Reactive oxygen species in legume and actinorhizal nitrogen-fixing symbioses: the microsymbiont's responses to an unfriendly reception

The plant responses to infection by pathogenic bacteria have been extensively reviewed in recent years, including the spatial and temporal production of reactive oxygen species (ROS). The immediate and localized release of ROS upon infection, known as the oxidative burst, was shown not only to be part of the hypersensitive response but also likely responsible for mediating, directly or via signal transduction pathways, other plant defence strategies. This paradigm inspired studies in nitrogen-fixing root nodule symbioses, and a parallelism is unavoidable. In rhizobia-legume symbioses, histochemical data revealed the presence of ROS in the host infection threads and in the root nodules primordia. On the other hand, in actinorhizal infections, it has been shown that Alnus glutinosa root exudates induce several oxidative stress response-related proteins in compatible Frankia. These data suggest that the nitrogen-fixing microsymbionts must have had to evolve adaptations to overcome and possibly regulate an unfriendly environment. In this review, particular emphasis will be given to the bacteria antioxidant mechanisms at different developmental stages of the nitrogen-fixing root nodule symbioses.