Diurnal and circadian expression of clock-associated pseudo-response regulators in Populus trichocarpa

The circadian clock is an autonomous oscillator that produces endogenous biological rhythms with a period of approximately 24 h. A number of circadian clock-associated factors have been intensely studied in the model plant Arabidopsis thaliana (At), including pseudo-response regulators (PRRs), which are key regulators of the circadian clock. In Populus trichocarpa (Pt), seven orthologs of the AtPRR genes have been identified. Here, the PtPRR family of genes, PtPRR1, PtPRR37, PtPRR5a, PtPRR5b, PtPRR73, PtPRR9la, and PtPRR9lb, were analyzed for circadian expression at the transcriptional level. These genes were expressed diurnally in the following order: PtPRR9la/PtPRR9lb -> PtPRR37/PtPRR73 -> PtPRR5a/'PtPRR5b and PtPRR1, with the PtPRR mRNAs starting to accumulate sequentially in 2-3-h intervals. These sequential transcriptional events, termed `circadian waves of PtPRR,' were not signicantly affected by the photoperiod conditions. All PtPRR genes were shown to be primarily expressed in mature leaves. These results suggest that members of the PtPRR family play important roles in mechanisms underlying the poplar circadian clock.