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THE ROLE OF CHLOROPLAST-GENERATED REACTIVE OXYGEN SPECIES IN PLANT-PATHOGEN INTERACTIONS Molinari P.E., Delprato M. L., Krapp A. R. and Carrillo N.Instituto de Biologia Molecular y Celular de Rosario (IBR?CONICET-UNRPlants respond to attack by pathogens displaying a series of complex responses that involve the expression of multiple genes and the implementation of the hypersensitive response (HR) at the site of infection. As a result of the recognition of the pathogen, reactive oxygen-derived species (ROS) are generated in several subcellular compartments, with chloroplasts being the main source of production only if the light stimulus is present.In this work were used tobacco plants expressing a protein directed to chloroplasts from cyanobacteria called Flavodoxin. These transgenic lines present a wide range of tolerance to adverse environmental conditions and a lower accumulation of ROS, because flavodoxin behaves like a general antioxidant. Therefore, these plants were used to study whether the light requirement of the HR is associated with the chloroplast production of ROS. A phenotypic and biochemical characterization of the interaction between these plants and different pathovars of the bacterium Pseudomonas syringae that has a biotrophic lifestyle was performed, and they represent three different types of plant-pathogen interaction (non-host, virulent and avirulent), in presence and absence of light, and the progress of bacterial growth in plant was evaluated. The determination of ROS by fluorescence microscopy allowed to demonstrate that the transgenic plants accumulate these species in significantly lower amounts than the wildtype, independently of the kind of interaction, as long as the light stimulus is present. These data contribute to the models that affirm that the ROS generated in chloroplasts participate in plant defense against pathogens