Singlet oxygen signaling: from intimate to global.

Reactive oxidizing species (ROS), such as hydrogen peroxide, nitric oxide, superoxide anion, and singlet oxygen, can damage cells as well as initiate responses such as new gene expression. The cell response evoked is strongly dependent on several factors. The subcellular location for formation of an ROS may be especially important for a highly reactive ROS, because it diffuses only a very short distance before reacting with a cellular molecule. How does a short-lived, locally acting ROS trigger responses at distant subcellular sites? This issue is discussed in light of a study of gene expression initiated by singlet oxygen, a ROS that exists for less than 100 ns in cells. Singlet oxygen is generated after a photosensitizer absorbs light energy and transfers the energy to molecular oxygen. To assess the role of singlet oxygen in light-induced gene expression in Arabidopsis, a mutant was used that accumulates the photosensitizer protochlorphyllide in chloroplasts. Three mechanisms are discussed that may connect the site of singlet oxygen formation to the signal transduction components: (i) direct oxidation of a signaling component by singlet oxygen, (ii) formation of oxidation products near the sites of singlet oxygen formation that diffuse to and react with signaling components, and (iii) alteration of the redox balance of the cell to a more oxidized state such that a greater proportion of a signaling pathway component is oxidized.