Horizontal gene transfer from transgenic plants to terrestrial bacteria – a rare event?

Abstract

Today, 12 years after the first field release of a genetically modified plant (GMP), over 15,000 field trials at different locations have been performed. As new and unique characteristics are frequently introduced into GMPs, risk assessment has to be performed to assess their ecological impact. The possibilities of horizontal gene transfer (HGT; no parent-to-offspring transfer of genes) from plants to microorganisms are frequently evaluated in such risk assessments of GMPs before release into the field. In this review we indicate why putative HGT from plants to terrestrial (soil and plant associated) bacteria has raised concern in biosafety evaluations. Further, we discuss possible pathways of HGT from plants to bacteria, outline the barriers to HGT in bacteria, describe the strategies used to investigate HGT from plants to bacteria and summarize the results obtained. Only a few cases of HGT from eukaryotes such as plants to bacteria have been reported to date. These cases have been ascertained after comparison of DNA sequences between plants and bacteria. Although experimental approaches in both field and laboratory studies have not been able to confirm the occurrence of such HGT to naturally occurring bacteria, recently two studies have shown transfer of marker genes from plants to bacteria based on homologous recombination. The few examples of HGT indicated by DNA sequence comparisons suggest that the frequencies of evolutionarily successful HGT from plants to bacteria may be extremely low. However, this inference is based on a small number of experimental studies and indications found in the literature. Transfer frequencies should not be confounded with the likelihood of environmental implications, since the frequency of HGT is probably only marginally important compared with the selective force acting on the outcome. Attention should therefore be focused on enhancing the understanding of selection processes in natural environments. Only an accurate understanding of these selective events will allow the prediction of possible consequences of novel genes following their introduction into open environments.

The use of chemical profiling for monitoring metabolic changes in artificial soil slurries caused by horizontal gene transfer

“…Earthworm casts have been shown to be sites of extensive plasmid conjugation between bacteria (Daane et al, 1996(Daane et al, , 1997, and may represent specific niches conducive also to HGT. Conversely the highly active microbial community in the earthworm cast environment could present obstacles to HGT by degrading/ adsorbing naked DNA or competing with the rare transformants (Nielsen et al, 1998). In the present experiments slurries derived from earthworm casts thus provided a challenging natural environment to study the utility of FT-IR for monitoring changes in global chemical profile resulting from a shift in catabolic function caused by HGT.…”

Biosafety and risk assessment framework for selectable marker genes in transgenic crop plants: a case of the science not supporting the politics

“…There is no recognized mechanism through which foreign genes, including antibiotic resistance genes integrated in plants, could be transferred from the plant genome to gut microorganisms, even if they could survive digestion. Several deliberate attempts have been made to transform naturally competent bacteria with transgenic plant DNA from different plants, but no gene transfer has been found to occur from genetically modified plants to soil bacteria despite the detection of the transgene in the soil (de Vries et al 2003;Dale et al 2002;Kay et al 2002;Smalla 1998, 1999;Nielsen et al 1998). Bennett et al (2004) considered bacterial DNA transfer systems and mechanisms of recombination that jointly might transfer antibiotic resistance genes from transgenic plants to bacteria.…”

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

“…Several studies have attempted to estimate the frequency of horizontal transfer of recombinant DNA from transgenic plants to bacteria and were summarized by Nielsen et al (1998). The data indicate that these events were so rare that they were below the detection limit and numerical estimates were often reached only by extrapolation (10 −13 to 10 −17 per cell).…”