Sulfur dioxide (SO2) supplies the basic sulfur element to promote plant growth, yet at the same time it is a harmful air pollutant. Currently, the mechanisms of plant adaptation to SO2 stress are largely unknown. Pathways of SO2 metabolism, a range of networks of interacting regulatory signals and defense mechanisms triggered in resistance to SO2 stress, have not yet been clarified. We performed transcriptome analysis of Arabidopsis plants fumigated with 30 mg m(-3) SO2 for 72 h and untreated controls using microarrays. This identified 2,780 significantly up- or down-regulated genes in plants response to SO2 stress, indicating a possible genome-scale reprogramming of the transcriptome. Significant changes in the transcript abundance of genes that participated in SO2 metabolic pathways indicated that numerous sulfites were involved in sulfur assimilatory pathways directly and away from sulfite oxidative pathways. Furthermore, the up-regulation of components involved in reactive oxygen species generating and scavenging pathways demonstrated altered redox homeostasis. Transcripts encoding key components in nitric oxide biosynthesis pathways were simultaneously up-regulated by SO2 exposure. In addition, transcripts associated with putative biotic stress were also up-regulated. Therefore, SO2 evokes a comprehensive reprogramming of metabolic pathways, consistent with up-regulation of transcripts involved in tolerance and defense mechanisms, in Arabidopsis.