The circadian clock, entrained by our planet’s 24 h rotation on its tilted axis, plays crucial roles in the synchronization of the performance of organisms with daily cycles of light and temperature, enabling organisms to regulate activities at the correct time of a day . For instance, the endogenous clock in plants influences various biological processes including leaf movements, hypocotyl growth, floral transition, and abiotic and biotic stress resistance [2–4].
The circadian rhythmicity and molecular mechanisms underlying the circadian clock have been investigated in many organisms including Drosophila melanogaster, Neurospora crassa, Synechoccocus elongatus, and mice [5–7]. In general, several interconnected transcription/translation feedback loops participate to establish central clock oscillations [8–10]. In plants, circadian rhythmicity is extensively investigated in a dicotyledonous model plant, Arabidopsis thaliana, and a ‘three-loop model’ has been proposed .
TIMING OF CAB EXPRESSION 1/PSEUDO-RESPONSE REGULATOR 1 (TOC1/PRR1) and two partially redundant MYB transcription factors, CIRCADIAN CLOCK-ASSOCIATED 1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY) comprise a central oscillation loop . CCA1 and LHY repress transcript expression of TOC1 by binding directly to its promoter, and as shown recently TOC1 negatively regulates the transcription of CCA1 and LHY [13, 14], establishing negative feedback loop [11, 15]. The second loop is formed by TOC1 and the evening complex (EC) consisting of EARLY FLOWERING 3 (ELF3), ELF4, and LUX ARRHYTHMO (LUX) . In addition, the third loop is established by negative feedback between PRR7/PRR9 and LHY/CCA1 [16, 17].
Protein turnover regulation provides another layer of sophistication in the regulation of the circadian clock. Notably, the TOC1 protein is regulated by proteasome-mediated protein degradation. The F-box protein ZEITLUPE (ZTL) in the E3 ubiquitin ligase SCF (SKP1-CUL1-F-box protein) complex ubiquitinates the TOC1 protein through direct physical interaction in a dark-dependent manner . Homologs of ZTL are also involved in circadian regulation of photoperiodic flowering [19, 20]. The FLAVIN-BINDING, KELCH REPEAT, F-BOX 1/ADAGIO3 (FKF1/ADO3) interacts with GI in a blue light-dependent manner and regulates CONSTANS expression by degrading a Dof transcription factor, CYCLING DOF FACTOR 1 . In addition, it has been reported that ZTL, FKF1, and LOV KELCH PROTEIN 2 together regulate TOC1 and PRR5 degradation, contributing to the circadian oscillation .
Recent works has shown that many of these circadian clock components can also be found in diverse plant species including rice, soybean, maize, and poplar [22–25]. CCA1/LHY genes are widely conserved in eudicotyledonous (eudicots) and monocotyledonous (monocots) plants [26–29], and CCA1/LHY and TOC1 feedback loops are thought to play a central role in the clock’s function in these plant species. Functional homologs of ZTL have also been found in several plant species [30, 31], further supporting that circadian clock components are fairly well-conserved in plants.
Despite the important role of the endogenous clock in entraining physiological processes to environmental signals, how the circadian clock regulates ecological performance of a plant in its natural habitat is largely unknown. Only a few studies have shown that the endogenous clock allows plants to maximize photosynthetic capacity and reproductive success [32–34]. In order to expand our understanding of the clock function in biotic and abiotic interactions, we identified three core clock components (LHY, TOC1, and ZTL) in a wild tobacco, Nicotiana attenuata, which has been developed as a model system for understanding ecological performance in native habitats, in particular the Great Basin desert in Utah. N. attenuata is a seasonal solanaceous plant, completing its life cycle during spring and summer to lie dormant in the seed bank for the many years between fires in its native habitat. These results provide additional evidence of the conservation of the circadian clock genes and set the stage for future studies to unravel the ecological relevance of the clock.