Circadian Rhythm in Plants

How do plants ‘tell time’ for circadian rhythms based on a ~24 cycle?

Plants have a remarkable ability to ‘tell time’ by synchronizing their biological processes with the daily cycle of about 24 hours. This phenomenon, known as circadian rhythm, allows plants to anticipate environmental changes and adjust their physiology accordingly. Circadian rhythms are regulated by internal molecular clocks that are highly conserved across various organisms, including plants.

At the core of this timekeeping mechanism in plants are specific genes called ‘clock genes. ‘ These genes produce proteins known as transcription factors that regulate the expression of other genes involved in various cellular processes. The key clock genes in plants are called TOC1 (Timing of CAB Expression 1), CCA1 (Circadian Clock Associated 1), and LHY (Late Elongated Hypocotyl)

The circadian clock in plants is entrained or synchronized primarily by light and temperature cues. Light is the most potent environmental signal affecting the plant’s biological clock. The primary photoreceptor responsible for this light response is called phytochrome. It perceives both red and far-red light and plays a crucial role in synchronizing the circadian rhythm with the light-dark cycle

In the morning, when the light intensity is low, phytochrome perceives the increasing red light and triggers the expression of CCA1 and LHY genes. These genes encode transcription factors that repress the expression of TOC1. As daylight intensifies, phytochrome switches to the active form and inhibits the activity of CCA1 and LHY, leading to increased expression of TOC1. TOC1 promotes the expression of other clock genes, completing a feedback loop

This feedback loop and the interconnected network of clock genes allow plants to maintain approximately 24-hour oscillations in gene expression and physiological processes. During the day, the expression of genes involved in photosynthesis, growth, and development is activated, while at night, genes associated with energy conservation and stress responses become more active

In addition to light cues, temperature fluctuations also play a role in regulating circadian rhythms. Temperature-sensitive proteins called warm-sensitive period (WSP) and cold-sensitive period (CSP) proteins affect the activity of key clock genes, modulating the circadian clock’s pace depending on the temperature changes

Together, the interplay between light and temperature cues, and the complex gene regulatory network, enable plants to ‘tell time’ and adjust their physiology to optimize growth, metabolism, and reproduction patterns. The circadian rhythms allow plants to anticipate and adapt to changes in the environment, such as the onset of dawn, fluctuations in temperature, and even seasonal variations

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