Submitted to: Journal of Bacteriology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/20/1996
Publication Date: N/A
Interpretive Summary: Interpretive Summary: Bradyrhizobium japonicum are beneficial bacteria which form symbiotic associations with soybeans that supply 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 B-glucans are small molecules composed solely of glucose and are important in the plant-microbe interactions, but the mode of action is not known. This paper describes the identification of a new gene (ndvC) involved in the synthesis of these molecules. Two genes are now known to be involved in synthesis and a mutation in ndvC leads to synthesis of an abnormal cyclic B-glucan; one which does not allow development of a normal symbiosis. This provides evidence for structural specificity of these molecules for plant-microbe interactions. This work is important for scientists who are working to develop biological methods to improve soybean productivity and to develop biological methods to control plant pathogens.
Technical Abstract: Bradyrhizobium japonicum synthesizes periplasmic cyclic B-(1- 3),B-(1-6)-D-glucans during growth in hypoosmotic environments and evidence is growing that these molecules may have a specific function during plant-microbe interactions in addition to osmoregulation. Site-directed Tn5 mutagenesis of a DNA region upstream of ndvB resulted in identification of a new gene (ndvC) involved in B-(1-3),B-(1-6)-glucan synthesis and in nodule development. The predicted translation product was a polypeptide (ca. 77 kDa) with several hydrophobic domains. It contained a consensus nucleoside-sugar binding motif and had 47 % similarity with an endo-B-(1-3)-glucanase from Saccharomyces cerevisiae. B. japonicum carrying a Tn5 insertion in ndvC resulted in synthesis of altered cyclic B-glucans composed of almost entirely of B-(1- 3)-glycosyl linkages. The mutant strain was only slightly sensitive to hypoosmotic growth conditions as compared with the ndvB mutant but it was severely impaired in symbiotic interactions with soybean (Glycine max). Nodulation was delayed by eight to ten days and many small nodule-like structures were formed which apparently were devoid of viable bacteria. The genetic locus containing ndvB and ndvC , when expressed in an R. meliloti ndvB mutant, resulted in a symbiotically competent strain. This suggests that the structure of the B-glucans is important for a successful symbiotic interaction, that there may be host-specific structural requirements, and that they may have a specific function in addition to their role in hypoosmotic adaptation.