Submitted to: International Congress on Molecular Plant-Microbe Interactions
Publication Type: Abstract Only
Publication Acceptance Date: August 30, 1996
Publication Date: N/A
The soybean microsymbiont Bradyrhizobium japonicum synthesizes cyclic B-(1-3),B-(1-6)-glucans which function as osmoprotectants during hypoosmotic growth conditions. The microsymbiont synthesizes the glucans throughout nodule development. Mutants of B. japonicum defective in the synthesis of cyclic B-(1-3),(1-6)-glucans are unable to establish a successful symbiotic interaction with soybean. Although bradyrhizobial glucans share some structual similarity with one of the best characterized elicitors from Phytophthora sojae [cell-wall B-(1-3),(1-6)-branched glucans], they are weak inducers of phytoalexin synthesis. However when tested in combination, B. japonicum cyclic B-glucans inhibited stimulation of phytoalexin accumulation by fungal glucans in a concentration dependent manner. In assays with a radio-labeled photoaffinity conjugate of the hepta-B-glucoside elicitor and solubilized B-glucan binding protein (Mr=75 kDa) from soybean plasma membranes, the the cyclic B-glucans of B. japonicum competitively inhibited binding of the fungal elicitor. The ndvC mutant of B. japonicum produces glucans predominantly of B-(1-3)-linkages, with a ring containing 6 to 8 glucose residues and DP=12 (cyclolaminarinose). This mutant formed nodule-like structures containing phytoalexin levels four-times higher than wild-type. Binding studies of cyclic B-glucan binding protein will be reported which which suggest a novel role for the microsymbiont's cyclic B-glucans as suppressors of the host defense response.