A Knockout of the Photoreceptor PtAureo1a Results in Altered Diel Expression of Diatom Clock Components

Plants and algae use light not only for driving photosynthesis but also to sense environmental cues and to adjust their circadian clocks via photoreceptors. Aureochromes are blue-light-dependent photoreceptors that also function as transcription factors, possessing both a LOV and a bZIP domain. Aureochromes so far have only been detected in Stramenopile algae, which include the diatoms. Four paralogues of aureochromes have been identified in the pennate model diatom Phaeodactylum tricornutum: PtAureo1a, 1b, 1c, and 2. While it was shown recently that diatoms have a diel rhythm, the molecular mechanisms and components regulating it are still largely unknown. Diel gene expression analyses of wild-type P. tricornutum, a PtAureo1a knockout strain, and the respective PtAureo1 complemented line revealed that all four aureochromes have a different diel regulation and that PtAureo1a has a strong co-regulatory influence on its own transcription, as well as on that of other genes encoding different blue-light photoreceptors (CPF1, 2 and 4), proteins involved in photoprotection (Lhcx1), and specific bHLH transcription factors (RITMO1). Some of these genes completely lost their circadian expression in the PtAureo1a KO mutant. Our results suggest a major involvement of aureochromes in the molecular clock of diatoms.