Abstract
The structural variation of symbiotic signals released by rhizobia determines the specificity of their interaction with legume plants. Previous studies showed that Sinorhizobium strains from different phylogenetic lineages had different symbiotic performance on certain cultivated soybeans. Whether they released similar or different symbiotic signals remained unclear. In this study, we compared their nod and exo gene clusters and made a detailed structural analysis of Nod factors and EPS by ESI-MS/MS and two dimensions NMR. Even if there are some differences among nod or exo gene clusters; they produced much conserved Nod factor and EPS compositions. The Nod factors consist of a cocktail of β-(1, 4)-linked tri-, tetra-, and pentamers of N-acetyl-D-glucosamine (GlcNAc). The C2 position on the non-reducing terminal end is modified by a lipid chain that contains 16 or 18 atoms of carbon–with or without unsaturations-, and the C6 position on the reducing residue is decorated by a fucose or a 2-O-methylfucose. Their EPS are composed of glucose, galactose, glucuronic acid, pyruvic acid in the ratios 5:1:2:1 or 6:1:2:1. These findings indicate that soybean cultivar compatibility of Sinorhizobium strains does not result from Nod factor or EPS structure variations. The structure comparison of the soybean microbionts with other Sinorhizobium strains showed that Nod factor structures of soybean microbionts are much conserved, although there are no specific genes shared by the soybean microsymbionts. EPS produced by Sinorhizobium strains are different from those of Bradyrhizobium. All above is consistent with the previous deduction that Nod factor structures are related to host range, while those of EPS are connected with phylogeny.
Highlights
Symbiotic nitrogen fixation is a critical process for the legume plant when the soil is poor in nitrogen
III CCBAU05631) were isolated from nodules of soybean distributed in Huang-HuaiHai Plain, where Sinorhizobium strains were reported as being the dominant population of the soybean microbionts in such region (Li et al, 2011a,b; Zhang et al, 2011, 2012; Tian et al, 2012)
To investigate whether the strains having different soybean cultivar host range release different symbiotic signals, the genes involved in synthesis of Nod factors were first extensively studied
Summary
Symbiotic nitrogen fixation is a critical process for the legume plant when the soil is poor in nitrogen. This occurs within the plant roots in a specialized organ: the nodule (Murray, 2011). When legume plants secret flavonoids into the soil, nodulation genes in rhizobia can be induced to synthesize the Nod factors (lipo-chitin oligosaccharides: LCOs) (Firmin et al, 1986). Such molecules have been identified for plenty of rhizobial species, as N-acetyl-D-Glucosamine (GlcNAc) oligosaccharides bearing diverse decorations (Perret et al, 2000). Some of them were found to be involved in the specific nodulation of the host cultivars (Lopez-Baena et al, 2016)
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