Structure of the unusual Sinorhizobium fredii HH103 lipopolysaccharide and its role in symbiosis

Abstract

Rhizobia are soil bacteria that form important symbiotic associations with legumes, and rhizobial surface polysaccharides, such as K-antigen polysaccharide (KPS) and lipopolysaccharide (LPS), might be important for symbiosis. Previously, we obtained a mutant of Sinorhizobium fredii HH103, rkpA, that does not produce KPS, a homopolysaccharide of a pseudaminic acid derivative, but whose LPS electrophoretic profile was indistinguishable from that of the wild-type strain. Also, we previously demonstrated that the HH103 rkpLMNOPQ operon is responsible for Pse5NAc7(3OHBu) production and is involved in HH103 KPS and LPS biosynthesis and that an HH103 rkpM mutant cannot produce KPS and displays an altered LPS structure. Here, we analyzed the LPS structure of HH103 rkpA, focusing on the carbohydrate portion and found that it contains a highly heterogeneous lipid A and a peculiar core oligosaccharide composed of an unusually high number of hexuronic acids and containing β-configured 5-acetamido-3,5,7,9-tetradeoxy-7-(3-hydroxybutyramido)-L-glycero-L-manno-nonulosonic acid [β-Pse5NAc7(3OHBu)]. This pseudaminic acid derivative, in its α-configuration, was the only structural component of the S. fredii HH103 KPS and, to the best of our knowledge, has never been reported from any other rhizobial LPS. We also show that Pse5NAc7(3OHBu) is the complete or partial epitope for a monoclonal antibody, NB6-228.22, that can recognize the HH103 LPS, but not those of most of the S. fredii strains tested here. We also show that the LPS from HH103 rkpM is identical to that of HH103 rkpA, but devoid of any Pse5NAc7(3OHBu) residues. Notably, this rkpM mutant was severely impaired in symbiosis with its host Macroptilium atropurpureum.

Rhizobial Exopolysaccharides: Genetic Regulation of Their Synthesis and Relevance in Symbiosis with Legumes

“…The other gene that is present in the rkp-2 region, lpsL, codes for a UDP-glucuronate 4-epimerase which catalyzes the conversion of GlcA into GalA. Both uronic acids are present in the S. fredii HH103 LPS [162], which justifies the fact that HH103 mutants in either rkpK or lpsL exhibited altered forms of LPS. As mentioned before, the R. leguminosarum bv.…”

Rhizobial Exopolysaccharides: Genetic Regulation of Their Synthesis and Relevance in Symbiosis With Legumes

“…The other gene that is present in the rkp-2 region, lpsL, codes for a UDP-glucuronate 4epimerase which catalyzes the conversion of GlcA into GalA. Both uronic acids are present in the S. fredii HH103 LPS [154], which justifies the fact that HH103 mutants in either rkpK or lpsL exhibited altered forms of LPS. As mentioned before, the R. leguminosarum bv.…”

Unraveling the sugar code: the role of microbial extracellular glycans in plant–microbe interactions

“…Furthermore, purified LPSs from phytopathogenic Xylella fastidiosa evoke defense responses independent of the presence of the O-antigen polysaccharide, while in vivo studies demonstrated the importance of this polysaccharide chain in shielding immunogenic surface structures and, thereby, delaying immunorecognition ( Rapicavoli et al ., 2018 ). Additionally, the rhizobial LPS from Sinorhizobium fredii HH103 is unique in its structure and was recently reported to have a role in the symbiotic establishment with its host legume Macroptilium atropurpureum and Cajanus cajan ( Di Lorenzo et al ., 2020 ). Sinorhizobium fredii HH103 core oligosaccharide differs from previously described rhizobial core oligosaccharides by the presence of a high number of hexuronic acids and a β-configured pseudaminic acid derivative.…”

“…Sinorhizobium fredii HH103 core oligosaccharide differs from previously described rhizobial core oligosaccharides by the presence of a high number of hexuronic acids and a β-configured pseudaminic acid derivative. LPS from the S. fredii HH103 rkpM mutant lacks the pseudaminic acid derivative residues, leading to severely impaired nodule formation and symbiosis ( Di Lorenzo et al ., 2020 ). While previous studies already reported symbiotic impairments of the S. fredii HH103 rpkM strain with different legume hosts, now the responsible structural motifs were described ( Margaret et al ., 2012 ; Acosta-Jurado et al ., 2016 ; Di Lorenzo et al ., 2020 ).…”

“…LPS from the S. fredii HH103 rkpM mutant lacks the pseudaminic acid derivative residues, leading to severely impaired nodule formation and symbiosis ( Di Lorenzo et al ., 2020 ). While previous studies already reported symbiotic impairments of the S. fredii HH103 rpkM strain with different legume hosts, now the responsible structural motifs were described ( Margaret et al ., 2012 ; Acosta-Jurado et al ., 2016 ; Di Lorenzo et al ., 2020 ). Although these data suggest recognition of LPS sugar modules, questions regarding the components mediating their recognition and signaling remain.…”