Most information on the role of plasmids in the ecology of rhizobia in soil has been obtained using rhizobial derivatives lacking a single plasmid only. Rhizobium leguminosarum biovar trifolii strain W14-2 contains four plasmids (a, b, c, d). The role of plasmids in saprophytic characteristics and sodium chloride tolerance of W14-2 in vitro was investigated using a library of plasmid-cured W14-2 derivatives that includes derivatives containing only one plasmid and no plasmid, as well as complemented derivatives. Plasmid involvement in certain phenotypes such as swarming (plasmid a), utilization of glycerol (plasmid a), adonitol (plasmid b), rhamnose and sorbitol (plasmid c) was tracked to single plasmids and was not influenced significantly by the other plasmids present in the cell. Presence of plasmid b or c was necessary for the utilization of arabinose and raffinose, whereas presence of both plasmids b and d was required for tolerance to high levels of sodium chloride. The latter phenotype may correspond to tolerance to chloride toxicity or to osmotolerance. Plasmids influenced growth of the strain in liquid HP (peptone-yeast extract-tryptone), a complex medium that was investigated as it contains a wide range of carbon substrates. Positive contributions of plasmids a (shorter duration of the lag phase and increased maximum density of cells), b and c (shorter log-phase generation time) were shown. Plasmid d was detrimental to the strain in HP when it was the only plasmid present in the cell, especially regarding log-phase generation time. Similarly, the derivative with only plasmid d was more sensitive to sodium toxicity than any other derivative, including the plasmidless derivative. Interactions between plasmids influenced growth in HP broth and salt tolerance, and detrimental effects of plasmid d were little expressed if any when the cell contained one plasmid or more besides plasmid d. No plasmid-cured derivative grew better than the wild-type overall, suggesting that the extrachromosomal genome contributes to the saprophytic competence of W14-2 in soil.
