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

Singlet oxygen (O-1(2)) refers to the lowest excited electronic state of molecular oxygen. It easily oxidizes biological molecules and, therefore, is cytotoxic. In plant cells, O-1(2) is formed mostly in the light in thylakoid membranes by reaction centers of photosystem II. In high concentrations, O-1(2) destroys membranes, proteins and DNA, inhibits protein synthesis in chloroplasts leading to photoinhibition of photosynthesis, and can result in cell death. However, O-1(2) also acts as a signal relaying information from chloroplasts to the nucleus, regulating expression of nuclear genes. In spite of its extremely short lifetime, O-1(2) can diffuse from the chloroplasts into the cytoplasm and the apoplast. As shown by recent studies, O-1(2)-activated signaling pathways depend not only on the levels but also on the sites of O-1(2) production in chloroplasts, and can activate two types of responses, either acclimation to high light or programmed cell death. O-1(2) 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 O-1(2) generation in plants, on O-1(2)-activated pathways of retrograde- and cellular signaling, and on the methods to study O-1(2) production in plants.