COMMUNICATION AND SIGNAL EXCHANGE IN THE RHIZOBIUM BRADYRHIZOBIUM LEGUME SYSTEM

A new comprehensive communication concept in the Rhizobium/Bradyrhizob ium legume symbiosis was developed. It includes a root zone specific f lavonoid exudation, the differential activity of phenylpropane/acetate pathway derivatives on chemotaxis, nod-gene inducing activity and phy toalexin resistance induction on the microsymbiont side (Bradyrhizobiu m). Nod factor production from the microsymbiont affects the host plan t in root hair curling and meristem induction. Phytoalexin production in the host plant is also an early response, however repressed to a lo w level after a few hours. Another strategy of the microsymbiont to ov ercome phytoalexin effects is degradation of phytoalexins in Rhizobium leguminosarum bv. vicieae. Competitiveness within the same infection group of the microsymbiont was studied with gus-gene fusion, using the blue coloured nodules to easily discriminate marked strains from unma rked competitors. New exopolysaccharide (EPS) mutants of Bradyrhizobiu m japonicum were reconstructed homologous with a DNA region to exoB ge ne of Rhizobium meliloti. Their clearly reduced competitiveness of nod ulation, demonstrates that exopolysaccharides of Bradyrhizohium japoni cum also have an important function during the early stages of this sy mbiotic interaction.