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“Background Nitrogen-fixing symbiotic bacteria, commonly known as rhizobia, employ a variety of strategies which allow them to exist in the soil and adapt to various environmental conditions

prior to infecting leguminous plant hosts. Rhizobial cell surface components, exopolysaccharide (EPS) and lipopolysaccharide (LPS), play an important role in determining the symbiotic competence of rhizobia, root tissue invasion and induction of nitrogen-fixing nodules on host plants forming indeterminate-type nodules, such as Pisum, Trifolium, Vicia, and Medicago spp. [1–4]. Acidic EPSs secreted in large amounts by rhizobia Edoxaban are species-specific compounds consisting of common sugars substituted with non-carbohydrate residues [1, 4–6]. EPS of Rhizobium leguminosarum is a heteropolymer consisting of octasaccharide subunits composed of five glucose residues, one galactose, and two glucuronic acid residues, additionally decorated with acetyl, pyruvyl, and 3-hydroxybutyryl groups [7, 8]. EPS-deficient mutants or those with an altered LPS structure are impaired in nodule cell invasion and nitrogen fixation [1, 6, 9–11]. Biosynthesis of EPS in R. leguminosarum is a multi-step process requiring the expression of several pss genes, located in the major EPS cluster on the chromosome [12, 13].