title: Probing entrainment of Ostreococcus tauri circadian clock by green and blue light through a mathematical modeling approach.
abstract: Most organisms anticipate daily environmental variations and orchestrate cellular functions thanks to a circadian clock which entrains robustly to the day/night cycle, despite fluctuations in light intensity due to weather or seasonal variations. Marine organisms are also subjected to fluctuations in light spectral composition as their depth varies, due to differential absorption of different wavelengths by sea water. Studying how light input pathways contribute to circadian clock robustness is therefore important. Ostreococcus tauri, a unicellular picoplanktonic marine green alga with low genomic complexity and simple cellular organization, has become a promising model organism for systems biology. Functional and modeling approaches have shown that a core circadian oscillator based on orthologs of Arabidopsis TOC1 and CCA1 clock genes accounts for most experimental data acquired under a wide range of conditions. Some evidence points at putative light input pathway(s) consisting of a two-component signaling system (TCS) controlled by the only two histidine kinases (HK) of O. tauri. LOV-HK is a blue light photoreceptor under circadian control, that is required for circadian clock function. An involvement of Rhodopsin-HK (Rhod-HK) is also conceivable since rhodopsin photoreceptors mediate blue to green light input in animal circadian clocks. Here, we probe the role of LOV-HK and Rhod-HK in mediating light input to the TOC1-CCA1 oscillator using a mathematical model incorporating the TCS hypothesis. This model agrees with clock gene expression time series representative of multiple environmental conditions in blue or green light, characterizing entrainment by light/dark cycles, free-running in constant light, and resetting. Experimental and theoretical results indicate that both blue and green light can reset O. tauri circadian clock. Moreover, our mathematical analysis suggests that Rhod-HK is a blue-green light receptor and drives the clock together with LOV-HK.
authors.affiliation: Laboratoire de Physique, Lasers, Atomes, Molécules, Université Lille 1 Sciences et Technologies, Centre National de la Recherche Scientifique, Unité Mixte de Recherche 8523 Villeneuve d'Ascq, France.
authors.forename: Quentin
authors.initials: Q
authors.lastname: Thommen
authors.affiliation: Laboratoire de Physique, Lasers, Atomes, Molécules, Université Lille 1 Sciences et Technologies, Centre National de la Recherche Scientifique, Unité Mixte de Recherche 8523 Villeneuve d'Ascq, France.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Pfeuty
authors.affiliation: Unité Mixte de Recherche 7621, Laboratoire d'Océanographie Microbienne, Observatoire Océanologique de Banyuls, Centre National de la Recherche Scientifique, Université Pierre et Marie Curie (Paris 06), Sorbonne Universités Banyuls sur Mer, France.
authors.forename: Philippe
authors.initials: P
authors.lastname: Schatt
authors.affiliation: Unité Mixte de Recherche 7621, Laboratoire d'Océanographie Microbienne, Observatoire Océanologique de Banyuls, Centre National de la Recherche Scientifique, Université Pierre et Marie Curie (Paris 06), Sorbonne Universités Banyuls sur Mer, France.
authors.forename: Amandine
authors.initials: A
authors.lastname: Bijoux
authors.affiliation: Unité Mixte de Recherche 7621, Laboratoire d'Océanographie Microbienne, Observatoire Océanologique de Banyuls, Centre National de la Recherche Scientifique, Université Pierre et Marie Curie (Paris 06), Sorbonne Universités Banyuls sur Mer, France.
authors.forename: François-Yves
authors.initials: FY
authors.lastname: Bouget
authors.affiliation: Laboratoire de Physique, Lasers, Atomes, Molécules, Université Lille 1 Sciences et Technologies, Centre National de la Recherche Scientifique, Unité Mixte de Recherche 8523 Villeneuve d'Ascq, France.
authors.forename: Marc
authors.initials: M
authors.lastname: Lefranc
keywords: Ostreococcus tauri; circadian clock; entrainment; light quality; photoreceptors