A MOLECULAR CHARACTERIZATION OF THE GUNNERA-NOSTOC SYMBIOSIS - COMPARISON WITH RHIZOBIUM-PLANT AND AGROBACTERIUM-PLANT INTERACTIONS

The Gunnera-Nostoc symbiosis is a N-2-fixing endosymbiosis between an angiosperm and a prokaryote, in some respects showing similarities to legume-rhizobial symbioses as well as to pathogenic Agrobacterium-plan t interactions. We here describe the results of a molecular comparison of these systems, along with a characterization of the Gunnera mucila ge which is important for Gunnera-Nostoc communication. Nostoc DNA seq uences homologous to the rhizobial nodEF, nodMN, exo Y genes, along wi th the nod box, were identified by heterologous hybridization, whereas no homologues to either nodABC, nodD1 or nodD2 were found. Neverthele ss, no anti-Nod protein antibodies recognized any Nostoc protein, eith er in normally grown or mucilage-induced cells. Furthermore, probes co ntaining the agrobacterial chvA, chvB and picA genes also hybridized w ith Nostoc DNA, whereas virA and virG genes did not. Using rhizobia wi th nod gene-lacZ fusions as reporters, Gunnera mucilage and seed rinse were shown to possess nod gene-inducing activity. The mucilage secret ed by the Gunnera stem gland, the organ susceptible to Nostoc infectio n, was composed of arabinose and galactose in relative molar proportio ns of 1.00:0.25, with an additional 0.13 molar parts of glucuronic aci d. It showed a reaction on dot blot assays with the arabinogalactan pr otein-specific monoclonal antibody MAC 207, indicating that it contain s arabinogalactan proteins.