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In the rhizobia-legume symbiotic interaction, bacterial surface polysaccharides, such as exopolysaccharide (EPS), lipopolysaccharide (LPS), K-antigen polysaccharide (KPS) or cyclic glucans (CG), appear to play crucial roles either acting as signals required for the progression of the interaction and/or preventing host defence mechanisms. The symbiotic significance of each of these polysaccharides varies depending on the specific rhizobia-legume couple. In this work we show that the production of exopolysaccharide by  Sinorhizobium fredii  HH103, but not by other  S .  fredii  strains such as USDA257 or NGR234, is repressed by  nod  gene inducing flavonoids such as genistein and that this repression is dependent on the presence of a functional NodD1 protein. In agreement with the importance of EPS for bacterial biofilms, this reduced EPS production upon treatment with flavonoids correlates with decreased biofilm formation ability. By using quantitative RT-PCR analysis we show that expression of the  exoY2  and  exoK  genes is repressed in late stationary cultures of  S .  fredii  HH103 upon treatment with genistein. Results presented in this work show that in  S .  fredii  HH103 EPS production is regulated just in the opposite way than other bacterial signals such as Nod factors and type 3 secreted effectors: it is repressed by flavonoids and NodD1 and enhanced by the  nod  repressor NolR. These results are in agreement with our previous observations showing that lack of EPS production by  S .  fredii  HH103 is not only non-detrimental but even beneficial for symbiosis with soybean.

Exopolysaccharide Production by Sinorhizobium fredii HH103 Is Repressed by Genistein in a NodD1-Dependent Manner