OxyR-Dependent Transcription Response ofSinorhizobium melilotito Oxidative Stress
ABSTRACTReactive oxygen species such as peroxides play an important role in plant development, cell wall maturation, and defense responses. During nodulation with the host plantMedicago sativa,Sinorhizobium meliloticells are exposed to H2O2in 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).S. meliloticells likely also experience oxidative stress, from both internal and external sources, during life in the soil. Here, we present microarray transcription data forS. melilotiwild-type cells compared to a mutant deficient in the key oxidative regulatory protein OxyR, each in response to H2O2treatment. Several alternative sigma factor genes are upregulated in the response to H2O2; the stress sigma generpoE2shows OxyR-dependent induction by H2O2, whilerpoH1expression is induced by H2O2irrespective of theoxyRgenotype. The activity of the RpoE2 sigma factor in turn causes increased expression of two more sigma factor genes,rpoE5andrpoH2. Strains with deletions ofrpoH1showed improved survival in H2O2as well as increased levels ofoxyRand total catalase expression. These results imply that ΔrpoH1strains are primed to deal with oxidative stress. This work presents a global view ofS. melilotigene expression changes, and of regulation of those changes, in response to H2O2.IMPORTANCELike all aerobic organisms, the symbiotic nitrogen-fixing bacteriumSinorhizobium melilotiexperiences oxidative stress throughout its complex life cycle. This report describes the global transcriptional changes thatS. melilotimakes in response to H2O2and the roles of the OxyR transcriptional regulator and the RpoH1 sigma factor in regulating those changes. By understanding the complex regulatory response ofS. melilotito oxidative stress, we may further understand the role that reactive oxygen species play as both stressors and potential signals during symbiosis.