Dose-dependent effects of 1O2 in chloroplasts are determined by its timing and localization of production

Abstract

In plants, highly reactive singlet oxygen (¹O₂) is known to inhibit photosynthesis and to damage the cell as a cytotoxin. However, more recent studies have also proposed ¹O₂ as a signal. In plants under stress, not only ¹O₂ but also other reactive oxygen species (ROS) are generated simultaneously, thus making it difficult to link a particular response to the release of ¹O₂ and establish a signaling role for this ROS. This obstacle has been overcome by the identification of conditional mutants of Arabidopsis thaliana that selectively generate ¹O₂ and trigger various ¹O₂-mediated responses. In chloroplasts of these mutants, chlorophyll or its biosynthetic intermediates may act as a photosensitizer and generate ¹O₂. These ¹O₂-mediated responses are not only dependent on the dosage of ¹O₂ but also are determined by the timing and suborganellar localization of its production. This spatial- and temporal-dependent variability of ¹O₂-mediated responses emphasizes the importance of ¹O₂ as a highly versatile and short-lived signal that acts throughout the life cycle of a plant.