More information on HGT from Biotech Crops to Soil Microbes

Nielsen et al. (1998, 2001) and Droge et al. (1998) have published reviews of the work on horizontal gene transfer from biotech plants to terrestrial bacterial.

A number of experiments have been undertaken to determine if, under non-laboratory conditions, DNA can move from biotech plants to soil bacteria and fungi (Paget et al., 1998; Gebhard and Smalla, 1999).

Experimental attempts to establish microbial transformation - the most likely mode of HGT from plants to prokaryotes - with transgenic DNA under non-laboratory conditions have been uniformly negative. To date, no experiment has provided evidence for the transfer of either transgenes or other genes from plants to soil microbes under non-lab conditions.

These experimental findings confirm results of investigations of HGT during and after field tests of biotech crops. Numerous field releases of a variety of biotech crops have been made over the past 14 years. Follow-up studies to assess HGT in the field have revealed no detectable transgene transfer to soil microbes under natural conditions (Smalla et al., 1994; Paget and Simonet, 1994; Badosa et al., 2004).

In the laboratory, HGT from biotech plants to microorganisms has been demonstrated under optimized conditions (Hoffmann et al., 1994; Schluter et al., 1995; Gebhard and Smalla, 1998; De Vries and Wackernagel, 1998; Nielsen et al., 2000; Meier and Wackernagel, 2003). Using a model system based on biotech Brassica plants and Aspergillus niger, a pathogenic fungus that infects brassicas, Hoffman et al. (1994) demonstrated successful transformation of A. niger with an antibiotic resistance marker gene. The rate of successful transfer was too low to calculate a frequency of HGT, however. Interestingly, the fungi that acquired the antibiotic resistance gene during co-cultivation with biotech plant tissue lost the resistance during further strain cultivation, even under continuous selective pressure. In addition, an intact antibiotic resistance gene was never identified in the resistant colonies.

Utilizing a different model system, Schluter et al. (1995) attempted to stimulate HGT between biotech potatoes and Erwinia chrysanthemum, a pathogen tightly associated with potatoes, in order to calculate rates of HGT under a wide variety of conditions. Under conditions they describe as "idealized" natural conditions, they calculated a HGT frequency of 2 X 10 -17, a rate they describe as "so rare as to be essentially irrelevant in any realistic risk assessment of biotech crops." These idealized natural conditions included using a bacterial marker gene linked to a functional origin of replication. It is important to note that most transformed plants do not contain origins of replication in their transgenes. Therefore, one might expect the frequency of transformation with transgenes to be even lower than the 2 X 10 -17 calculated by Schluter et al. (1995).

Several studies were performed in which transgenic plant DNA with a functional antibiotic resistance gene (nptII) was incubated with natural transformable bacteria (Acinetobacter sp. or Pseudomonas stutzeri) with a non-functional copy of nptII (317 or 10 bp deletion, respectively) (Gebhard and Smalla, 1998; De Vries and Wackernagel, 1998; Nielsen et al., 2000; Meier and Wackernagel, 2003). The degree of HGT was determined by the restoration of antibiotic resistance and transfer frequencies of 1.4 x 10 -8 to 5.4 x 10 -9 were detected. The laboratory conditions were set up to produce maximum competence of bacteria for DNA uptake and restoration of antibiotic resistance required homologous integration of the biotech plant derived nptII into the bacterial DNA. Even under optimized conditions, homologous integration could not be demonstrated in the laboratory in the absence of homologous sequences (De Vries et al., 2001).

De Vries et al. (2004) used a similar approach to study the probability of HGT from transgenic plant DNA to Acinetobacter sp. DNA extracted from tissues of transplastomic (transgenic DNA integrated in the plastid genome) tobacco containing the aadA gene (resistance against spectinomycin and streptomycin) was exposed to Acinetobacter cells with a non-functional aadA copy. Depending on the degree of homology of the aadA bordering sequences between the plant and Acinetobacter DNA, transformation frequencies of "not detectable" to 1.4 x 10 -4 were observed. This is a higher transformation frequency compared to the experiments described above. The authors suggest that the higher copy number of plastid genes compared with nuclear genes is the reason for this. Previous observations that sequence homology is essential for transgenic plant DNA integration were confirmed in this study.

Some laboratory studies have failed to demonstrate HGT under optimized, laboratory conditions. Broer et al. (1996) infected transgenic tobacco plants with Agrobacterium tumefaciens but could find no instance of HGT from plant to bacterium at a detection level of 6 x 10 -12. Nielsen et al. (1997) examined the frequency of HGT between transgenic plants and Acinetobacter under laboratory conditions and found no transformation with plant DNA.

However, as mentioned above, evolutionary evidence indicates the probable horizontal transfer of a few genes from plants to bacteria via transformation. Therefore, genes can move from plants to bacteria under natural conditions. However, altering the genetics of soil microorganisms via transformation from transgenic plants would appear to be ecologically insignificant since the elements in transgenes already occur and are available for transformation from other organisms.

The more important question is not whether transgenes in plants can move, at some low frequency, to soil organisms but, rather, whether there is sufficient selective advantage to maintain these transgenes over other endogenous genes found in the environment. Concerns about environmental impacts of HGT are germane only in those instances where the transgene spreads through the population.