Whole-Genome Resequencing of Spontaneous Oxidative Stress-Resistant Mutants Reveals an Antioxidant System of Bradyrhizobium japonicum Involved in Soybean Colonization

Abstract

Soybean is the most inoculant-consuming crop in the world, carrying strains belonging to the extremely related species Bradyrhizobium japonicum and Bradyrhizobium diazoefciens. Currently, it is well known that B. japonicum has higher efciency of soybean colonization than B. diazoefciens, but the molecular mechanism underlying this diferential symbiotic performance remains unclear. In the present study, genome resequencing of four spontaneous oxidative stress–resistant mutants derived from the commercial strain B. japonicum E109 combined with molecular and physiological studies allowed identifying an antioxidant cluster (BjAC) containing a transcriptional regulator (glxA) that controls the expression of a catalase (catA) and a phosphohydrolase (yfbR) related to the hydrolysis of hydrogen peroxide and oxidized nucleotides, respectively. Integrated synteny and phylogenetic analyses supported the fact that BjAC emergence in the B. japonicum lineage occurred after its divergence from the B. diazoefciens lineage. The transformation of the model bacterium B. diazoefciens USDA110 with BjAC from E109 signifcantly increased its ability to colonize soybean roots, experimentally recapitulating the benefcial efects of the occurrence of BjAC in B. japonicum. In addition, the glxA mutation signifcantly increased the nodulation competitiveness and plant growth–promoting efciency of E109. Finally, the potential applications of these types of non-genetically modifed mutant microbes in soybean production worldwide are discussed.