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

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