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 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. IMPORTANCE: Chemotaxis is crucial for the establishment of beneficial plant-microbe associations, yet mechanistic studies of chemotaxis have been limited to a handful of soil bacterial models, namely Azospirillum brasilense , Sinorhizobium meliloti , and Rhizobium leguminosarum . These three models represent only a fraction of the diversity found among plant- beneficial bacteria and agricultural inoculants. 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.