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

Abstract

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 freeliving 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.

SydR, a redox-sensing MarR-type regulator ofSinorhizobium meliloti, is crucial for symbiotic infection ofMedicago truncatularoots

“…By contrast, Δ sydR mutant induces the formation of nitrogen-fixing nodules with M. sativa . These data are similar to the results of Zhang et al , (17) with a SMa2020 deletion mutant generated in Rm1021 background. They show the importance of plant species ( M. truncatula versus M. sativa) in the outcome of interaction.…”

“…Likewise, the repression of MtLYK3 expression that is directly linked to infection, did not occur in roots inoculated by one or the other mutant (26, 28). Finally, it can be assumed that the infection process in ΔsydR-inoculated roots arrests earlier than the process in exoA-inoculated roots since, unlike the latter, it lost the ability to trigger MtN20 These data are similar to the results of Zhang et al, (17) with a SMa2020 deletion mutant generated in Rm1021 background. They show the importance of plant species (M. truncatula versus M. sativa) in the outcome of interaction.…”

“…In S. meliloti , OhrR regulates ohr shown to be expressed in symbiosis, but neither ohrR nor ohr inactivation affects symbiosis efficiency (16). Finally, while this work was in progress, Zhang et al , (17) published that a deletion in SMa2020, an OhrR-like encoding gene, reduced IT formation and plant growth with no reported effect on the number of nodules in S. meliloti / M. sativa interaction.…”

MarR Family Transcriptional Regulators and Their Roles in Plant-Interacting Bacteria

“…However, plants inoculated with an ohr mutant of S. meliloti do not present nodulation defects, while a single deletion of the ohr gene in Azorhizobium caulinodans leads to a significant decrease in nitrogen fixation [ 57 ], suggesting the existence of functional equivalents of the Ohr enzyme in S. meliloti such as the AhpC alkyl hydroperoxidase [ 76 , 117 ]. Nevertheless, a recent study suggests that triple deletion in paralogous ohrR genes in S. meliloti affects symbiotic performance during interactions with Medicago sativa [ 118 ].…”