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 beta-(1 --> 3),beta-(1 --> 6)-D-glucan synthesis locus of Brady- rhizobium japonicum is composed of at least two genes, ndvB and ndvC. Mutat ion in either gene affects glucan synthesis, as well as the ability of the bacterium to establish a successful symbiotic interaction with the legume h ost soybean (Glycine max). B. japonicum strain AB-14 (ndvB::Tn5) does not s ynthesize P-glucans, and strain AB-1 (ndvC::Tn5) synthesizes a cyclic beta- glucan lacking beta-(1 --> 6)-glycosidic bonds. We determined that the stru cture of the glucan synthesized by strain AB-1 is cyclodecakis-(1 --> 3)-be ta-D-glucosyl, a cyclic beta-(1 --> 3)-linked decasaccharide in which one o f the residues is substituted in the 6 position with beta-laminaribiose. Cy clodecakis-(1 --> 3)-beta-D-glucosyl did not suppress the fungal P-glucan-i nduced plant defense response in soybean cotyledons and had much lower affi nity for the putative membrane receptor protein than cyclic beta-(1 --> 3), beta-(1 --> 6)-glucans produced by wild-type B. japonicum. This is consiste nt with the hypothesis presented previously that the wild-type cyclic p-glu cans may function as suppressors of a host defense response.