Emerging concepts of reactive oxygen species functions in plants

Reactive oxygen species (ROS) are ubiquitous signalling molecules that serve to integrate developmental, metabolic and stress signals to shape adaptive outcomes, linking energy metabolism to plant physiology, growth and stress resilience in a changing world. Resolving the factors and mechanisms involved in ROS-mediated control has proved to be far from trivial, not least because ROS are produced by every compartment of plant cells, serving multiple functions with numerous points of reciprocal control between phytohormones and other signalling pathways. While many enzymes produce hydrogen peroxide (H2O2) directly, other ROS sources such as respiratory burst oxidase homologues (RBOH) produce superoxide as a primary product. Key questions remain concerning the respective roles of superoxide and H2O2 in redox regulation of plant growth, development and defence, and how plant cells can differentiate between ROS produced in different cellular compartments. One solution concerns cysteine (Cys) molecular switches, which are specialised protein thiols that operate as highly sensitive ROS sensors in different locations, transducing changes in oxidation status to the nucleus and facilitating functional changes in protein activity, structure, and localisation. In addition, it is likely that the localisation and positions of many redox proteins, such as catalase and RBOH, are not as fixed as initially proposed, allowing plasticity of function in different compartments. This review discusses current concepts in plant ROS biology, highlighting novel aspects that permeate every aspect of plant biology.