The Gunnera-Nostoc symbiosis is a N2-fixing endosymbiosis between an angiosperm and a prokaryote, in some respects showing similarities to legume-rhizobial symbioses as well as to pathogenic Agrobacterium-plant interactions. We here describe the results of a molecular comparison of these systems, along with a characterization of the Gunnera mucilage which is important for Gunnera–Nostoc communication.
Nostoc DNA sequences homologous to the rhizobial nodEF, nodMN, exoY genes, along with the nod box, were identified by heterologous hybridization, whereas no homologues to either nodABC, nodD1 or nodD2 were found. Nevertheless, no anti-Nod protein antibodies recognized any Nostoc protein, either in normally grown or mucilage-induced cells. Furthermore, probes containing the agrobactenal chvA, chvB and picA genes also hybridized with Nostoc DNA, whereas virA and virG genes did not.
Using rhizobia with nod gene–lacZ fusions as reporters, Gunnera mucilage and seed rinse were shown to possess nod gene-inducing activity.
The mucilage secreted by the Gunnera stem gland, the organ susceptible to Nostoc infection, was composed of arabinose and galactose in relative molar proportions of 1.00:0.25, with an additional 0.13 molar parts of glucuronic acid. It showed a reaction on dot blot assays with the arabinogalactan protein-specific monoclonal antibody MAC 207, indicating that it contains arabinogalactan proteins.