Functions of the <i>Sinorhizobium meliloti</i> LsrB Substrate-Binding Domain in Oxidized Glutathione Resistance, Alfalfa Nodulation Symbiosis, and Growth

To successfully colonize legume root nodules, rhizobia must effectively evade host-generated reactive oxygen species (ROS). LsrB, a redox regulator from <i>Sinorhizobium meliloti</i>, is essential for symbiosis with alfalfa (<i>Medicago sativa</i>). The three cysteine residues in LsrB's substrate domain play distinct roles in activating downstream redox genes. The study found that LsrB's substrate-binding domain, dependent on the cysteine residue Cys146, is involved in oxidized glutathione (GSSG) resistance and alfalfa nodulation symbiosis. LsrB homologues from other rhizobia, with Cys172/Cys238 or Cys146, enhance GSSG resistance and complement <i>lsrB</i> mutant's symbiotic nodulation. Substituting amino acids in <i>Azorhizobium caulinodans</i> LsrB with Cys restores <i>lsrB</i> mutant phenotypes. The <i>lsrB</i> deletion mutant shows increased sensitivity to NCR247, suggesting an interaction with host plant-derived NCRs in alfalfa nodules. Our findings reveal that the key cysteine residue in the LsrB's substrate domain is vital for rhizobium-legume symbiosis.