Clocks in the Green Lineage: Comparative Functional Analysis of the Circadian Architecture of the PicoeukaryoteOstreococcus 

Abstract

Biological rhythms that allow organisms to adapt to the solar cycle are generated by endogenous circadian clocks. In higher plants, many clock components have been identified and cellular rhythmicity is thought to be driven by a complex transcriptional feedback circuitry. In the small genome of the green unicellular alga Ostreococcus tauri, two of the master clock genes Timing of Cab expression1 (TOC1) and Circadian Clock-Associated1 (CCA1) appear to be conserved, but others like Gigantea or Early-Flowering4 are lacking. Stably transformed luciferase reporter lines and tools for gene functional analysis were therefore developed to characterize clock gene function in this simple eukaryotic system. This approach revealed several features that are comparable to those in higher plants, including the circadian regulation of TOC1, CCA1, and the output gene Chlorophyll a/b Binding under constant light, the relative phases of TOC1/CCA1 expression under light/ dark cycles, arrhythmic overexpression phenotypes under constant light, the binding of CCA1 to a conserved evening element in the TOC1 promoter, as well as the requirement of the evening element for circadian regulation of TOC1 promoter activity. Functional analysis supports TOC1 playing a central role in the clock, but repression of CCA1 had no effect on clock function in constant light, arguing against a simple TOC1 /CCA1 one-loop clock in Ostreococcus. The emergence of functional genomics in a simple green cell with a small genome may facilitate increased understanding of how complex cellular processes such as the circadian clock have evolved in plants.

Seaweed reproductive biology: environmental and genetic controls

“…For example, despite clear indications that photoperiod plays an important role in controlling reproduction, circadian clock genes have hardly been characterized from seaweeds, which is in contrast to the extensive knowledge on clocks in some unicellular algae (e.g. Chlamydomonas, Ostreoccocus: see Mittag et al 2005, Corellou et al 2009). In flowering plants, circadian clock genes control flower development, among other processes.…”

Central role for ferritin in the day/night regulation of iron homeostasis in marine phytoplankton

“…In this study, we used O. tauri, a species which has emerged as a model organism thanks to the ease of culture and techniques for functional analyses such as genetic transformation and gene targeting by homologous recombination (28)(29)(30). We investigated the diel regulation of ferritin and iron-binding proteins in the field and under laboratory conditions.…”

Transient Gene Expression System Established in Porphyra yezoensis Is Widely Applicable in Bangiophycean Algae

“…Stable transformation systems have already been established in microalgae (Hallmann 2007), such as volvocine green algae, unicellular Chlamydomonas reinhardtii (Debuchy et al 1989;Kindle et al 1989), Dunaliella salina (Geng et al 2003(Geng et al , 2004) and multicellular Volvox carteri (Schiedlmeier et al 1994), all of which have long been favorite experimental organisms for genetic and molecular biological studies. Recently, eukaryotic green alga Ostreococcus tauri, which is an attractive microalgal model because of its minimal cellular organization and compact genome, has also become transformable (Corellou et al 2009;Moulager et al, 2010;Heidje et al, 2010). Furthermore, stable transformation is now available in diatom Phaeodactylum tricornutum (Apt et al 1996;Zaslavskaia et al 2000Zaslavskaia et al , 2001 and unicellular red alga Cyanidioschyzon merolae (Minoda et al 2004).…”