Identification of an Exopolysaccharide Biosynthesis Gene in Bradyrhizobium diazoefficiens USDA110.

Chunxia Xu,Weijun Dai,Christian Staehelin,Wenjie Cai,Huaqin Ruan

doi:10.3390/microorganisms9122490

Abstract

Exopolysaccharides (EPS) play critical roles in rhizobium-plant interactions. However, the EPS biosynthesis pathway in Bradyrhizobium diazoefficiens USDA110 remains elusive. Here we used transposon (Tn) mutagenesis with the aim to identify genetic elements required for EPS biosynthesis in B. diazoefficiens USDA110. Phenotypic screening of Tn5 insertion mutants grown on agar plates led to the identification of a mutant with a transposon insertion site in the blr2358 gene. This gene is predicted to encode a phosphor-glycosyltransferase that transfers a phosphosugar onto a polyprenol phosphate substrate. The disruption of the blr2358 gene resulted in defective EPS synthesis. Accordingly, the blr2358 mutant showed a reduced capacity to induce nodules and stimulate the growth of soybean plants. Glycosyltransferase genes related to blr2358 were found to be well conserved and widely distributed among strains of the Bradyrhizobium genus. In conclusion, our study resulted in identification of a gene involved in EPS biosynthesis and highlights the importance of EPS in the symbiotic interaction between USDA110 and soybeans.

Highlights

The EPS of Bradyrhizobium diazoefficiens (B. japonicum) USDA110 was found to play an important role in the symbiosis with soybean plants [8,9,10]
Our study suggests that Blr2358 participates in EPS biosynthesis, and affects the symbiotic interaction between B. diazoefficiens and soybeans
To identify genetic elements of the entire genome involved in polysaccharide biosynthesis, we conducted a Tn5 mutagenesis screen in B. diazoefficiens strain USDA110

Summary

Introduction

Nodule formation requires rhizobial exopolysaccharides (EPS) or EPS-derived oligosaccharides [1,2,3,4,5,6,7]. EPS is dispensable for the symbiosis between Sinorhizobium (Ensifer) fredii HH103 and soybeans [11]. The absence of EPS or the synthesis of altered oligosaccharides can impair the rhizobial root hair infection process, suggesting that EPS contributes to the suppression of plant defense reactions in compatible interactions [14]. EPS or oligosaccharides derived from EPS possess a signaling role during rhizobial infection. The LysM receptor kinase EPR3 in L. japonicus has been identified as a first EPS receptor, suggesting that EPS affects symbiotic signaling in host plants [12]

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Results

Conclusion