Infection Mutants of Rhizobium Meliloti are Altered in Acidic Exopolysaccharide Production

Rhizobium meliloti induces the formation of root nodules on its symbiotic partner alfalfa (Medicago sativa). The morphogenesis of the nodule is a complex interaction between the host plant and the invading bacteria. Genetic analysis of the microsymbiont has revealed interesting details about this symbiotic association. A very large plasmid (pSYM) has bee identified in R. meliloti which was found to carry symbiotic genes, such as common nodulation, host specificity and fixation genes (Rosenberg et al 1981; Banfalvi et al 1981; Kondorosi et al 1982). Recently, it was found that R. meliloti strain 2011 contains two Megaplasmids (Banfalvi et al 1985; Simon 1984) of 1200 and 1500 Kd (Burkardt et al submitted). The smaller plasmid is identical to the symbiotic plasmid pSYM whereas the larger plasmid is involved in exopolysaccharide biosynthesis (Hynes et al 1986). In this paper we report the isolation and characterization of infection mutants which are altered in acidic exopolysaccharide production. The mapping of the infection genes is described and experiments are presented showing that infection and nodulation mutants can complement each other on a cellular level.