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June 12, 2009

All in the timing

Knocking out a clock gene in plant cells interrupts mitochondrial function and energy release

Figure 1: An artistic representation of the plant circadian clock as a mandala. This clock, previously known to regulate many developmental and physiological processes, including photosynthesis and stress response, has now been linked to mitochondrial function. (Blue and red circles represent the tricarboxylic acid cycle in Arabidopsis and mitochondria, respectively.)

Image courtesy of Hiroyuki Hanamura

A RIKEN-led group of molecular biologists has established the first direct link between the circadian clock mechanism in flowering plants and the functioning of the mitochondria, where energy is generated in the cells.

Daily rhythms in the biochemical or metabolic activity of cells have long been known across all biological kingdoms. They are governed by the oscillating activity of clock genes, the impairment of which has been shown in mice to be related lifestyle diseases such as obesity. In plants, production of plant biomass is likely to be linked with clock genes.

Recent studies in the genetic model plant Arabidopsis have revealed three key genes involved in the timing mechanism—CCA1, LHY and TOC1. These genes form the centerpiece of several interlocked feedback loops which establish and adjust the daily oscillation pattern.

Kazuki Saito and colleagues from the RIKEN Plant Science Center in Yokohama and Nagoya University studied the molecular impact of mutations in these key clock genes. They analyzed not only the direct changes in the nucleic acid and protein products generated by mutant genes, but they also looked at the differences in the downstream metabolic products formed. Details of their work were published recently in the Proceedings of the National Academy of Sciences1.

TOC1 is one of five related proteins known as the pseudo-response regulator (PRR) family. Previous work has shown them to be important components in adjusting the circadian system to changes in temperature and light. The researchers focused on a triple mutant of PRR9, 7 and 5 which leads to inability to establish a circadian rhythm under constant light. In previous work the research group demonstrated a strong link between this mutant and stress response in plants (Fig. 1).

The triple mutant leads to late-flowering plants with dark green leaves. They are similar in appearance to those generated when the CCA1 gene becomes overactive. But the researchers found the metabolic details of two plant forms to be utterly different. In particular, they were surprised to find that the triple mutant led to a build-up of three key intermediate compounds of the tri-carboxylic acid pathway, the standard energy release process which takes place in the mitochondria of all higher organisms. The impact of the mutant PRR clock genes on the mitochondria was direct and unequivocal.

“We now want to determine the molecular components involved in this link between the clock genes and metabolism,” says Saito.

References

  1. Fukushima, A., Kusano, M., Nakamichi, N., Kobayashi, M., Hayashi, N., Sakakibara, H., Mizuno, T. & Saito, K. Impact of clock-associated Arabidopsis pseudo-response regulators in metabolic coordination. Proceedings of the National Academy of Sciences USA 106, 7251–7256 (2009). |  | (Link)