The Two Chemotaxis Gene Clusters of Ensifer alkalisoli YIC4027(T), a Symbiont of Sesbania cannabina, Play Different Roles in Chemotaxis and Competitive Nodulation

Ensifer alkalisoli YIC4027(T) is a dominant rhizobium that has been isolated from the root nodules of Sesbania cannabina. Motility and chemotaxis are critical to maintaining competitiveness in establishing the symbiotic relationship. E. alkalisoli carries two gene clusters, che1 and che2, containing chemotaxis-related gene homologues. To determine the respective role of each gene cluster, we constructed mutants and compared them with the wild type in a free-living state and in symbiosis with the host plant. A swimming analysis revealed that the che1 cluster was the major pathway controlling the chemotaxis and swimming bias, while the che2 cluster had a minor role in these behaviors. However, the Delta che2 mutant was impaired in exopolysaccharide (EPS) production. During symbiosis, the Delta che1 mutant was more severely impaired in its competitive root colonization and nodulation ability than the Delta che2 mutant. Taken together, our data strongly suggested that both of the che clusters contribute to the competitive symbiotic association, the che1-like homologue being the main regulator of the chemotactic response and the che2 cluster regulating EPS production. These data illustrated a novel strategy of motile rhizobia bacteria to utilize the two pathways containing the homologous genes to enhance the efficiency of nodule formation by regulating distinct motility parameters or other cellular functions.