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 (O-1(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 O-1(2), are two beta-carotene molecules that quench O-1(2) to ground-state oxygen. O-1(2) can oxidise beta-carotene to release beta-cyclocitral, which has emerged as a O-1(2)-mediated stress signal in the plant Arabidopsis thaliana. We investigated if beta-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 beta-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 beta-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 beta-cyclocitral-treated and O-1(2)-treated C. reinhardtii. Furthermore, exogenous treatment with beta-cyclocitral did not induce tolerance to O-1(2). We conclude that while beta-cyclocitral may down-regulate chlorophyll synthesis, it does not seem to contribute to O-1(2)-mediated high light stress signalling in algae.

Automated summary

The study found that singlet oxygen (O-1(2)) partially regulates light stress signaling in algae and plants, and beta-carotene molecules can quench O-1(2). In the unicellular alga Chlamydomonas reinhardtii, beta-cyclocitral may down-regulate chlorophyll synthesis, but it does not seem to contribute to O-1(2)-mediated high light stress signaling.