Altered exopolysaccharides of Bradyrhizobium japonicum mutants correlate with impaired soybean lectin binding, but not with effective nodule formation |
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Authors: | Dale B Karr Rong-Ti Liang Bradley L Reuhs David W Emerich |
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Institution: | (1) Department of Biochemistry, 117 Schweitzer Hall, University of Missouri-Columbia, Columbia, MO 65211, USA, US;(2) Complex Carbohydrate Research Center, University of Georgia, Athens, GA 30602-4712, USA, GE |
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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.
Received: 6 October 1999 / Accepted: 18 November 1999 |
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Keywords: | : Bradyrhizobium Exopolysaccharide Glycine (nodulation) Lectin binding (soybean) Symbiosis |
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