Characterization of chemotaxis in soybean symbiont Bradyrhizobium diazoefficiens

Symbiotic relationships between nitrogen-fixing soil bacteria and legumes provide nearly half of all biologically fixed nitrogen on Earth, playing a crucial role in sustainable agriculture. These relationships rely on bacterial navigation of complex, dynamic soil environments to reach their plant hosts. Central to this behavior are bacterial motility and chemotaxis, the ability to sense and move toward host-derived signals in the rhizosphere. In the soybean symbiont Bradyrhizobium diazoefficiens USDA110, motility is controlled by dual flagellar systems, and this strain contains three putative but uncharacterized chemotaxis operons (che1, che2, and che3). Using targeted deletions of all three predicted cheA genes, we show that cheA2 is the primary driver of chemotaxis toward soybean seed exudate in soft agar assays, and that the greater contribution of cheA2 vs. cheA1 in soft agar chemotaxis is due to its genomic context. Interestingly, we also found that B. diazoefficiens mutants that are incapable of chemotaxis in semisolid media retain wild type-like swimming speeds in aqueous media. These findings provide insight into how the agricultural inoculant B. diazoefficiens coordinates its chemosensory systems to respond to its host plant.