Submitted to: Nitrogen Fixation International Congress
Publication Type: Proceedings
Publication Acceptance Date: November 20, 1999
Publication Date: N/A
Interpretive Summary: Bradyrhizobium japonicum are beneficial bacterial 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 plant-microbe interactions especially in suppressing the plant's defense response, thereby allowing the development of a effective symbiosis. Previously, we described two genes required for the synthesis of beta-glucan. This paper describes a third gene that may be involved in control of the process. These results are important for scientists who are working to improve symbiotic nitrogen fixation and for understanding plant-microbe interactions.
The soybean microsymbiont, Bradyrhizobium japonicum, synthesizes cyclic B-(1,3),(1,6)-glucans during nodule development. The function of cyclic glucans in free-living bacteria is periplasmic osmoprotection. The role in symbiotic interactions is not yet clear, but we have postulated a role in the suppression of host-defense responses. We have isolated and characterized the cyclic B-glucan synthesis locus from B. japonicum and have identified two genes (ndvB, C) which are required for glucan synthesis and for symbiotic N-fixation. Mutation of ndvB abolished glucan synthesis altogether, while mutation of ndvC resulted in glucans with altered glycosidic linkages. Involvement of another gene (ORF1) located upstream of ndvB was determined by mutagenesis, deletion analysis, and complementation. ORF1, which we named ndvD, is required for glucan synthesis and for successful interactions with soybean. Analysis of the deduced amino acid sequence suggests a regulatory function for ndvD.