| Summary: | Light stress signalling in algae and plants is partially orchestrated by singlet oxygen (<sup>1</sup>O<sub>2</sub>), a reactive oxygen species (ROS) that causes significant damage within the chloroplast, such as lipid peroxidation. In the vicinity of the photosystem II reaction centre, a major source of <sup>1</sup>O<sub>2</sub>, are two β-carotene molecules that quench <sup>1</sup>O<sub>2</sub> to ground-state oxygen. <sup>1</sup>O<sub>2</sub> can oxidise β-carotene to release β-cyclocitral, which has emerged as a <sup>1</sup>O<sub>2</sub>-mediated stress signal in the plant <i>Arabidopsis thaliana</i>. We investigated if β-cyclocitral can have similar retrograde signalling properties in the unicellular alga <i>Chlamydomonas reinhardtii</i>. Using RNA-Seq, we show that genes up-regulated in response to exogenous β-cyclocitral included <i>CAROTENOID CLEAVAGE DIOXYGENASE 8</i> (<i>CCD8</i>), while down-regulated genes included those associated with porphyrin and chlorophyll anabolism, such as tetrapyrrole-binding protein (<i>GUN4</i>), magnesium chelatases (<i>CHLI1</i>, <i>CHLI2</i>, <i>CHLD</i>, <i>CHLH1</i>), light-dependent protochlorophyllide reductase (<i>POR1</i>), copper target 1 protein (<i>CTH1</i>), and coproporphyrinogen III oxidase (<i>CPX1</i>). Down-regulation of this pathway has also been shown in β-cyclocitral-treated <i>A. thaliana</i>, indicating conservation of this signalling mechanism in plants. However, in contrast to <i>A. thaliana</i>, a very limited overlap in differential gene expression was found in β-cyclocitral-treated and <sup>1</sup>O<sub>2</sub>-treated <i>C. reinhardtii</i>. Furthermore, exogenous treatment with β-cyclocitral did not induce tolerance to <sup>1</sup>O<sub>2</sub>. We conclude that while β-cyclocitral may down-regulate chlorophyll synthesis, it does not seem to contribute to <sup>1</sup>O<sub>2</sub>-mediated high light stress signalling in algae.
|
|---|