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.