EVIDENCE FOR INVOLVEMENT OF A THIRD GENE IN CYCLIC B-GLUCAN SYNTHESIS IN BRADYRHIZOBIUM JAPONICUM.

Authors: CHEN, RONGJI - GUANGXI UNIV., PRC; BHAGWAT, ARVIND - UNIVERSITY OF MARYLAND; KEISTER, DONALD

Submitted to: Plant Physiology
Publication Type: Abstract Only
Publication Acceptance Date: 7/26/1999
Publication Date: N/A
Citation: N/A

Interpretive Summary:

Technical Abstract: The soybean microsymbiont, Bradyrhizobium japonicum, synthesizes cyclic beta-(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 beta-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 at ndvC resulted in glucans with altered glycosidic linkages. In addition to the symbiotic defect, the ndvB mutant was non-motile and hyposmotically sensitive. Recently, we have isolated motility revertants of the ndvB mutant. Although the revertants are motile they remained symbiotically defective, providing additional support for a specific role for cyclic beta-glucans in symbiotic interactions. Involvement of another gene (ORF1) located upstream of ndvB was determined by mutagenesis and deletion analysis. It appears that ORF1 is required for initiation of glucan synthesis as well as for establishing successful symbiotic interaction with soybean. Glucan synthesis could not be initiated in genetic constructs lacking ORF1 and the cells remained non-motile and sensitive to hypoosmotic growth conditions. Genetic constructs of B. japonicum with single and double deletions in the ndv genes and their symbiotic phenotypes will be presented.