Extrinsic and intrinsic factors regulate the spatio-temporal localisation of the Arabidopsis circadian component ELF3

Circadian clocks are biological timekeeping mechanisms that integrate environmental signals to coordinate internal responses to occur at the most optimal time of the day. The plant circadian clock is composed of a series of autoregulatory feedback loops. Within these loops, EARLY FLOWERING3 (ELF3) has emerged as a critical regulatory hub. ELF3 recruits ELF4 and LUX ARRYTHMO (LUX) to form the evening complex (EC), a protein complex that represses gene expression. The EC is essential for the functionality and integration of environmental stimuli into the plant circadian clock. In recent years, we have developed a strong understanding of how the EC represses gene expression. In contrast, how the activity of the EC is regulated has remained poorly understood.

Changing the localisation of proteins within the cell is an emerging regulatory mechanism in light, temperature and circadian signalling in plants. In this thesis, I explored how the cellular dynamics of ELF3 are regulated. I found that the nuclear and sub-nuclear localisation of ELF3 is responsive to red (RL), blue and far-red light and warm temperature pulses. Further analysis of how RL-pulses controlled ELF3 localisation revealed two separate signalling pathways, one which is dependent on phytochromeB (phyB), while the other occurs independently of phyB.

I also investigated the contribution of ELF4 in controlling the localisation of ELF3 in response to light and warm temperature pulses. For all pulses that were tested, I observed no specific requirement for ELF4. However, I did find that ELF4 has a general role in promoting both the nucler and sub-nuclear localisation of ELF3. The importance of ELF4 was tissue-specific, with ELF4 having a more critical role in hypocotyl nuclei than root nuclei. Together, this thesis provides a new interface to understand how environmental signals are integrated into the oscillator and regulate the activity of plant circadian clock components.