CONICET | Buscador de Institutos y Recursos Humanos

Bradyrhizobium diazoefficiens is a motile soil a-proteobacterium that can live in free-living state or in N2-fixing symbiosis with soybean. Thanks to this ability, B. diazoefficiens is used as commercial inoculants for soybean crops. Previous works in our lab showed that motility and distribution of theserhizobia in soil are determining factors of their symbiotic efficiency.There are diverse modes of bacterial motility among which swimming of individual cells in liquid media, and swarming of multicellular layers on solid or semi-solid surfaces. Both are propelled by flagella.B. diazoefficiens has two different flagellar systems: a subpolar system that is constitutively expressed and a lateral system that is inducible. Each system is composed of different sets of proteins, and both contribute to swimming and swarming.Lateral flagella induction is affected by the type of carbon source, viscosity, pH, or growth in liquid or semi-solid agar. FliL is a flagellar protein that is important for bacterial motility, whose role remainsunclear despite it has been studied in several bacterial species. It seems to be associated to the flagellar basal body and would contribute to the stability of the flagellar structure when flagellar load increases. B. diazoefficiens USDA 110 have two putative fliL, each one related to the subpolar or lateral flagellar gene clusters (fliL1 and fliL2, respectively). According to TMHMM 2.0 server, both FliL have a single trans-membrane domain in the N-terminal position, but it seems that FliL2 is periplasmic, whereas all FliL1 appear to reside in the cytoplasm.In order to study their roles, we obtained unmarked in-frame deletional mutants for each gene in the B. diazoefficiens type strain USDA 110. These two mutants (named LP5826 and LP6868 for fliL1 and fliL2 deletions) were evaluated for swimming in liquid, semi-solid and viscous media. Also, LP6868 was analyzed for swarming.In semi-solid medium, only the diameter of LP6868 swimming halo was smaller than the wild-type, even when the subpolar and lateral filaments were expressed normally, as judged by SDS-PAGE of flagellins and electron microscopy of whole cells. This difference in motility was steeper in viscous medium (5% PVP)These results suggest that in this fliL2 deletion mutant the lateral flagella were stopped and/or had less torque than the wild-type flagella.In contrast, LP6868 can swarm a little more than the wild-type, and the edge of swarming colony presented elongated cells. Similar results were obtained in a previous work in Proteus mirabilis fliL mutants.In summary, our results showed that FliL2, but not FliL1, have effects on B. diazoefficiens swimming and swarming behavior. The role of FliL1 in subpolar flagellum remains unclear.