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The majority of metabolic processes and stress responses in plant cell involve oxidation-reduction reactions, thus redox signaling could be termed as the key element of cell regulation. Photosynthesis is a predominant process in the control of plant growth, development and stress responses. On the other hand, photosynthesis generates redox intermediates with extraordinarily negative redox potentials, thus bearing a high risk for photo-oxidative damage. In the absence of available electron acceptors, or even in competition with them, O2 can be reduced in photosynthetic electron transport chain (PETC) via the Mehler reaction. The relative rate and role of this process in vivo has been discussed over several decades in a number of papers, but its significance is not clear yet. Several authors have postulated that the Mehler reaction is unlikely to support a significant flow of electron transport and represents only about 10% of total electron flow in C3 plants. However, there is evidence corroborating the correlation between environmental stresses and increase in O2 photoreduction via the Mehler reaction of up to the 20-30% of total electron flow in C3 plants. The same high rates of electron flow to O2 can be drawn for some algae, C4 plants and especially for cyanobacteria. Current data on wide range of plant responses triggered by ROS and redox changes in several antioxidant systems allow proposing that the Mehler reaction plays an important role in redox signaling under stress conditions.