Oxygen processing in photosynthesis: regulation and signalling

The gradual but huge increase in atmospheric O-2 concentration that followed the evolution of oxygenic photosynthesis is one consequence that marks this event as one of the most significant in the earth's history. The high redox potential of the O-2/water couple makes it an extremely powerful electron sink that enables energy to be transduced in respiration. In addition to the tetravalent interconversion of O-2 and water, there exist a plethora of reactions that involve the partial reduction of O-2 or photodynamic energy transfer to produce active oxygen species (AOS). All these redox reactions have become integrated during evolution into the aerobic photosynthetic cell. This review considers photosynthesis as a whole-cell process, in which O-2 and AOS are involved in reactions at both photosystems, enzyme regulation in the chloroplast stroma, photorespiration, and mitochondrial electron transport in the light. In addition, oxidants and antioxidants are discussed as metabolic indicators of redox status, acting as sensors and signal molecules leading to acclimatory responses. Our aim throughout is to assess the insights gained from the application of mutagenesis and transformation techniques to studies of the role of O-2 and related redox components in the integrated regulation of photosynthesis.