Plants are often exposed to external conditions that adversely affect their growth, development or productivity. Such unfavourable environmental stress factors may result in rapid and transient increases of intracellular concentrations of reactive oxygen species (ROS) that are chemically distinct and impact plants either by being cytotoxic or by acting as a signal. Because different ROS are generated simultaneously in different cellular and extracellular compartments, it is almost impossible to link a particular ROS to a specific stress response and to determine its mode of action. The conditional flu mutant of Arabidopsis has been used to determine the biological role of singlet oxygen. Immediately after a dark/light shift of the flu mutant, singlet oxygen is generated within the plastids activating several stress responses that include growth inhibition of mature plants and seedling lethality. These stress responses do not result from physicochemical damage caused by singlet oxygen, but are attributable to the activation of a genetically determined stress response programme triggered by the Executer1 protein. Singlet oxygen-mediated stress responses at the transcriptional level necessitate a retrograde transduction of signals from the chloroplast to the nucleus that activate distinct sets of genes different from those that are induced by superoxide/hydrogen peroxide. Hence, the biological activities of these two types of ROS are distinct from each other. Whether they act independently or interact is not known yet and is the topic of our current research.