PLASMIDS AND SAPROPHYTIC GROWTH OF RHIZOBIUM-LEGUMINOSARUM BV TRIFOLII W14-2 IN SOIL

The role of plasmids in the saprophytic growth of Rhizobium is mostly unknown. Plasmid-cured and complemented derivatives of R. leguminosaru m by. trifolii strain W14-2 were used to investigate the role of plasm ids in the growth of this strain in sterile soil incubated under favor able moisture and temperature conditions. Strain W14-2 contains four p lasmids (a,b,c,d). Absence of single plasmids in plasmid-cured derivat ives generally did not reduce growth in soil when compared to the wild -type but absence of plasmid a delayed growth. Derivatives were unable to grow in soil when only plasmids a or d were present in cells. When only plasmids b or c were present, growth was delayed and the final p opulation in 7 days was approximately 10% of the wild-type population. When the wild-type was co-inoculated at equal population into soil wi th derivatives lacking plasmids a, c, or d, the population of the wild -type at 7 days incubation was approximately 10 times larger than thos e of the derivatives. Elimination of only plasmid b did not reduce the ability of the strain to grow in soil when competing with the wild-ty pe. Plasmids were involved in saprophytic growth of strain W14-2 in so il and may be important to the ecology of Rhizobium.