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March 18, 2010

The proteins that help plants keep time

Researchers at the RIKEN Plant Science Center have clarified the function of three proteins that play a central role in the circadian clock in plants. The finding, to appear in the journal The Plant Cell, opens the door to the engineering of plant clock systems, with powerful applications to agriculture.

Circadian rhythms of a circadian clock

The circadian clock, a 24-hour biological cycle governing everything from seasonal flowering to hormone secretion, has been the focus of intense attention in plant science research for its wide-reaching implications to growth and development (Figure 1). At the heart of this clock is a feedback loop of gene expression known as the 'central oscillator', whose interaction is thought to regulate biological rhythms governing various physiological processes.

With their finding, the researchers have clarified the way in which this oscillator adjusts its activity throughout the day. They show that the three proteins studied, the Pseudo-Response Regulators PRR5, PRR7 and PRR9, associate with promoter regions of the genes CCA1 and LHY to repress transcription of these genes at different times (Figure 2). Collectively, this sequential repression shapes the clock's activity over the 16-hour period from day to night (Figure 3).

An essential component of the central oscillator, this mechanism of gene repression fills a crucial gap in our understanding of circadian clock function in plants. Artificial manipulation of the three proteins enables control over time-specific components of the clock system connected to properties such as plant size and stress tolerance, with significant potential benefits to agriculture.

Contact

Norihito Nakamichi
Biodynamics Research Team
RIKEN Plant Science Center
Tel: +81-(0)48-503-9576 / Fax: +81-(0)48-503-9609

Jens Wilkinson
RIKEN Global Relations and Research Coordination Office
Tel: +81-(0)48-462-1225 / Fax: +81-(0)48-463-3687
Email: pr@riken.jp

Plant circadian clock

Figure 1: Plant circadian clock

(A) The circadian clock creates circadian rhythms in organisms. The rhythms are self-sustaining even in conditions without time cues from the environment (e.g., changes in lighting or temperature).

(B) Physiological events governed by clock regulation in plants

Binding of PRR proteins to LHY promoter from morning until midnight in vivo

Figure 2: Binding of PRR proteins to LHY promoter from morning until midnight in vivo

(A) PRR9, PRR7, and PRR5 proteins bind to LHY promoter from 2-h until 8-h, from 4-h until 14-h, and from 8-h until 16-h after light on (light on is time 0).

(B) LHY mRNA expression is repressed over period from 2-h until 18-h after light on (red arrow).

Diminished repression state of LHY expression in prr-double mutants

Figure 3: Diminished repression state of LHY expression in prr-double mutants

(A) PRR9 protein expression and LHY mRNA expression in prr7 prr5 mutant. Derepression of LHY in night-time (during 12-h until 18-h after light on) is observed.

(B) PRR5 protein expression and LHY mRNA expression in prr9 prr7 mutant. Derepression of LHY in day-time (over period from 4-h until 12-h after light on) is observed.