Circadian rhythm is a roughly 24-hour cycle in the biochemical, physiological, or behavioral processes of living entities, including both plants and animals. The circadian clock plays a crucial role in plant biology, and hence in agriculture, because outputs controlled by the clock include the timing of germination, diurnal optimization of photosynthesis, and floral transition. Each has been shown to be crucial for plant fitness. The circadian rhythms can be synchronized by the day/night cycle so that plants can correctly anticipate dawn and dusk. Although the photoreceptors involved in perceiving the light signals have been well established, how the light signals are transduced from the photoreceptors to the central clock remain largely unknown. In addition, little is known about how the rhythmic expression pattern of central clock genes and cyclic output responses (such as growth and photosynthetic activity of plants) are generated and controlled. The goal of this project is to investigate the role of a set of light- and circadian clock-regulated transcription factors (FHY3, FAR1, HY5, CCA1 and LHY) in transcriptional regulation of EARLY FLOWERING 4 (ELF4, a central clock component) gene expression and resetting of the circadian clock in response to changes in daily light conditions. It is expected that results from this study will provide fresh insights into the transcriptional circuitry underlying circadian clock regulation and lead to a better understanding of the physiology and reproduction in higher plants. In turn, the obtained information has the potential to be used for future application in optimizing growth of crop plants (such as increased fitness, increased hybrid vigor and latitudinal growth limit). This project will also provide excellent training opportunities for school children, undergraduate, graduate students and postdoc fellows.
In response to the daily nutation of the earth around the sun (near 24 h day-night cycles), most living organisms on earth (including humans) have developed an internal time-tracking mechanism, the circadian clock. The clock enables the plants to anticipate the daily changes in the light conditions (such as sun rise in the early morning and sun set in the late afternoon) and adjust their growth and development accordingly. The clock also enable the plants to know the seasonal changes and help them to decide when to flowering. Understanding how plant have gained such a power to sense the changes in light conditions is a major interest to plant scientists. Plants use a series of photoreceptors to perceive the light signals and through the "input pathway", transduce the light signal to the "central oscillator" (like the central processor of a computer) of the clock, to regulate the rhythmic expression of key components of the central oscillator, and through the "output pathway" to regulate various rhythmic responses (such as seed germination, hypocotyl growth, photosynthesis, stomata movement, flowering etc.). In Arabidopsis, EARLY FLOWERING 4 (ELF4) encodes a central clock component of the central oscillator of the clock. Its rhythmic expression (low in the morning and high in the evening) is essential for proper clock function gene and entrainment of the clock in response to changes in daily light conditions. How its cyclic expression is regulated by the light signals has remained unclear. In this project, we have demonstrated that, through a series of molecular, genetic and biochemical studies, three light-regulated transcriptional activators, FHY3, FAR1, and HY5, bind to the ELF4 promoter and activate its expression, whereas two morning-expressed clock components, CCA1 and LHY directly binds to ELF4 promoter and repress its expression. Thus, ELF4 expression is the product of interplay between these three activators and the CCA1/LHY repressors. Our findings not only revealed a new biological function of FHY3 and FAR1 in regulating circadian clock and flowering time, but more importantly, for the first time, we provide substantial evidence that a set of light- and circadian clock-regulated transcription factors act coordinately at the target gene promoter to generate the robust oscillations of central clock genes, and establish a potential molecular link connecting environmental light-dark cycle to the central oscillator to regulate a diverse set of physiological responses. These results helped us to better understand how the clock was set up and regulated by the environmental light signals, and may have applications in manipulating the clock for improved agronomic traits of crops.
