Submitted to: Canadian Journal of Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/24/2002
Publication Date: N/A
Interpretive Summary: Bradyrhizobium japonicum are beneficial bacteria that 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 if 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 an effective symbiosis. Previously, we described two genes, ndvB,C required for the synthesis of beta-glucans. This publication describes the discovery and characterization of a third gene involved in the synthesis of these interesting compounds. We have formulated a hypothesis for the mechanism of their synthesis. This work is a step in the elucidation of how the bacteria suppress a plant defense response during the development of a nitrogen-fixing symbiosis and this is important in our efforts to improve the symbiotic association. This work will be used by scientists working to improve agricultural productivity.
Technical Abstract: Previously, we identified two genes in Bradyrhizobium japonicum (ndvB, ndvC) that are required for beta-glucan synthesis and successful symbiotic interactions with soybean. In this study, we report a new open reading frame (ORF1) located in the intergenic region between ndvB and ndvC, which is essential for beta-glucan synthesis and effecive nodulation of Glycine max (soybean). This gene is designated ndvD (nodule development). The ndvD translation product has a predicted molecular weight of 26.4 kDa with one transmembrane domain. Genetic experiments involving gene deletion, Tn5 insertion, and gene complementation revealed that mutation of ndvD generated pleiotropic phenotypes including hypoosmotic sensitivity, reduced motility, and defects in conjugative gene transfer. Although deficient in in vivo beta-glucan synthesis, the membrane preparations from the ndvD mutant synthesized neutral beta-glucans in vitro. Therefore, ndvD does not appear to be a structural gene for B-glucan synthesis. Our hypothesis for the mechanism of beta-glucan synthesis is presented.