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Endogenous circadian clock integrates cyclic signals of environment and daily and seasonal behaviors of organisms to achieve spatiotemporal synchronization, which greatly improves genetic diversity and fitness of species. This review addresses recent studies on the plant circadian system in the field of chronobiology, covering topics on molecular mechanisms, internal and external Zeitgebers, and hierarchical regulation of physiological outputs. The architecture of the circadian clock involves the autoregulatory transcriptional feedback loops, post‐translational modifications of core oscillators, and epigenetic modifications of DNA and histones. Here, light, temperature, humidity, and internal elemental nutrients are summarized to illustrate the sensitivity of the circadian clock to timing cues. In addition, the circadian clock runs cell‐autonomously, driving independent circadian rhythms in various tissues. The core oscillators responds to each other with biochemical factors including calcium ions, mineral nutrients, photosynthetic products, and hormones. We describe clock components sequentially expressed during a 24‐h day that regulate rhythmic growth, aging, immune response, and resistance to biotic and abiotic stresses. Notably, more data have suggested the circadian clock links chrono‐culture to key agronomic traits in crops. [ABSTRACT FROM AUTHOR]

The circadian clock allows plants to actively adapt to daily environmental changes through temporal regulation of physiological traits. In response to drought stress, circadian oscillators gate ABA signalling, but the molecular mechanisms remain unknown, especially in crops. Here, we investigated the role of soybean circadian oscillators GmLCLa1, GmLCLa2, GmLCLb1 and GmLCLb2 in leaf water stress response. Under dehydration stress, the GmLCL quadruple mutant had decreased leaf water loss. We found that the dehydration treatment delayed the peak expression of GmLCL genes by 4 hr. In addition, the circadian clock in hairy roots also responded to ABA, which led to a free‐running rhythm with shortened period. Importantly, in the gmlclq quadruple mutant, diurnal expression phases of several circadian‐regulated ABA receptor, ABA catabolism and ABA signalling‐related genes were shifted significantly to daytime. Moreover, in the gmlclq mutant leaf, expression of GmPYL17, GmCYP707A, GmABI2 and GmSnRK2s was increased under water dehydration stress. In summary, our results show that GmLCLs act as negative regulators of ABA signalling in leaves during dehydration response. We investigated the molecular characteristics and the physiological function of circadian oscillators in soybean. We found the GmLCL quadruple mutant had decreased leaf water loss under dehydration stress, and GmLCLs act as negative regulators of ABA signalling in leaves during dehydration response. [ABSTRACT FROM AUTHOR]

As sessile organisms, plants perceive, respond, and adapt to the environmental changes for optimal growth and survival. The plant growth and fitness are enhanced by circadian clocks through coordination of numerous biological events. In legume species, nitrogen‐fixing root nodules were developed as the plant organs specialized for symbiotic transfer of nitrogen between microsymbiont and host. Here, we report that the endogenous circadian rhythm in nodules is regulated by MtLHY in legume species Medicago truncatula. Loss of function of MtLHY leads to a reduction in the number of nodules formed, resulting in a diminished ability to assimilate nitrogen. The operation of the 24‐h rhythm in shoot is further influenced by the availability of nitrogen produced by the nodules, leading to the irregulated nyctinastic leaf movement and reduced biomass in mtlhy mutants. These data shed new light on the roles of MtLHY in the orchestration of circadian oscillator in nodules and shoots, which provides a mechanistic link between nodulation, nitrogen assimilation, and clock function. [ABSTRACT FROM AUTHOR]

The circadian pacemaker in plants is a hierarchical multioscillator system that directs and maintains a 24‐hr oscillation required for organism homeostasis and environmental fitness. Molecular clockwork within individual tissues and organs acts cell autonomously, showing differences in circadian expression of core oscillators and their target genes; there are functional dominance and coupling in the complex regulatory network. However, molecular characteristics of organ‐specific clocks are still unknown. Here, we showed the detached shoot and root possess dynamic circadian protein–protein interactions between clock core components, periodicity in organs exhibits a difference. The period length difference between shoot and root was not remarkable in prr7‐3 and prr7‐3 prr9‐1 mutants. In addition, the phase transition curve indicated that shoot and root clock respond differently to the resetting cues of ambient temperature. PRR9 and PRR7 compensate circadian period between 22°C and 28°C in shoot, not in root. The circadian rhythms of PRR9 or PRR7 transcript accumulation showed no difference at 22°C and 28°C in shoot, but differences were observed in root. In summary, our results reveal the specificity of dynamic circadian protein–protein interactions in organ‐autonomous clocks and the critical roles of PRR9 and PRR7 in mechanisms regulating temperature compensation in aerial shoot system. We found the difference of circadian period length of dynamic circadian protein–protein interactions between shoot and root. PRR9 and PRR7 were found to contribute significantly to organ‐specific circadian periodicity maintenance and in the temperature compensation of aerial shoot clock. To the best of our knowledge, this is the first example in plants monitoring clock protein interactions in real time in individual organs. Having demonstrated the uniqueness and consistency of clock components function in aerial shoot and subterranean root, we are able to pose a variety of intriguing questions about the adaptive evolution and hierarchical coupling in the plant clock system. [ABSTRACT FROM AUTHOR]

In plants, the spatiotemporal expression of circadian oscillators provides adaptive advantages in diverse species. However, the molecular basis of circadian clock in soybean is not known. In this study, we used soybean hairy roots expression system to monitor endogenous circadian rhythms and the sensitivity of circadian clock to environmental stimuli. We discovered in experiments with constant light and temperature conditions that the promoters of clock genes GmLCLb2 and GmPRR9b1 drive a self‐sustained, robust oscillation of about 24‐h in soybean hairy roots. Moreover, we demonstrate that circadian clock is entrainable by ambient light/dark or temperature cycles. Specifically, we show that light and cold temperature pulses can induce phase shifts of circadian rhythm, and we found that the magnitude and direction of phase responses depends on the specific time of these two zeitgeber stimuli. We obtained a quadruple mutant lacking the soybean gene GmLCLa1, LCLa2, LCLb1, and LCLb2 using CRISPR, and found that loss‐of‐function of these four GmLCL orthologs leads to an extreme short‐period circadian rhythm and late‐flowering phenotype in transgenic soybean. Our study establishes that the morning‐phased GmLCLs genes act constitutively to maintain circadian rhythmicity and demonstrates that their absence delays the transition from vegetative growth to reproductive development. We investigated the molecular characteristics and the physiological function of circadian oscillators in soybean. We found that soybean circadian clock is very sensitive to ambient light/temperature signals, and GmLCL genes are required for self‐sustained circadian period length and the timing of flowering. [ABSTRACT FROM AUTHOR]

It was noted that circadian components function in plant adaptation to diurnal temperature cycles and freezing tolerance. Our genome-wide transcriptome analysis revealed that evening-phased COR27 and COR28 mainly repress the transcription of clock-associated evening genes PRR5, ELF4 and cold-responsive genes. Chromatin immunoprecipitation indicated that CCA1 is recruited to the site containing EE elements of COR27 and COR28 promoters in a temperature-dependent way. Further genetic analysis shows COR28 is essential for the circadian function of PRR9 and PRR7. Together, our results support a role of COR27 and COR28 as nighttime repressors integrating circadian clock and plant cold stress responses. [ABSTRACT FROM AUTHOR]