HORIZONTAL GENE-TRANSFER FROM TRANSGENIC PLANTS TO TERRESTRIAL BACTERIA - A RARE EVENT

Today, 12 years after the first field release of a genetically modifie d plant (GMP), over 15 000 field trials at different locations have be en performed. As new and unique characteristics are frequently introdu ced into GMPs, risk assessment has to be performed to assess their eco logical 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 relea se into the field. In this review we indicate why putative HGT from pl ants to terrestrial (soil and plant associated) bacteria has raised co ncern in biosafety evaluations. Further, we discuss possible pathways of I-IGT from plants to bacteria, outline the barriers to HGT in bacte ria, describe the strategies used to investigate HGT from plants to ba cteria and summarize the results obtained. Only a few cases of HGT fro m eukaryotes such as plants to bacteria have been reported to date. Th ese cases have been ascertained after comparison of DNA sequences betw een plants and bacteria. Although experimental approaches in both fiel d 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 homol ogous 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 infer ence is based on a small number of experimental studies and indication s found in the literature. Transfer frequencies should not be confound ed with the likelihood of environmental implications, since the freque ncy of HGT is probably only marginally important compared with the sel ective force acting on the outcome. Attention should therefore be focu sed on enhancing the understanding of selection processes in natural e nvironments. Only an accurate understanding of these selective events will allow the prediction of possible consequences of novel genes foll owing their introduction into open environments, (C) 1998 Published by Elsevier Science B.V. All rights reserved.