Research Summary

One of the first reactions of plants under stress is the enhanced production of chemically distinct reactive oxygen species (ROS). A major difficulty in elucidating the biological activity of ROS during stress stems from the fact that not only one but several chemically distinct ROS are generated simultaneously, thus making it very difficult to link a particular stress response to a specific ROS. This problem has been alleviated by using the conditional flu mutant of Arabidopsis that allows to induce the production of only one ROS, singlet oxygen, within plastids in a non-invasive, controlled manner (Fig. 1). In the dark the flu mutant accumulates protochlorophyllide (Pchlide), a potent photosensitizer that upon illumination generates singlet oxygen. Several singlet oxygen-mediated stress responses have been distinguished during re-illumination of the flu mutant. Inactivation of nuclear genes encoding the two closely related plastid proteins Executer1 and Executer2 has been shown to be sufficient to abrogate these stress responses despite the ongoing release of singlet oxygen. By varying the length of the dark period, one can adjust the level of the photosensitizer Pchlide and define conditions that minimize the cytotoxicity of singlet oxygen and either endorse acclimation in flu plants exposed to a very short dark period as one extreme or promote a genetically controlled cell death response in plants shifted for a longer period to the dark as another extreme (Fig. 2). This activity of singlet oxygen assigns a new function to the chloroplast, namely that of a sensor of environmental changes that activates a broad range of stress responses, known to be activated also by abiotic and biotic stressors. Our work is aimed at dissecting the complexity of singlet oxygen signalling and understanding and eventually also modifying the genetic constraints that determine the adaptability of plants to environmental changes.