Stress-triggered redox signalling: what's in pROSpect?

Reactive oxygen species (ROS) have a profound influence on almost every aspect of plant biology. Here, we emphasize the fundamental, intimate relationships between light-driven reductant formation, ROS, and oxidative stress, together with compartment-specific differences in redox buffering and the perspectives for their analysis. Calculations of approximate H2 O2 concentrations in the peroxisomes are provided, and based on the likely values in other locations such as chloroplasts, we conclude that much of the H2 O2 detected in conventional in vitro assays is likely to be extracellular. Within the context of scant information on ROS perception mechanisms, we consider current knowledge, including possible parallels with emerging information on oxygen sensing. Although ROS can sometimes be signals for cell death, we consider that an equally important role is to transmit information from metabolism to allow appropriate cellular responses to developmental and environmental changes. Our discussion speculates on novel sensing mechanisms by which this could happen and how ROS could be counted by the cell, possibly as a means of monitoring metabolic flux. Throughout, we place emphasis on the positive effects of ROS, predicting that in the coming decades they will increasingly be defined as hallmarks of viability within a changing and challenging environment.