IN-SITU TRANSFER OF AN EXOGENOUSLY ISOLATED PLASMID BETWEEN PSEUDOMONAS SPP IN SUGAR-BEET RHIZOSPHERE

A method for studying plasmid transfer in the rhizosphere is described . This work demonstrates plasmid transfer in an unenclosed rhizosphere under field conditions. The donor (Pseudomonas marginalis 376N) and r ecipient (Pseudomonas aureofaciens 381R) bacteria and the conjugative mercury resistance plasmid (pQBR11) studied were all isolated from the bacterial community indigenous to sugar beet rhizosphere. Spontaneous nalidixic acid and rifampicin resistant mutants of these bacteria wer e used as donors and recipients of pQBR11 for in situ matings. Fresh f ield soil was mixed with donors and recipients to give a soil mating m ix (SMM) which was placed underground on the surface of a sugar beet r oot storage organ. Plasmid transfer in the SMM was determined after 24 h at frequencies between 5.1 x 10(-5) and 1.3 x 10(-8) transconjugant s per recipient. Higher transfer frequencies (1.3 x 10(-2) to 1.7 x 10 (-6)) were recorded on the peel adjacent to the SMM. No transfer of me rcury resistance was detected in SMM controls incubated at 20 degrees C in vitro or placed in soil at distances of more than 5 cm from plant s.