Reactive oxygen species can arise from normal metabolic activity such as organelle-based electron transport or be intermediates in signal transduction pathways activated by plant respiratory burst oxidase homologs (Rboh). Divergent stress including temperature drought and UV-B exposure yield overlapping transcriptome response profiles whose origin can be traced to the use of reactive oxygen signaling intermediates. However, ROS signaling is ubiquitous and contributes in pathways controlled by hormones and developmental cues as well. Scavenging systems and NO are likely to temper its signaling properties and help contribute to the specificity of particular responses.