DEMONSTRATION OF TRA(-BEET - GENE-TRANSFER TO A RECOMBINANT PSEUDOMONAD() PLASMID ACTIVITY IN BACTERIA INDIGENOUS TO THE PHYLLOSPHERE OF SUGAR)

The presence of transfer proficient plasmids in bacteria isolated from the leaves of sugar beet (Beta vulgaris L.) was studied. Of 435 bacte ria sampled 79 (18%) contained plasmids. Pseudomonads (30%), Erwinia ( 12%) and Klebsiella (9%) were the largest populations sampled of which 22%, 33% and 29%, respectively, contained plasmids. The ability of th ese plasmids to self-transfer or mediate the mobilization of the tra-m ob+ broad host range IncQ plasmid R300B was determined. R300B was main tained in 61/79 natural plasmid containing isolates, the Gram positive isolates could not support R300B. Pseudomonas aureofaciens SBW25, iso lated from sugar beet leaves, was chromosomally marked with a tetracyc line resistance gene and used as a recipient (SBW25ETc). Five isolates of Erwinia herbicola and one of Erwinia salicis containing natural pl asmids were able to mobilize R300B into the recombinant, SBW25ETc. The se mobilizing (tra+) plasmids were not maintained in transconjugant SB W25 cells. Analysis of the fragment patterns of PstI digested plasmid DNA demonstrated that four (pSB139, pSB140, pSB142, pSB146; 110 kb) we re identical, one (pSB153; 65 kb) was common to a subset of fragments in these four and another (pSB169; 100 kb) was unique. Other natural i solates were able to transfer copper resistance (Erwinia rhapontici, 2 strains) or mercury resistance (Pseudomonas fluorescens SBW340) to a rifampicin resistant recipient Pseudomonas putida UWC1 but not to SBW2 5ETc. These self-transferable plasmids were not able to mobilize R300B . These data demonstrate that the phyllosphere supports indigenous mic robial populations which have the capacity to transfer genetic materia l between bacteria of different genera.