Traditionally, reactive oxygen intermediates (ROIs) were considered to be toxic by-products
of aerobic metabolism, which were disposed of using antioxidants. However, in recent
years, it has become apparent that plants actively produce ROIs as signaling molecules
to control processes such as programmed cell death, abiotic stress responses, pathogen
defense and systemic signaling. Recent advances including microarray studies and the
development of mutants with altered ROI-scavenging mechanisms provide new insights
into how the steady-state level of ROIs are controlled in cells. In addition, key
steps of the signal transduction pathway that senses ROIs in plants have been identified.
These raise several intriguing questions about the relationships between ROI signaling,
ROI stress and the production and scavenging of ROIs in the different cellular compartments.