Implication of reactive oxygen species and mitochondrial dysfunction in the early stages of plant programmed cell death induced by ultraviolet-C overexposure

Recent studies have suggested that ultraviolet-C (UV-C) overexposure induces programmed cell death (PCD) in Arabidopsis thaliana (L.) Heynh, and this process includes participation of caspase-like proteases, DNA laddering as well as fragmentation of the nucleus. To investigate possible early signal events, we used microscopic observations to monitor in vivo the behaviour of mitochondria, as well as the production and localization of reactive oxygen species (ROS) during protoplast PCD induced by UV-C. A quick burst of ROS was detected when the protoplasts were kept in continuous light after UV-C exposure, which was restricted in chloroplasts and the adjacent mitochondria. Pre-incubation with ascorbic acid (AsA, antioxidant molecule) or 3-(3, 4-dichlorophenyl)-1, 1-dimethylurea (DCMU, an inhibitor of photosynthetic electron transport) decreased the ROS production and partially protected protoplasts from PCD. A mitochondrial transmembrane potential (MTP) loss occurred prior to cell death; thereafter, the mitochondria irregularly clumped around chloroplasts or aggregated in other places within the cytoplasm, and the movement of mitochondria was concomitantly blocked. Pre-treatment with an inhibitor of mitochondrial permeability transition pores (MPTP), cyclosporine (CsA), effectively retarded the decrease of MTP and reduced the percentage of protoplasts undergoing PCD after UV-C overexposure. Our results suggest that the MTP loss and the changes in distribution and mobility of mitochondria, as well as the production of ROS play important roles during UV-induced plant PCD, which is in good accordance with what has been reported in many types of apoptotic cell death, both in animals and plants.