ACRE Advice

ACRE's response to concerns raised in written representations and submissions associated with the CHARDON LL public hearing and to statements made at ACRE's open hearing relating to the safety assessment of T25 GM maize conducted under Directive 90/220/EEC

Background to the transgenic trait

At this point it is helpful to explain that the herbicidal compound L- phosphinothricin (L-PPT) to which T25 maize is tolerant, is the active ingredient in a number of herbicides with different names. Some of these have been referred to in the CHARDON LL and ACRE hearings. To avoid any confusion the names of these herbicides and the genes that confer tolerance to them are described below.

There are two species of soil bacteria known to synthesise PPT and it is the L form of this amino acid that is toxic to plants. L-PPT is the active ingredient of a number of broad spectrum herbicides, these include glufosinate ammonium and bialaphos, both of which have been referred to in association with T25 maize. Liberty® is the commercial name for the glufosinate ammonium herbicide that is linked to this crop.

Herbicides based on L-PPT act by inhibiting glutamine synthetase, an enzyme which catalyses the first reaction in the pathway that assimilates inorganic nitrogen into organic compounds. When L-PPT inhibits glutamine synthetase, toxic levels of ammonia accumulate in plant cells causing cell death. This is not the case in mammals (including humans) treated with the herbicide, since mammals have different metabolic pathways that prevent ammonia from building up and imbalances in amino acid levels from occurring ⁹.

Traditional breeding programmes to generate crop plants tolerant to glufosinate ammonium have not been successful. However, using genetic modification techniques, genes from bacteria that confer tolerance to L-PPT have been transferred into a variety of plants. These genes encode enzymes that inactivate the herbicidal activity of L-PPT. The pat (phosphinothricin N-acetyl transferase) and bar (bialaphos resistance) genes both encode phosphinothricin acetyl transferases (PATs ¹⁰) that catalyse the detoxification of L-PPT. In the presence of acetyl-CoA, PAT enzymes catalyse the acetylation of the free amino group of L-PPT to yield N-acetyl-L-PPT, a compound that does not inactivate glutamine synthetase. The PAT proteins that pat and bar genes encode are considered to be functionally equivalent for the purpose of conferring tolerance to L-PPT (Wehrmann et al 1996 ¹¹). Both forms of the enzyme are highly specific for L-PPT and do not acetylate other L-amino acids.

T25 maize contains the pat gene from a laboratory strain of a naturally occurring soil bacterium, Streptomyces viridochromogenes ¹²,Tu494 (Wohlleben et al. 1988 ¹³).

9 Hack R., Ebert E., Ehling G. and Leist K-H. (1994). Glufosinate ammonium - some aspects of its mode of action in mammals. Food Chem. Toxicol. 32: 461 -470.

10 The PPT acetyltransferase encoded by the bar gene is sometimes referred to as a BAR, rather than a PAT enzyme.

11 Wehrmann A., Van Vliet A., Opsomer C., Botterman J. and Schulz A. (1996). The similarities of bar and pat gene products make them equally applicable for plant engineers. Nature Biotech. 14: 1274 - 1278.

12 This species of Streptomyces is a saprophytic, soil-borne microbe and is not considered a pathogen of plants, humans, or other animals.

13 Wohlleben W., Arnold W,. Broer I., Hillemannn D,. Strauch E. and Puhler A. (1988). Nucleotide sequence of the phosphinothricin N-acetyltransferase from Streptomyces viridochromogenes Tu494 and its expression in Nicotiana tabacum. Gene 70: 25-37.