Plasmids influence growth of rhizobia in the rhizosphere of clover

The importance of plasmids in the colonization of the rhizosphere by Rhizobium leguminosarum bv. trifolii is largely unknown. We investigated the ability of rhizobial strain W14-2, which contains four plasmids, and its plasmid-cured derivatives, including a plasmidless derivative, to grow in the rhizosphere of crimson clover (Trifolium incarnatum L.). Clover seeds were planted in sand and inoculated either with a single plasmid-cured derivative of strain W14-2 or with a mixture of the derivative and the wild-type. Changes in population size of the genotypes in the rhizosphere were monitored by plate counts on selective media. Derivatives cured of single plasmids achieved population numbers similar to those of the wild-type when inoculated alone. When co-inoculated with the wild-type, they did not achieve as high population numbers as the wild-type. Curing of plasmid a resulted in a population one-tenth that of the wild-type. Derivatives containing plasmids b or c achieved population numbers similar to the wild-type when inoculated singly although there was a prolonged period before growth began. Populations of derivatives containing no plasmid or only plasmid a or d decreased with time in the rhizosphere of clover. All of the plasmids contributed to the ability of strain W14-2 to grow in the rhizosphere of clover.