PLASMIDS INFLUENCE GROWTH OF RHIZOBIA IN THE RHIZOSPHERE OF CLOVER

The importance of plasmids in the colonization of the rhizosphere by R hizobium leguminosarum by. trifolii is largely unknown. We investigate d 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 sin gle plasmid-cured derivative of strain W14-2 or with a mixture of the derivative and the wild-type. Changes in population size of the genoty pes in the rhizosphere were monitored by plate counts on selective med ia. Derivatives cured of single plasmids achieved population numbers s imilar to those of the wild-type when inoculated alone. When co-inocul ated with the wild-type, they did not achieve as high population numbe rs 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 ac hieved population numbers similar to the wild-type when inoculated sin gly although there was a prolonged period before growth began. Populat ions of derivatives containing no plasmid or only plasmid a or d decre ased with time in the rhizosphere of clover. All of the plasmids contr ibuted to the ability of strain W14-2 to grow in the rhizosphere of cl over.