The Mehler reaction is the major source of superoxide anion radicals (O2•−) and hydrogen peroxide molecules in the chloroplast. In terms of mechanism, the Mehler reaction starts as a one-electron oxidation of electron transfer components of the photosynthetic apparatus by molecular oxygen producing O2•−, which is further reduced to H2O2, the stable product of the Mehler reaction. This review covers recent data on the mechanisms of O2 photoreduction in the photosynthetic electron transport chain, with particular emphasis on photosystem I (PSI), which has long been considered as the major site of O2•− production in the chloroplast. Despite the long history of the study, until recently, there was some contradiction as to the precise location(s) of O2 photoreduction within PSI. Here, I explain some controversies that still exist and discuss evolutionary trends toward optimizing PSI activity in the presence of O2.
- Mehler reaction
- Photosynthetic electron transport chain
- Photosystem I
- Superoxide radical