glutathione peroxidase homologous gene Gpxh in Chlamydomonas reinhardtii is upregulated by singlet oxygen produced in photosystem II

The expression of the glutathione peroxidase homologous gene Gpxh, known to be specifically induced by the formation of singlet oxygen (¹O₂), was analyzed in cells of Chlamydomonas reinhardtii exposed to environmental conditions causing photoinhibition. Illumination with high light intensities, leading to an increased formation of ¹O₂ in photosystem II, continuously induced the expression of Gpxh in cell for at least 2 h. Phenolic herbicides like dinoterb, raise the rate of ¹O₂ formation by increasing the probability of charge recombination in photosystem II via the formation of the primary radical pair and thereby ³P₆₈₀ formation (Fufezan C et al. 2002, FEBS Letters 532, 407-410). In the presence of dinoterb the light-induced loss of the D1 protein in C. reinhardtii was increased and the high light-induced Gpxh expression was further stimulated. DCMU, a urea-type herbicide, causing reduced ¹O₂ generation in photosystem II, protected the D1 protein slightly against degradation and downregulated the expression of the Gpxh gene compared to untreated cells exposed to high light intensities. This indicates that the Gpxh expression is induced by ¹O₂ under environment conditions causing photoinhibition.