Quorum Sensing Regulator CinR Directly Activates the Catalase-Peroxidase Gene katG to Alleviate Oxidative Stress and Promote Symbiotic Nitrogen Fixation in Rhizobium etli CFN42.

Many rhizobia use quorum sensing (QS) systems to detect their population density and modify their symbiotic behavior with the legume host. There are three LuxRI-type QS systems in Rhizobium etli CFN42, and CinR plays a key role in symbiotic performance. However, the details of how CinR regulates the symbiotic process remain unknown. In this study, we employed the RNA-Seq method to screen differentially expressed genes between the wild-type strain and the ΔcinR mutant of R. etli CFN42. We found that most of the genes related to reactive oxygen species (ROS) were expressed at lower levels in the ΔcinR mutant than in CFN42. We also found that the ΔcinR mutant was more sensitive to H2O2 than to CFN42. We then showed that CinR positively regulated katG expression and possessed an affinity to bind the katG promoter in the absence of the AHL ligand. The addition of AHLs promoted CinR binding to the katG promoter and enhanced katG expression. Accumulation of H2O2 and O2•- was observed in root nodules formed by the ΔcinR mutant. Crucially, katG overexpression rescued the H2O2-sensitive phenotype in vitro and partially restored defective symbiotic performance in nodules formed by the ΔcinR mutant on the common bean. These results suggest that CinR globally regulates ROS scavenging gene expression in order to balance oxidative stress within root nodules, promoting nitrogenase activity of R. etli CFN42.