Sinorhizobium meliloti ohrR genes affect symbiotic performance with alfalfa (Medicago sativa)

Sinorhizobium meliloti infects the host plant alfalfa to induce formation of nitrogen-fixation root nodules, which inevitably elicit reactive oxygen species (ROS) bursts and organic peroxide generation. The MarR family regulator OhrR regulates the expression of chloroperoxidase and organic hydrogen resistance protein, which scavenge organic peroxides in free-living S. meliloti cells. The single mutant of ohrR genes SMc01945 (ohrR1) and SMc00098 (ohrR2) lacked symbiotic phenotypes. In this work, we identified the novel ohrR gene SMa2020 (ohrR3) and determined that ohrR genes are important for rhizobial infection, nodulation and nitrogen fixation with alfalfa. By analysing the phenotypes of the single, double and triple deletion mutants of ohrR genes, we demonstrate that ohrR1 and ohrR3 slightly affect rhizobial growth, but ohrR2 and ohrR3 influence cellular resistance to the organic peroxide, tert-butyl hydroperoxide. Deletion of ohrR1 and ohrR3 negatively affected infection thread formation and nodulation, and consequently, plant growth. Correspondingly, the expression of the ROS detoxification genes katA and sodB as well as that of the nitrogenase gene nifH was downregulated in bacteroids of the double and triple deletion mutants, which may underlie the symbiotic defects of these mutants. These findings demonstrate that OhrR proteins play a role in the S. meliloti-alfalfa symbiosis.