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

Abstract

The Rhizobium meliloti cell surface components, the exopolysaccharides (EPS I & EPS II) and the lipopolysaccharide (LPS), are known to be important in different stages of the establishment of the Medicago sativa (alfalfa) symbiosis (Müller et al., 1988, Lagares et al., 1992). From R. meliloti mutants defective in the EPS/LPS biosynthesis, we speculated that these compounds have a function in the suppression of plant defence reactions against the symbiont (Niehaus et al. 1993). Since EPS defective mutants of R. meliloti induce the defence system of the host plant, elicitors must be present. These elicitors could derive from hydrolytic activities of the rhizobial cells during the infection process (Fig. 1).