bridges the interaction between ELF4 and LUX, thereby forming the repressive EC. GI subnuclear localization is also modulated by ELF4. (b) Protein stability and turnover sets the pace of the clock. Left, in the afternoon, TOC1 is protected from ZTL-mediated proteasomal degradation through its interaction with PRR3 (which hinders ZTL access) and PRR5 (which promotes TOC1 translocation to the nucleus), as well as by blue-light-dependent GI-mediated ZTL stabilization. Right, progressive phosphorylation of PRR5 and TOC1 enhances their binding to ZTL, which promotes their degradation later in the evening. In addition, GI proteasomal degradation is promoted through ELF3-mediated interaction with COP1 in the dark; this interaction also triggers the degradation of ELF3.