Oxygen plays the important role not only in respiration of the organisms. The interaction of oxygen molecules with cell components produces the reactive oxygen species (ROS). ROS is always present in biological cells and their concentration is maintained in continuous level under normal conditions. But the level of ROS increases significantly under stress conditions which may leads to the cells death. However besides the destructive processes in biological structures ROS do execute the role of signal molecules, providing a co-ordination of intercellular processes as well as the adequate response of organism to a disease and environment changes. Hydrogen peroxide is the stable mobile ROS was recognised as most important signal-transducing molecule. The photosynthetic electron transport chain of chloroplast thylakoids is one of the main sources of ROS production in plants but the detailed mechanism of the process remains unknown. Thereby the understanding how these ROS serve as signal molecules and which step of this signal is limit for cells live may help to save the plants from premature death. The main aim of the proposed work is to study how ROS are produced in plants and to understand how these ROS function as the signalling molecules in plants. The site and the molecular mechanism of singlet oxygen, superoxide and hydrogen peroxide formation in the photosynthetic electron transport chain will be investigated in vitro and in plants in vivo. Also the amount of ROS and ROS localization in vivo and the ROS signaling pathways will be investigated in the proposed work.