Direct conjugal transfers of Ti plasmid to soil microflora

The bacterial species in soil that can receive a Ti plasmid by conjugation from Agrobacterium spp. were investigated. In order to have direct access t o the potential reservoir of Ti plasmid amongst soil microflora, the conjug al system consisting of a multiply auxotrophic derivative of C58 (ST-96-4) and a derivative of pTiC58 Delta accR (pSTiEGK) containing a triple antibio tic-resistance cassette in traM was used to transfer the Ti plasmid in a co mplex soil microflora used as the recipient. Numerous transconjugants were obtained by this method but none was identified as Agrobacterium. This coul d be explained by the low density of Agrobacterium in the tested soil. As i ndicated by analysis of the ribosomal gene ws, transconjugants recovered di rectly from soil were found to be new bacterial species which appeared to b e closely related to Sinorhizobium spp.