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 (Glycine max [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.

Improved Soybean Root Association of N-Starved Bradyrhizobium japonicum

“…1). This growth rate agrees with previous reports of B. japonicum growth with mannitol as the carbon source (25). Rhizobia are able to satisfy their N needs in very scarce environments by means of their two GS enzymes (12), which together form a highly efficient N-assimilating system.…”

Production of exopolysaccharide by Rhizobium leguminosarum bv. trifolii and its role in bacterial attachment and surface properties

“…The authors tested mannitol and malate as carbon sources and indicated that the amount of synthesized EPS was higher in the presence of the former than the latter carbon source, whereas EPS composition was the same with both these carbon sources. In contrast, Karr et al (2000) found for another B. japonicum strain 2143 that the type of the carbon source may also affect the composition of this polysaccharide. Similarly, Staudt et al (2012) provided evidence for Rhizobium tropici CIAT899, indicating that the chemical composition of EPS can vary with changes to the growth environment.…”

Alfalfa Root Nodule Invasion Efficiency Is Dependent on Sinorhizobium meliloti Polysaccharides

“…For example, Bradyrhizobium japonicum strain 2143 and two derivative strains are capable of producing three exopolysaccharides that appear to be involved in the efficiency of their symbioses with Glycine max (18), and B. japonicum strain USDA 123 produces two structurally distinct polysaccharides, one when outside the nodule and the second when inside the nodule (2). Additionally, the symbiotic defects of EPS-deficient mutants of B. japonicum strain 110spc4 are host dependent, differing markedly on the hosts Glycine max and Glycine soja (33), and there are many other rhizobia for which production of specific polysaccharides is important for symbiosis, including Rhizobium sp.…”