Reactive oxygen species such as peroxides play an important role in plant development, cell wall maturation, and defense responses. During nodulation with the host plant , cells are exposed to HO in infection threads and developing nodules (R. Santos, D. Hérouart, S. Sigaud, D. Touati, and A. Puppo, Mol Plant Microbe Interact 14:86-89, 2001, https://doi.org/10.1094/MPMI.2001.14.1.86). cells likely also experience oxidative stress, from both internal and external sources, during life in the soil. Here, we present microarray transcription data for wild-type cells compared to a mutant deficient in the key oxidative regulatory protein OxyR, each in response to HO treatment. Several alternative sigma factor genes are upregulated in the response to HO; the stress sigma gene shows OxyR-dependent induction by HO, while expression is induced by HO irrespective of the genotype. The activity of the RpoE2 sigma factor in turn causes increased expression of two more sigma factor genes, and Strains with deletions of showed improved survival in HO as well as increased levels of and total catalase expression. These results imply that Δ strains are primed to deal with oxidative stress. This work presents a global view of gene expression changes, and of regulation of those changes, in response to HO Like all aerobic organisms, the symbiotic nitrogen-fixing bacterium experiences oxidative stress throughout its complex life cycle. This report describes the global transcriptional changes that makes in response to HO and the roles of the OxyR transcriptional regulator and the RpoH1 sigma factor in regulating those changes. By understanding the complex regulatory response of to oxidative stress, we may further understand the role that reactive oxygen species play as both stressors and potential signals during symbiosis.