Singlet Oxygen in Plants: Mechanistic Insights into Production, Oxidative Damage, and Stress-Response Signaling

Singlet oxygen (1O2), a highly reactive oxygen species generated in illuminated chloroplasts, is a major contributor to photooxidative damage in plants, particularly under high-light stress. This review summarizes the major sites of 1O2 production, its damage mechanisms, and the antioxidant defense and scavenging strategies that limit 1O2 accumulation. Particular emphasis is placed on recent advances in 1O2-mediated signal transduction. EX1 and β-carotene-derived oxidation products have been proposed to act as key components in 1O2 perception and signaling, mediating signal transmission from distinct subchloroplast locations and contributing to a spatially coordinated 1O2 signaling network. Plants counteract 1O2-induced damage through a multilayered defense–perception–signal transduction system. The functional partitioning of 1O2 signaling between grana margins and grana cores, together with the coordinated action of multiple signaling molecules, provides important insights into the mechanisms of plant photoprotection and offers potential strategies for improving crop tolerance to photooxidative stress.