β-Cyclocitral Does Not Contribute to Singlet Oxygen-Signalling in Algae, but May Down-Regulate Chlorophyll Synthesis

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