Microbial horizontal gene transfer and the DNA release from transgenic crop plants

Abstract

Intraspecific and interspecific horizontal gene transfers among prokaryotes by mechanisms like conjugation, transduction and transformation are part of their life style. Experimental data and nucleotide sequence analyses show that these processes appear to occur in any prokaryotic habitat and have shaped microbial genomes throughout evolution over hundreds of million years. Here we summarize studies with a focus on the possibility of the transfer of free recombinant DNA released from transgenic plants to microorganisms by transformation. A list of 87 species capable of natural transformation is presented. We discuss monitoring techniques which allowed detection of the spread of intact DNA from plants during their growth, in the process of decay and by pollen dispersal including novel biomonitoring assays for measuring the transforming potential of DNA in the environment. Also, studies on the persistence of free DNA in soil habitats and the potential of bacteria to take up DNA in soil are summarized. On the other hand, the various barriers evolved in prokaryotes which suppress interspecific gene transfer and recombination will be addressed along with studies aiming to estimate the chance of a gene transfer from plant to microbe. The results suggest that, although such transfers could be possible in principle, each of the many steps involved from the release of intact DNA from a plant cell to integration into a prokaryotic genome has such a low probability that a successful transfer event be extremely rare. Further, interspecies transfer of chromosomal DNA is mostly negative for the recipient, and, if not, in the absence of a selective advantage the transformant will be lost. It is stressed that the nucleotide sequences introduced into transgenic plants are much less likely to be captured from the transgenic plants than directly from those organisms (often bacteria or viruses) from which they were originally derived.

Assessment of transformability of bacteria associated with tomato and potato plants

“…The estimated frequency of transformation for both isolates was in the range between 10 −8 and 10 −9 transformants per μg of DNA, slightly above the frequency of occurrence of spontaneous mutation to Rp-resistance. Successful transformation with DNA carrying small mutations also has been reported in different species (De Vries and Wackernagel, 2004;Nielsen et al, 1998;Thomas and Nielsen, 2005). Our results suggest that highly competent naturally transformable isolates like A. balylyi ADP1 are not commonly present in the culturable fraction of potato-and tomato-associated communities.…”

“…Transformation of plant-associated bacteria by DNA originating from plant cells might occur (De Vries and Wackernagel, 2004;Nielsen et al, 2000b;Thomas and Nielsen, 2005). A factor that may spur the dissemination of inserts, via integration by homologous recombination, is the presence of DNA sequences in plant cell organelles of prokaryotic origin, such as chloroplast and mitochondrial genomes.…”

“…The estimated potential frequency of transformation of plant-associated bacteria by DNA released from plants is extremely low, i.e. on the order of 10 −13 -10 −17 per cell (De Vries and Wackernagel, 2004;Nielsen et al, 1998). Transformation of plantassociated bacteria by plant DNA has never been demonstrated in open environments such as agricultural fields.…”

“…Transformation has been demonstrated in a wide range of other prokaryotic species exposed to environmental conditions (De Vries and Wackernagel, 2004). The state of competence leading to natural transforma-bility is commonly related to the presence of competence genes, like the com genes present in Thermus thermophilus and Acinetobacter baylyi (Friedrich et al, 2001 and.…”

Lateral genetic transfer: open issues

“…by nucleases or humic acids. Some 90 species of prokaryotes, including some that live in soil or water, are known to be naturally competent (Nielsen et al 1998;de Vries & Wackernagel 2005). Potential hot spots in the environmental for recombination and LGT include biofilms, the rhizosphere, decomposing material, guts of soil animals and within bacterivorous protozoa.…”