Bradley L. Reuhs

Chronic intracellular infection of alfalfa nodules by Sinorhizobium meliloti requires correct lipopolysaccharide core.

Our analyses of lipopolysaccharide mutants of Sinorhizobium meliloti offer insights into how this bacterium establishes the chronic intracellular infection of plant cells that is necessary for its nitrogen-fixing symbiosis with alfalfa. Derivatives of S. meliloti strain Rm1021 carrying an lpsB mutation are capable of colonizing curled root hairs and forming infection threads in alfalfa in a manner similar to a wild-type strain. However, developmental abnormalities occur in the bacterium and the plant at the stage when the bacteria invade the plant nodule cells. Loss-of-function lpsB mutations, which eliminate a protein of the glycosyltransferase I family, cause striking changes in the carbohydrate core of the lipopolysaccharide, including the absence of uronic acids and a 40-fold relative increase in xylose. We also found that lpsB mutants were sensitive to the cationic peptides melittin, polymyxin B, and poly-l-lysine, in a manner that paralleled that of Brucella abortus lipopolysaccharide mutants. Sensitivity to components of the plants innate immune system may be part of the reason that this mutant is unable to properly sustain a chronic infection within the cells of its host-plant alfalfa.

Lipopolysaccharides and K-Antigens: Their Structures, Biosynthesis, and Functions

The bacterial surface is the first line of defense against antimicrobial molecules and stress caused by changes in the environment surrounding the bacterium. In the case of plant- and animal-microbe interactions, many bacterial cell surface molecules are important virulence determinants. Thus, in order to understand the molecular basis for bacterial-plant interactions, it is important to characterize the molecular architecture of the bacterial cell surface, and how the bacterium modifies this architecture in response to its different environments, including its in planta environment.

The presence of a novel type of surface polysaccharide in Rhizobium meliloti requires a new fatty acid synthase-like gene cluster involved in symbiotic nodule development

Bacterial exopolysaccharide (EPS) and lipopolysaccharide (LPS) molecules have been shown to play important roles in plant‐bacterium interactions. Here we have demonstrated that the fix‐23 loci, which compensate for exo mutations during symbiotic nodule development, are involved in the production of a novel polysaccharide that is rich in 3‐deoxy‐D manno‐2‐octulosonic acid (Kdo) but is not the classical LPS. This molecule is likely to be a surface antigen since antiserum to whole Rhizobium meliloti cells reacts strongly with it, and since mutations in fix‐23 result in an inability to produce this polysaccharide and to bind bacteriophage 16‐3. It is likely that this Kdo‐rich polysaccharide is analogous to certain Escherichia coli K‐antigens which are anchored to the membrane via a phospholipid moiety. DNA sequence analysis of one gene cluster of this region revealed that the predicted protein products of six genes exhibit a high degree of homology and similar organization to those of the rat fatty acid synthase multifunctional enzyme domains.

Altered exopolysaccharides of Bradyrhizobium japonicum mutants correlate with impaired soybean lectin binding, but not with effective nodule formation.

Abstract. The exact mechanism(s) of infection and symbiotic development between rhizobia and legumes is not yet known, but changes in rhizobial exopolysaccharides (EPSs) affect both infection and nodule development of the legume host. Early events in the symbiotic process between Bradyrhizobium japonicum and soybean (Glycinemax [L.] Merr.) were studied using two mutants, defective in soybean lectin (SBL) binding, which had been generated from B. japonicum 2143 (USDA 3I-1b-143 derivative) by Tn5 mutagenesis. In addition to their SBL-binding deficiency, these mutants produced less EPS than the parental strain. The composition of EPS varied with the genotype and with the carbon source used for growth. When grown on arabinose, gluconate, or mannitol, the wild-type parental strain, B. japonicum 2143, produced EPS typical of DNA homology group I Bradyrhizobium, designated EPS I. When grown on malate, strain 2143 produced a different EPS composed only of galactose and its acetylated derivative and designated EPS II. Mutant 1252 produced EPS II when grown on arabinose or malate, but when grown on gluconate or mannitol, mutant 1252 produced a different EPS comprised of glucose, galactose, xylose and glucuronic acid (1:5:1:1) and designated EPS III. Mutant 1251, grown on any of these carbon sources, produced EPS III. The EPS of strain 2143 and mutant 1252 contained SBL-binding polysaccharide. The amount of the SBL-binding polysaccharide produced by mutant 1252 varied with the carbon source used for growth. The capsular polysaccharide (CPS) produced by strain 2143 during growth on arabinose, gluconate or mannitol, showed a high level of SBL binding, whereas CPS produced during growth of strain 2143 on malate showed a low level of SBL binding. However, the change in EPS composition and SBL binding of strain 2143 grown on malate did not affect the wild-type nodulation and nitrogen fixation phenotype of 2143. Mutant 1251, which produced EPS III, nodulated 2 d later than parental strain 2143, but formed effective, nitrogen-fixing tap root nodules. Mutant 1252, which produced either EPS II or III, however nodulated 5–6 d later and formed few and ineffective tap root nodules. Restoration of EPS I production in mutant 1252 correlated with restored SBL binding, but not with wild-type nodulation and nitrogen fixation.

Rhizobial Capsular and Lipopolysaccharides: Evidence for their Importance in Rhizobium-Legume Symbiosis

The cell surface of rhizobia is comprised of a number of polysaccharides; extracellular (EPSs), capsular (KPSs), and lipopolysaccharides (LPSs). Each is important in forming an effective nitrogen-fixing symbiosis. Defective mutants are unable to invade the host root cortical cells in a normal manner, or the infection thread is aborted prior to cortical cell invasion (1,2).

Correlation of Lipopolysaccharide Structural Defects with Genetic Lesions in Rhizobium Etli Ce3

Rhizobial extracellular, capsular, and lipopolysaccharides (EPSs, CPSs, and LPSs, respectively) have all been shown to play essential roles in the symbiotic infection of the host legume [1]. In this brief report, the LPSs from Rhizobium etli CE3 and several mutants defective in symbiosis and in LPS structure are discussed.