Interkingdom gene transfer is limited by a combination of physical, biological, and genetic barriers. The results of greenhouse experiments involving transplastomic plants (genetically engineered chloroplast genomes) cocolonized by pathogenic and opportunistic soil bacteria demonstrated that these barriers could be eliminated. The Acinetobacter sp. strain BD413, which is outfitted with homologous sequences to chloroplastic genes, coinfected a transplastomic tobacco plant with Ralstonia solanacearum and was transformed by the plant's transgene (aadA) containing resistance to spectinomycin and streptomycin. However, no transformants were observed when the homologous sequences were omitted from the Acinetobacter sp. strain. Detectable gene transfer from these transgenic plants to bacteria were dependent on gene copy number, bacterial competence, and the presence of homologous sequences. Our data suggest that by selecting plant transgene sequences that are nonhomologous to bacterial sequences, plant biotechnologists could restore the genetic barrier to transgene transfer to bacteria.The tremendous adaptation potential of prokaryotes is mainly related to their ability to exchange genes by specific mechanisms such as conjugation, transduction, and transformation (22). The efficiency of such mechanisms during bacterial incorporation of genes from transgenic plants, and particularly those encoding antibiotic resistance, is difficult to assess (15), although the occurrence of such transfers under natural soil conditions would remain rare (3). In soil, in spite of the persistence of plant DNA, there would be relatively few naturally transformable bacteria (13, 17) and these prokaryotes would rarely find the required conditions to develop competence (4), thus apparently significantly reducing the probability of gene transfer. For those bacteria that have developed specific symbiotic or pathogenic relationships with plants, conditions for gene transfer could be favorable, as shown with the plant pathogen Ralstonia solanacearum (2). This bacterium multiplied in its host plant, disorganized tissues, and colonized the plant via the vascular tissue, leading to the development of a competence stage of active transformability in planta (2). Although bacteria-bacteria gene transfer occurred between R. solanacearum in planta, no gene transfer from the transgenic plant to the R. solanacearum was detected. This lack of detection was not necessarily due to the lack of transfer but was possibly due in part to the low transformation efficiency of R. solanacearum and the dilution of the transgene by the entire plant genome (3). The ratio between target versus non-target DNA sequences on which homologous recombination can occur was very low, thereby possibly preventing the integration mechanism necessary to produce a measurable number of transformants.While the potential of plant environments to mediate plantbacteria gene exchange has not been established, two recent events have increased the likelihood. Genetic engineering of the chlorop...
The behavior of the soil bacterium Acinetobacter sp. BD413 was monitored in Ralstonia solanacearum-infected and non-infected tomato plants after direct injection into the stem or natural infection by roots. In healthy plants, Acinetobacter sp. BD413 failed to colonize plant tissue. In plants infected simultaneously by the pathogen R. solanacearum,the Acinetobacter population increased linearly to about 3.1 x 10(7) cells per gram plant material and was maintained at a high level until the death of the plant. Moreover, Acinetobacter sp. BD413 was found to develop a competent state when multiplying in planta, indicating it could possibly be transformed by bacterial or plant DNA.
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