Functional integration of the Bradyrhizobium diazoefficiens flagellar systems

J.I. QUELAS; M.J. ALTHABEGOITI; E.J. MONGIARDINI; A.A. MELGAREJO; A.R. LODEIRO; C. JIMENEZ-SANCHEZ; J.J. ORTEGA-CALVO

Budapest

Congreso; 12th European Nitrogen Fixation Conference.; 2016

Biologicalperformance is often characterized by expressing emergent properties thatcannot be deduced from the functions of individual parts of the system.Flagella-driven swimming motility is among the most important performance ofbacterial cells, in terms of its energy requirement and its relevance forcolonization of diverse environments. Most bacterial species endowed withflagella possess one flagellar system, either monotrichous, lophotrichous orperitrichous, while some possess two independent flagellar systems in the samecell. An interesting genus in this regard is Bradyrhizobium, which includes the N2-fixing symbiontsof soybean. While B. diazoefficiens andB. japonicum possess a constitutivesubpolar system and an inducible lateral system, B. elkanii possesses only the subpolar system, and B. liaoningense lacks functionalflagella. However, all these species share the same soil environment andnodulate soybean. In search for the adaptive value of the dual flagellar systemof B. diazoefficiens, which may beexpressed simultaneously in liquid medium, we wondered whether their functions mightbe integrated for the single task of swimming in the water channels of the soil.By comparing the swimming behavior of the wild-type with that of mutantsexpressing either the subpolar or the lateral system, we observed that thewild-type swimming behavior does not arise from the additive effects of eachflagellar system. Moreover, the subpolar system is the main contributor topropulsion and chemotaxis, while the lateral system contributes to hold swimmingnear surfaces, in porous media, and in viscous environments. By artificialevolution we obtained a B. diazoefficiensderivative in which its subpolar flagellum contributed to higher swimmingspeed and its lateral flagella were expressed constitutively. While displayinga similar functional interaction of flagellar systems as the wild-type, thisderivative had higher dispersal in porous medium, higher swarming motility, aswell as it was more competitive for soybean nodulation in field assays andproduced higher grain yield. All these phenotypes were heritable because theywere maintained after passage through nodules and after more than 10 years ofuse in the laboratory. Hence, we decided to compare the complete genomesequence of this derivative against the wild-type, and found that none of thelateral flagellar genes was modified. By contrast, we found only 10 changes, 9of which were SNP and 1 was a small deletion. The changes occurred in ORFsencoding unknown proteins (3), transcription-related proteins (3), metabolicproteins (2), efflux transport proteins (1), and in an intergenic region (1).We are currently obtaining site-directed mutants in these loci to assess theireffects on bacterial motility and lateral flagella regulation.