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The role of glutathione reductase (GR; EC 1.6.4.2) in the tolerance of Chlamydomonas reinhardtii P.A. Dangeard to high‐intensity light stress (HL, 1400 μmol m−2 s−1) was examined. Cells survived under high light (HL) stress, although their growth was inhibited after long‐term treatment (9‐24 h). GR activity increased 1 h after HL treatment. The contents of total glutathione, reduced glutathione (GSH) and glutathione disulfide (GSSG) increased 1‐3 h after HL treatment and then decreased after 24 h, while the GSH:GSSG ratio (glutathione redox potential) decreased after 3–9 h and recovered after 24 h. The transcript abundance of GR, CrGR1 (Cre06.g262100) and CrGR2 (Cre09.g396252) as well as glutathione synthesis‐related genes, CrGSH1 (Cre02g077100.t1.1) and CrGSH2 (Cre17.g70800.t1.1), increased with a peak near 1 h after HL treatment. Except for enhanced glutathione synthesis, the GR‐mediated glutathione redox machinery is also critical for the tolerance of C. reinhardtii cells to HL stress. Therefore, GR was downregulated or upregulated to investigate the importance of GR in HL tolerance. The CrGR1 knockdown amiRNA line exhibited low GR transcript abundance, GR activity and GSH:GSSG ratio and could not survive under HL conditions. Over‐expression of CrGR1 or CrGR2 driven by a HSP70A:RBCS2 fusion promoter resulted in a higher GR transcript abundance, GR activity and GSH:GSSG ratio and led to cell survival when exposed to high‐intensity illumination, i.e. 1800 μmol m−2 s−1. In conclusion, GR‐mediated modulation of the glutathione redox potential plays a role in the tolerance of Chlamydomonas cells to photo‐oxidative stress.

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