The PRR family of transcriptional regulators reflects the complexity and evolution of plant circadian clocks

•PSEUDO-RESPONSE REGULATORS (PRRs) are conserved among plants.•PRRs are key components of circadian clocks in green alga and plants.•In Arabidopsis most PRRs are transcriptional repressors.•Transcription/translation circadian oscillators are not conserved among different taxa. Circadian clocks are internal time-keeping mechanisms that provide an adaptive advantage by enabling organisms to anticipate daily changes and orchestrate biological processes accordingly. Circadian regulated pseudo-response regulators are key components of transcription/translation circadian networks in green alga and plants. Recent studies in Arabidopsis thaliana have shown that most of them act as transcriptional repressors and directly regulate output pathways suggesting a close relationship between the central oscillator and circadian regulated processes. Moreover, phylogenetic studies on this small gene family have shed light on the evolution of circadian clocks in the green lineage.