Photorespiration and the Mehler reaction : cellular protection or redox perturbation?

Abstract

Two processes associated with photosynthesis make the major contribution to the oxidative load borne by leaf tissue in C3 plants. These are the Mehler reaction in the chloroplast and the glycollate oxidase reaction in the peroxisomes, both of which generate H2O2. Although often discussed as protective mechanisms, both the Mehler reaction and photorespiration can therefore influence cellular redox balance and related signalling. We have modelled the rate of photorespiratory H2O2 production from rates of net CO2 uptake measured in different conditions of irradiance and CO2 concentration. The data suggest that production of H2O2 in photorespiration exceeds that derived from the Mehler reaction under most conditions and that the total rate of H2O2 generation can approach or exceed the rate of net CO2 fixation. To avoid accumulation of H2O2, the chloroplast and the peroxisome have high antioxidant capacities. In addition to antioxidant enzymes such as superoxide dismutase, catalase, and peroxiredoxins, photosynthetic cells accumulate two low molecular weight antioxidants, ascorbate and glutathione. These antioxidants do not always interact in a coupled manner and the acorbate/ dehydroascorbate (DHA) redox pair may vary independently of the reduced (GSH)/oxidised glutathione (GSSG) redoz couple. This suggests that signals derived from changes in the rate of H2O2 generation are transmitted through perturbations in each antioxidant.