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Summary: The conditional flu mutant of Arabidopsis accumulates excess amounts of protochlorophyllide within plastid membranes in the dark and generates singlet oxygen upon light exposure. By varying the length of the dark period, the level of the photosensitizer protochlorophyllide may be modulated, and conditions have been established that either endorse the cytotoxicity of 1O2 or reveal its signaling role. Two criteria have been used to distinguish between these two modes of activity of 1O2: the impact of the EXECUTER1 mutation and the prevalence of either non-enzymatic or enzymatic lipid peroxidation. During illumination of etiolated flu seedlings, toxic effects of 1O2 prevail and non-enzymatic lipid peroxidation proceeds rapidly. In contrast, in light-grown flu plants that were subjected to an 8 h dark/light shift, lipid peroxidation occurred almost exclusively enzymatically. The resulting oxidation product, 13-hydroperoxy octadecatrienoic acid (13-HPOT), serves as a substrate for synthesis of 12-oxo phytodienoic acid (OPDA) and jasmonic acid (JA), both of which are known to control various metabolic and developmental processes in plants. Inactivation of the EXECUTER1 protein abrogates not only 1O2-mediated cell death and growth inhibition of flu plants, but also enzymatic lipid peroxidation. However, inactivation of jasmonate biosynthesis in the aos/flu double mutant does not affect 1O2-mediated growth inhibition and cell death. Hence, JA and OPDA do not act as second messengers during 1O2 signaling, but form an integral part of a stress-related signaling cascade activated by 1O2 that encompasses several signaling pathways known to be activated by abiotic and biotic stressors.

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