Characterization of surface motility in <Emphasis Type="Italic">Sinorhizobium meliloti</Emphasis>: regulation and role in symbiosis

Sinorhizobium meliloti can exhibit diverse modes of surface translocation whose manifestation depends on the strain. The mechanisms involved and the role played by the different modes of surface motility in the establishment of symbiosis are largely unknown. In this work, we have characterized the surface motility shown by two S. meliloti reference strains (Rm1021 and GR4) under more permissive conditions for surface spreading and analyzed the symbiotic properties of two flagella-less S. meliloti mutants with different behavior on surfaces. The use of Noble agar in semisolid minimal medium induces surface motility in GR4, a strain described so far as non-motile on surfaces. The motility exhibited by GR4 is swarming as revealed by the non-motile phenotype of the flagella-less flaAB mutant. Intriguingly, a flgK mutation which also abolishes flagella production, triggers surface translocation in GR4 through an as yet unknown mechanism. In contrast to GR4, Rm1021 moves over surfaces using mostly a flagella-independent motility which is highly reliant on siderophore rhizobactin 1021 production. Surprisingly, this motility is absent in a flagella-less flgE mutant. In addition, we found that fadD loss-of-function, known to promote surface motility in S. meliloti, exerts different effects on the two reference strains: while fadD inactivation promotes a flagella-independent type of motility in GR4, the same mutation interferes with the surface translocation exhibited by the Rm1021 flaAB mutant. The symbiotic phenotypes shown by GR4flaAB and GR4flgK, non-flagellated mutants with opposite surface motility behavior, demonstrate that flagella-dependent motility positively influences competitiveness for nodule occupation, but is not crucial for optimal infectivity.