Singlet Oxygen in Plants: Generation, Detection, and Signaling Roles

Singlet oxygen (¹O₂) refers to the lowest excited electronic state of molecular oxygen. It easily oxidizes biological molecules and, therefore, is cytotoxic. In plant cells, ¹O₂ is formed mostly in the light in thylakoid membranes by reaction centers of photosystem II. In high concentrations, ¹O₂ destroys membranes, proteins and DNA, inhibits protein synthesis in chloroplasts leading to photoinhibition of photosynthesis, and can result in cell death. However, ¹O₂ also acts as a signal relaying information from chloroplasts to the nucleus, regulating expression of nuclear genes. In spite of its extremely short lifetime, ¹O₂ can diffuse from the chloroplasts into the cytoplasm and the apoplast. As shown by recent studies, ¹O₂-activated signaling pathways depend not only on the levels but also on the sites of ¹O₂ production in chloroplasts, and can activate two types of responses, either acclimation to high light or programmed cell death. ¹O₂ can be produced in high amounts also in root cells during drought stress. This review summarizes recent advances in research on mechanisms and sites of ¹O₂ generation in plants, on ¹O₂-activated pathways of retrograde- and cellular signaling, and on the methods to study ¹O₂ production in plants.