Symbiotic Suppression of the Medicago sativa Plant Defence System by Rhizobium meliloti Oligosaccharides

The establishment of nitrogen-fixing root nodules in the Rhizobium-legume symbiosis is a complex, multistep interaction between the bacterium and the specific host plant. Major early events in this plant-microbe interaction are the perception of plant borne flavonids, the synthesis of a lipooligosaccharide, the nodulation factor, and the infection of the plant via a curled root hair by the microsymbiont. The infection of a curled root hair is characterized by the formation and sustained development of the infection thread, in which the bacteria multiply and grow towards the simultaniously initiated meristem within the root cortex (Hirsch 1992). Rhizobium meliloti mutants that fail to synthesize the exopolysaccharide succinoglycan (EPS I) were unable to infect the plant, nevertheless, they induce non-infected pseudonodules. A detailed analysis of these pseudonodules revealed strong evidence for the induction of a plant defence response by the mutated microsymbiont (Niehaus et al. 1993). This observation gave the experimental evidence that also symbiotic bacteria have to deal with the plant defence system. In analogy to compatible and incompatible interactions between pathogens and plants, a “gene for gene” hypothesis was also formulated for symbiotic interactions (Djordjevic et al. 1987). While specific recognition is the basis for incompatibility in plant-pathogen interactions, it appears that specific recognition is necessary for compatibility in symbiosis. Failure to communicate will lead either to a complete lack of interaction or the abortion of the infection process at various stages. So it has been suggested that successful infection of the host plant may also depend on the ability of rhizobia to escape or to suppress the induction of a plant defence reaction that normally serves to prevent infection by pathogens (Vance 1983, Niehaus et al. 1993).