Time to grow

Plants may not have any appointments to keep, but they still need to know what time it is, and proper growth and survival depend on the existence of a circadian ‘clock’ that modulates gene expression patterns over the course of the day and night (Fig. 1).

A team led by Norihito Nakamichi of the RIKEN Plant Science Center in Yokohama has now made important progress in exposing the ‘gears’ that power this clock in the thale cress, Arabidopsis thaliana, by clarifying the regulatory mechanism of three genes—PRR9, PRR7 and PRR5—that play a key role in regulating other circadian factors¹. “The function of the proteins encoded by the ‘clock genes’ was undetermined,” says Nakamichi. “We focused on the molecular function of these proteins because triple knockouts of these PRR genes result in disruption of the circadian clock.”

Previous studies have suggested that these proteins overlap functionally, and Nakamichi and his colleagues demonstrated that all three are equally effective at inhibiting expression of the Circadian Clock-Associated1 (CCA1) and Late Elongated Hypocotyl (LHY) clock genes. They subsequently identified two evolutionarily conserved stretches of amino acids that contribute to this gene repression, and determined that this activity appears to depend on physical recruitment of PRR9, PRR7 and PRR5 to DNA sequences that regulate CCA1 and LHY expression.

Although these three proteins appear to be functionally redundant, each is selectively produced at a specific time of day, and subsequent experiments helped to dissect the time-table governing each protein’s activity. The investigators generated strains of Arabidopsis that express only PRR9, PRR7 or PRR5—with expression peaks in the morning, from day to night, and at night, respectively—and noted the effects on CCA1 and LHY expression. Their results revealed a collaborative relationship between the three repressors; for example, PRR9 alone can inhibit CCA1 and LHY in the morning, but is insufficient to enforce night-time inhibition of these genes in the absence of PRR5 and PRR7.

Considering how many key processes related to plant growth and development are guided by this internal clock, Nakamichi predicts that these findings could eventually benefit the agricultural sector. “We have created a transcriptional activator version of PRR5; when it is over-expressed in Arabidopsis, it activates target genes by binding their promoters,” he says. “Using such constructs, we can cancel the function of redundant PRR proteins one at a time in useful plants such as crops and trees in order to improve their stress tolerance and increase their biomass.”;