Activated oxygen and free oxygen radicals in pathology: New insights and analogies between animals and plants

All organisms are subject to diseases and forced to adapt to unfavourable conditions. In this context, plants, animals and lower organisms are equally concerned. Both diseases and environmental adaptations are frequently connected with altered oxygen metabolism chemically characterized as a transition from heterolytic (two electron abstraction or donation) to increased homolytic (one electron transition) processes. Homolytic reactions generate radicals which may react in an aggressive manner, thus damaging cellular compartments, tissues and finally organisms and populations. Therefore, these reactions generating reactive oxygen species (ROS) are generally counterbalanced by an endogenous increase of antioxidative processes or externally by supportive medication. In the last twenty years, several unexpected similarities in plants and animals concerning oxygen activation and radical-driven processes have been found. In this review, homologous or corresponding processes and activities in plants and animals are compared.