SydR, a redox-sensing MarR-type regulator of Sinorhizobium meliloti, is crucial for symbiotic infection of Medicago truncatula roots

Rhizobia associate with legumes and induce the formation of nitrogen-fixing nodules. The regulation of bacterial redox state plays a major role in symbiosis and Reactive Oxygen Species (ROS) produced by the plant are known to activate signaling pathways. However, only a few redox-sensing transcriptional regulators (TRs) have been characterized in the microsymbiont. Here, we describe SydR, a novel redox-sensing TR of S. meliloti that is essential for the establishment of symbiosis with Medicago truncatula. SydR, a MarR-type TR, represses the expression of the adjacent gene SMa2023 in growing cultures, and this repression is alleviated by NaOCl, tert-butyl, or H2O2 treatment. Gels shift assays strongly suggest that SydR binds to TATCGCGATA motif in the sydR-SMa2023 intergenic region in a redox-dependent manner. Furthermore, site-directed mutagenesis demonstrated that the oxidative inhibition of SydR involves the formation of an intermolecular C16-C16 disulfide bond. The inactivation of sydR did not alter the sensitivity of S. meliloti to NaOCl, tert-butyl, or H2O2, nor did it affect the response to oxidants of the roGFP2-Orp1 redox biosensor expressed within bacteria. However, in planta, {Delta}sydR mutation impaired the formation of root nodules. Microscopic observations and analyses of marker gene expression showed that the {Delta}sydR mutant is arrested at an early stage of the bacterial infection process. Altogether, these results demonstrated that SydR is a redox sensing MarR-type TR that plays a key role in the regulation of symbiosis with M. truncatula. IMPORTANCEThe nitrogen-fixing symbiosis between rhizobia and legumes has an important ecological role in the nitrogen cycle, contributes to nitrogen enrichment of soils, and can improve plant growth in agriculture. This interaction is initiated in the rhizosphere by a molecular dialog between the two partners, resulting in plant root infection and formation of root nodules, where bacteria reduce the atmospheric nitrogen into ammonium. This symbiosis involves modifications of the bacterial redox state in response to reactive oxygen species produced by the plant partner. Here, we show that SydR, a transcriptional regulator of the Medicago symbiont Sinorhizobium meliloti, acts as a redox-responsive repressor that is crucial for the development of root nodules and contributes to the regulation of bacterial infection in S. meliloti / Medicago truncatula symbiotic interaction.