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

Abstract

ABSTRACTRhizobia associate with legumes and induce the formation of nitrogen-fixing nodules. The regulation of bacterial redox state plays a major role in symbiosis andReactiveOxygenSpecies (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 ofS. melilotithat is essential for the establishment of symbiosis withMedicago 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 H2O2treatment. Gels shift assays strongly suggest that SydR binds toTATCGCGATAmotif in thesydR-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 ofsydRdid not alter the sensitivity ofS. melilotito 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, ΔsydRmutation impaired the formation of root nodules. Microscopic observations and analyses of marker gene expression showed that the ΔsydRmutant 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 withM. 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 theMedicagosymbiontSinorhizobium meliloti, acts as a redox-responsive repressor that is crucial for the development of root nodules and contributes to the regulation of bacterial infection inS. meliloti / Medicago truncatulasymbiotic interaction.