Characterization of surface motility in Sinorhizobium meliloti: regulation and role in symbiosis

Abstract

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 flagellaindependent 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.

2‐Tridecanone impacts surface‐associated bacterial behaviours and hinders plant–bacteria interactions

“…Interestingly, the same effect was observed in response to airborne 2‐TDC generated from solutions applied onto the lid of the Petri dish. The stimulatory effect was more pronounced using less permissive conditions for surface migration MM (1% agar) (Bernabéu‐Roda et al ., ). Airborne 2‐TDC generated from solutions containing from 0.2 to 10 µmol of the MK was able to stimulate surface motility with similar dose‐dependent effects on GR4 and Rm1021 (Fig.…”

“…Airborne 2‐TDC generated from solutions containing from 0.2 to 10 µmol of the MK was able to stimulate surface motility with similar dose‐dependent effects on GR4 and Rm1021 (Fig. A), two S. meliloti strains which exhibit different modes of surface translocation (Bernabéu‐Roda et al ., ). In contrast, swimming motility remained unaffected in the presence of 2‐TDC (Supporting Information Fig.…”

“…We sought to investigate the mechanism by which 2‐TDC specifically promotes surface migration in S. meliloti . It is known that inactivation of the fadD gene promotes surface motility in S. meliloti GR4 and Rm1021 by stimulating a flagella‐independent mechanism exhibited by the surface spreading displayed by the flagella‐less GR4fadDflaAB and 1021fadDflaAB strains (Bernabéu‐Roda et al ., ; Fig. ).…”

“…Data about genetic determinants and environmental cues that control surface motility and biofilm formation in rhizobia, as well as the role that these two traits play in the plantmicrobe interaction, are scarce. Swarming motility has been reported for the alfalfa symbiont Sinorhizobium meliloti strains GR4 and Rm1021, although the latter can also translocate over surfaces using a type of flagellumindependent motility, which requires the amphiphilic siderophore rhizobactin 1021 (Nogales et al, 2010;Nogales et al, 2012;Bernab eu-Roda et al, 2015). Recent studies performed with these rhizobial strains revealed the existence of mechanisms which regulate swarming and biofilm formation inversely to promote efficient plant root colonization (Amaya-G omez et al, 2015).…”

Hormone modulation of legume‐rhizobial symbiosis

“…Strikingly, GR24 promoted swarming motility in S. meliloti in a dose‐dependent manner but had little effect on the growth rate of S. meliloti 1021 (Soto et al ; Pelaez‐Vico et al ). S. meliloti could exclusively use swarming to spread on surfaces and soil, and a nonmotile flaAB mutant showed a slight deficiency in this symbiotic phenotype with fewer nodules and less nodule occupation (Bernabéu‐Roda et al ). Based on these data, we speculate that NFs‐induction of SLs directly promotes rhizobial infection by increasing bacterial swarming motility.…”