Rhizobium plasmids in bacteria-legume interactions.

The functional analysis of plasmids in Rhizobium strains has concentrated mainly on the symbiotic plasmid (pSym). However, genetic information relevant to both symbiotic and saprophytic Rhizobium life cycles, localized on other 'cryptic' replicons, has also been reported. Information is reviewed which concerns functional features encoded in plasmids other than the pSym: biosynthesis of cell surface polysaccharides, metabolic processes, the utilization of plant exudates, aromatic compounds and diverse sugars, and features involved symbiotic performance. In addition, factors which affect plasmid evolution through their influence on structural features of the plasmids, such as conjugative transfer and genomic rearrangements, is discussed. Based on the overall data, we propose that together the plasmids and the chromosome constitute a fully integrated genomic complex, entailing structural features as well as saprophytic and cellular functions.