Chronicles of time: Unveiling circadian clocks in non-flowering plants

Abstract

Circadian clocks are fundamental to life, allowing organisms to adapt their physiology to the 24-h day-night cycle. The endogenous clock comprises three components: the input pathway, the central oscillator, and the output pathway. Extensive research has been conducted on circadian clock regulation in flowering plants (angiosperms); similarly, significant progress has been made in understanding circadian rhythms in cyanobacteria, highlighting the broader importance of these mechanisms across diverse organisms. However, much less attention has been directed towards other non-flowering plants, such as green algae, red algae, bryophytes, and gymnosperms. These ancient lineages, which represent critical steps in the evolution of plant life, also exhibit endogenous timekeeping mechanisms. Among them, cyanobacteria, often considered precursors to plant circadian systems, offer valuable insights into the origins of circadian rhythms in photosynthetic organisms. This review explores the current knowledge regarding circadian clock systems in non-flowering plants, examining key molecular components, regulatory pathways, and the physiological processes influenced by these clocks, such as growth, photosynthesis, and stress responses. A comparative analysis between non-flowering plants and angiosperms reveals the evolutionary insights into how timekeeping mechanisms have diversified and adapted into changing environments across various phylogenetic boundaries. This review emphasizes the importance of continued research into the circadian biology of non-flowering plants, which may reveal key insights into the adaptive evolution of timekeeping mechanisms in the plant kingdom. © 2025 The Authors