Symbiotic conditions induce structural modifications of Sinorhizobium sp. NGR234 surface polysaccharides

When the rhizosphere is starved of nitrogen, the soil bacteria Rhizobium are able to infect legume roots and invade root nodules, where they can fix atmospheric nitrogen. Nod boxes, the nod gene promoters located on the rhizobial symbiotic plasmid, are activated by means of flavonoids present in the legume root exudates, leading to the synthesis of lipochitooligomers: the Nod factors. Several recent works pointed out the importance of rhizobial surface polysaccharides in establishing the highly specific symbiosis between rhizobia and legumes. Lipopolysaccharides (LPSs) exhibit specific active roles in the later stages of the nodulation processes, such as the penetration of the infection thread into the cortical cells or the setting up of the nitrogen-fixing phenotype. The study reported here concerns the structural modifications affecting surface (lipo)polysaccharides when Sinorhizobium sp. NGR234 strains are grown with nod gene induction under nitrogen starvation. In the absence of induction, NGR234 only produces fast-migrating LPSs. When cultured in the presence of flavonoids, the same strain produces large quantities of a high-molecular-weight rhamnose-rich lipopolysaccharide (RLPS). Because the synthesis of this compound seems to be coded by the symbiotic plasmid under direct or indirect gene induction by flavonoids, this RLPS is thought to be biologically relevant.