Submitted to: International Congress on Molecular Plant-Microbe Interactions
Publication Type: Abstract Only
Publication Acceptance Date: 8/30/1996
Publication Date: N/A
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 the DNA region upsteam 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. 62 kDa) with several transmembrance domains. It contained a conserved sequence characteristic of a nucleoside-sugar binding motif and had 51% similarity with B-glucanosyl transferase from Candida albicans. B. japonicum carrying a Tn5 insertion in ndvC resulted in synthesis of altered cyclic B-glucans composed 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). This new class of glucan molecules (cyclolaminarinose) apparently functioned as osomolytes but not in supporting symbiotic effectiveness. Thus segregation of the pleiotropic phenotypes due to mutations in the glucan synthesis locus was observed for the first time. These results suggest that the structure of the B-glucan molecule is important for a successful symbiotic interaction, and that B-glucans may have a specific function in addition to their role in hypoosmotic adaptation.