Further studies of the role of cyclic beta-glucans in symbiosis. An NdvC mutant of Bradyrhizobium japonicum synthesizes cyclodecakis-(1-->3)-beta-glucosyl.

The cyclic β-(1→3),β-(1→6)-d-glucan synthesis locus of Bradyrhizobium japonicum is composed of at least two genes, ndvB and ndvC . Mutation in either gene affects glucan synthesis, as well as the ability of the bacterium to establish a successful symbiotic interaction with the legume host soybean ( Glycine max ). B. japonicum strain AB-14 ( ndvB ::Tn 5 ) does not synthesize β-glucans, and strain AB-1 ( ndvC ::Tn 5 ) synthesizes a cyclic β-glucan lacking β-(1→6)-glycosidic bonds. We determined that the structure of the glucan synthesized by strain AB-1 is cyclodecakis-(1→3)-β-d-glucosyl, a cyclic β-(1→3)-linked decasaccharide in which one of the residues is substituted in the 6 position with β-laminaribiose. Cyclodecakis-(1→3)-β-d-glucosyl did not suppress the fungal β-glucan-induced plant defense response in soybean cotyledons and had much lower affinity for the putative membrane receptor protein than cyclic β-(1→3),β-(1→6)-glucans produced by wild-type B. japonicum . This is consistent with the hypothesis presented previously that the wild-type cyclic β-glucans may function as suppressors of a host defense response.