Characterization of chemotaxis in soybean symbiont <i>Bradyrhizobium diazoefficiens</i>

ABSTRACT 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 che2 is the primary driver of chemotaxis toward soybean seed exudate in soft agar assays, and that the greater contribution of che2 vs. che1 to soft agar chemotaxis is not due to differences in CheA protein sequence. Interestingly, we also find that B. diazoefficiens mutants incapable of chemotaxis in semisolid media retain wild-type-like swimming speeds in aqueous media. These findings provide insight into how B. diazoefficiens coordinates its chemosensory systems to respond to its host plant. IMPORTANCE Chemotaxis is crucial for the establishment of beneficial plant-microbe associations, yet mechanistic studies of chemotaxis have been limited to a handful of bacterial models. The soybean symbiont Bradyrhizobium diazoefficiens USDA110 is a commonly used soybean inoculant with exceptional nitrogen fixation efficiency, but the genetic control of chemotaxis in B. diazoefficiens has not been examined. Establishing B. diazoefficiens as a model of chemotaxis provides an opportunity to understand how multiple chemotaxis systems coordinate root colonization in this major agricultural symbiont and can enable comparative analyses of plant-microbe recognition strategies across agricultural bacteria.