Detection of potential transgenic plant DNA recipients among soil bacteria

Abstract

The likelihood of gene transfer from transgenic plants to bacteria is dependent on gene number and the presence of homologous sequences. The large number of transgene copies in transplastomic (transgenes contained in the chloroplast genome) plant cells as well as the prokaryotic origin of the transgene, may thus significantly increase the likelihood of gene transfer to bacteria that colonize plant tissues. In order to assess the probability of such transfer, the length of homologous DNA sequences required between the transgene and the genome of the bacterial host was assessed. In addition, the probability that bacteria, which co-infect diseased plants, are transformable and have sequences similar to the flanking regions of the transgene was evaluated. Using Acinetobacter baylyi strain BD143 and transplastomic tobacco plants harboring the aadA gene (streptomycin and spectinomycin resistance), we found that sequences identical to the flanking regions containing as few as 55 nucleotides were sufficient for recombination to occur. Consequently, a collection of bacterial isolates able to colonize tobacco plant tissue infected by Ralstonia solanacearum strain K60 was obtained, screened for DNA sequence similarity with the chloroplastic genes accD and rbcL flanking the transgene, and tested for their ability to uptake extracellular DNA (broad host-range pBBR1MCS plasmids) by natural or electro-transformation. Results showed that among the 288 bacterial isolates tested, 8% presented DNA sequence similarity with one or both chloroplastic regions flanking the transgene. Two isolates, identified as Pseudomonas sp. and Acinetobacter sp., were able to integrate exogenous plasmid DNA by electro-transformation and natural transformation, respectively. Our data suggest that transplastomic plant DNA recipients might be present in soil bacterial communities.

Scientific Opinion on application (EFSA‐GMO‐BE‐2011‐101) for the placing on the market of herbicide‐tolerant genetically modified oilseed rape MON 88302 for food and feed uses, import and processing under Regulation (EC) No 1829/2003 from Monsanto

“…A single alignment with A. tumefaciens Ti-plasmid sequence of sufficient length and identity for HR (de Vries and Wackernagel, 2002;Monier et al, 2007;Hülter and Wackernagel, 2008;EFSA, 2009b;Overballe-Petersen et al, 2013) was identified, as expected. The alignment displayed 99.3 % identity over a 296 bp fragment of the Ti-plasmid containing the left border region used for transfer of the T-DNA.…”

Acinetobacter baylyi ADP1: Transforming the choice of model organism

“…In one study, transfer occurred with as few as 55 total bp of flanking identity (27). Foreign DNA can even be incorporated at a very low frequency in the absence of apparent homology (28).…”

Explanatory note on DNA sequence similarity searches in the context of the assessment of horizontal gene transfer from plants to microorganisms

“…Homologous recombination becomes increasingly inefficient with decreasing length of sequences with high identity (de Vries and Wackernagel, 2002;Monier et al, 2007;EFSA, 2009;Overballe-Petersen et al, 2013) and reaches under in vitro conditions a plateau phase with about 1 kb of sequences with high identity at both stretches of DNA (Kung et al, 2013). Based on the above-mentioned literature data, a 200-bp stretch of identical sequence can be considered as a minimum length of continuous DNA sequence to take into account in the analysis of identifying GM plant DNA regions with increased recombination potential with microbial genomes.…”