Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 1, 1998
Publication Date: N/A
Interpretive Summary: Bradyrhizobium japonicum are beneficial bacteria which form symbiotic associations with soybeans. This symbiosis supplies the plant with a source of nitrogen which can be used for protein synthesis. Improvement of the symbiotic association is important for decreasing the fertilizer nitrogen requirement of soybeans and for sustainable agriculture. Cyclic beta-glucans are small molecules composed solely of glucose, produced by the bacteria and are important in the plant-microbe interactions, but the mode of action is not known. How legume plants suppress the host defense response that normally occurs in response to microbial invasion during the process of symbiotic nodule development also is not known. This paper describes experiments that led us to hypothesize that the cyclic beta-glucans may function to suppress the soybean's defense response during nodule development. These results are important for scientists who are working to improve symbiotic nitrogen fixation and for understanding plant-microbe interactions.
Periplasmic cyclic B-(1 3),(1 6)-glucans of Bradyrhizobium japonicum were shown to suppress a fungal (Phytopthora sojae) B-glucan-induced host defense response in soybean cotyledon assays. B. japonicum strain AB-14 (ndvB::Tn5) which did not synthesize B-glucan and strain AB-1 (ndvC::Tn5) which synthesizes cyclic B-(1-3)-glucan (apparently defective in synthesis of B-(1 6)-glycosidic bonds in the cyclic B-glucan molecule), were both defective in nodule morphogenesis and had elevated levels of glyceollin in the nodule tissue. The structure of the glucan synthesized by strain AB-1 is a cyclic decasaccharide (cyclolaminarinose) in which one of the residues is substituted in its 6 position with B-laminaribiose. Consistent with the proposed suppressor function for the wild-type cyclic B-(1-3),(1-6)-glucans (Mithofer et al., Planta 199: 270-275, 1996), the branched cyclodecalaminarinose did not suppress a fungal B-glucan-induced plant defense response in soybean cotyledons. The data is consistent with the hypothesis that cyclic B- glucans of B. japonicum function to suppress the host s defense response during nodule development and symbiotic nitrogen fixation.