Independent regulation of the GPXH gene expression by primary and secondary effects of high light stress in Chlamydomonas reinhardtii

The expression of the GPXH gene, coding for a phospholipid hydroperoxide glutathione peroxidase homologous protein, was shown to be strongly induced in Chlamydomonas reinhardtii exposed to photooxidative stress conditions like high light illumination. Here, we show that the response of the GPXH gene to high-light conditions is induced by two independent signals and mechanisms: first, there is a fast but transient transcriptional induction by singlet oxygen formed after charge recombination and energy transfer from the overexcited photosystem II reaction center to molecular oxygen. Second, upon very strong or ongoing photooxidative stress, GPXH upregulation is further stimulated presumably by an mRNA stabilization mechanism as a result of lipid hydroperoxide formation. This could be confirmed by imitating the high-light response of GPXH by exposing cultures of C. reinhardtii to a mixture of the singlet oxygen producing photosensitizer rose bengal and organic hydroperoxides. Furthermore, determination of mRNA decay rates allowed simulating the GPXH responses to various stress conditions with a mathematical model. A preventive effect of singlet oxygen formation on the resistance against photooxidative stress and lipid peroxidation was shown by acclimation experiments. Thus, the consecutive regulation of GPXH expression by an early transcriptional upregulation by a primary (singlet oxygen formation) and a time-dependent stabilization of mRNA levels by a secondary effect (lipid peroxidation) of high light stress might be an efficient mechanism to control the stress response.