A Rhizobium leguminosarum Lipopolysaccharide Lipid-A Mutant Induces Nitrogen-Fixing Nodules with Delayed and Defective Bacteroid Formation

Abstract

Lipopolysaccharides from pea-nodulating strain Rhizobium leguminosarum by. viciae 3841, as all other members of the family Rhizobiaceae with the possible exception of Azorhizobium caulinodans, contains a very long chain fatty acid; 27-hydroxyoctacosanoic acid (27OHC28:0) in its lipid A region. The exact function and importance of this residue, however, is not known. In this work, a previously constructed mutant, Rhizobium leguminosarum by. viciae 22, deficient in the fatty acid residue, was analyzed for its symbiotic phenotype. While the mutant was able to form nitrogen-fixing nodules, a detailed study of the timing and efficiency of nodulation using light and electron microscopy showed that there was a delay in the onset of nodulation and nodule tissue invasion. Further, microscopy showed that the mutant was unable to differentiate normally forming numerous irregularly shaped bacteroids, that the resultant mature bacteroids were unusually large, and that several bacteroids were frequently enclosed in a single symbiosome membrane, a feature not observed with parent bacteroids. In addition, the mutant nodules were delayed in the onset of nitrogenase production and showed reduced nitrogenase throughout the testing period. These results imply that the lack of 27OHC28:0 in the lipid A in mutant bacteroids results in altered membrane properties that are essential for the development of normal bacteroids.

Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium –legume symbiosis

“…Bacteroids of LPS mutants are abnormal (29)(30)(31). The lpsB mutant of S. meliloti was found to be more sensitive to antimicrobial peptides (30).…”

Rhizobium etli CE3 Bacteroid Lipopolysaccharides Are Structurally Similar but Not Identical to Those Produced by Cultured CE3 Bacteria

“…In the case of indeterminate nodules, it is possible that the maintenance of single occupancy symbiosomes results from more intimate contact between the dividing bacterial cell and the PM, whereas the latter determinate process involves detachment of the bacterial and PM cell. In the former case, we have reported that an R. leguminosarum biovar viciae mutant that is defective in the synthesis of the very long fatty acid moiety of its lipid A is also defective in bacteroid formation, and results in multiple occupancy symbiosomes (52,53). It is important to investigate the characteristics of the surface polysaccharides of bacteroids in these and other symbiotic systems in order to be able to fully understand the mechanism of symbiosis.…”

Characterization of Galacturonosyl Transferase Genes rgtA, rgtB, rgtC, rgtD, and rgtE Responsible for Lipopolysaccharide Synthesis in Nitrogen-fixing Endosymbiont Rhizobium leguminosarum

“…Detailed microscopy studies on the development of nodules and bacteroids for each of these mutants is required to fully understand any possible symbiotic function of these GalA residues. These more detailed studies are needed particularly in view of the already published data showing that disruption of acpXL, the gene that encodes the acyl carrier protein required for the synthesis of the very long chain fatty acid that is present on the lipid A, also results in nitrogen-fixing nodules; however, microscopic examination showed that bacteroid formation was severely affected (31)(32)(33). This work is currently in progress for each of the rgt mutants and will be described in an additional manuscript.…”