Functions of the Sinorhizobium meliloti LsrB Substrate-Binding Domain\nin Oxidized Glutathione Resistance,\nAlfalfa Nodulation Symbiosis, and Growth

To successfully colonize legume root nodules, rhizobia\nmust effectively\nevade host-generated reactive oxygen species (ROS). LsrB, a redox\nregulator from Sinorhizobium meliloti, is essential for symbiosis with alfalfa (Medicago\nsativa). The three cysteine residues in LsrB’s\nsubstrate domain play distinct roles in activating downstream redox\ngenes. The study found that LsrB’s substrate-binding domain,\ndependent on the cysteine residue Cys146, is involved in oxidized\nglutathione (GSSG) resistance and alfalfa nodulation symbiosis. LsrB\nhomologues from other rhizobia, with Cys172/Cys238 or Cys146, enhance\nGSSG resistance and complement <i>lsrB</i> mutant’s\nsymbiotic nodulation. Substituting amino acids in Azorhizobium\ncaulinodans LsrB with Cys restores <i>lsrB</i> mutant phenotypes. The <i>lsrB</i> deletion mutant shows\nincreased sensitivity to NCR247, suggesting an interaction with host\nplant-derived NCRs in alfalfa nodules. Our findings reveal that the\nkey cysteine residue in the LsrB’s substrate domain is vital\nfor rhizobium-legume symbiosis.