GM Crop Database

Database Product Description

T25 (ACS-ZMØØ3-2)
Host Organism
Zea mays (Maize)
Trade Name
Liberty-Link™
Trait
Herbicide tolerant, glufosinate ammonium.
Trait Introduction
Chemically mediated introduction into protoplasts and regeneration.
Proposed Use

Production for human consumption and livestock feed.

Product Developer
Bayer CropScience USA LP

Summary of Regulatory Approvals

Country Food Feed Environment Notes
Argentina 1998 1998 1998
Australia 2002
Brazil 2007 2007 2007
Canada 1997 1997 1996
China 2004 2004
Colombia 2012 2011
European Union 1998 1998 1998
Japan 1997 1997 2004
Korea 2003 2004
Malaysia 2013 2013
Mexico 2007 2007
New Zealand 2002
Philippines 2003 2003
Russia 2007 2006
Singapore 2014 2014
South Africa 2001 2001
Taiwan 2002
Turkey 2015
United States 1995 1995 1995
Vietnam 2015 2015

Introduction Expand

Maize line T25 was developed through a specific genetic modification to allow the use of glufosinate ammonium, the active ingredient in phosphinothricin herbicides (Basta, Rely, Finale, and Liberty), as a weed control option in maize crops. The pat gene, conferring tolerance to glufosinate ammonium, was cloned from the common aerobic soil actinomycete, Streptomyces viridochromogenes, and encodes the enzyme phosphinothricin-N-acetyltransferase (PAT).

The PAT enzyme was used as a selectable marker enabling identification of transformed plant cells as well as a source of resistance to the herbicide phosphinothricin (also known as glufosinate ammonium). Phosphinothricin containing herbicides, such as glufosinate ammonium, act by inhibiting glutamine synthetase resulting in the accumulation of toxic levels of ammonia that kill the plant within hours of application. The PAT enzyme detoxifies phosphinothricin by acetylation into an inactive compound. The modified corn permits farmers to use phosphinothricin-containing herbicides for weed control in the cultivation of corn.

Summary of Introduced Genetic Elements Expand

Code Name Type Promoter, other Terminator Copies Form
pat Phosphinothricin N-acetyltransferase HT

Cauliflower mosaic virus 35S

Cauliflower mosaic virus 35S poly(A) signal

1

Modified for enhanced expression; PEG-mediated uptake into plant protoplasts

Characteristics of Zea mays (Maize) Expand

Center of Origin Reproduction Toxins Allergenicity

Mesoamerican region, now Mexico and Central America

Cross-pollination via wind-borne pollen is limited, pollen viability is about 30 minutes. Hybridization reported with teosinte species and rarely with members of the genus Tripsacum.

No endogenous toxins or significant levels of antinutritional factors.

Although some reported cases of maize allergy, protein(s) responsible have not been identified.

Donor Organism Characteristics Expand

Latin Name Gene Pathogenicity
Streptomyces viridochromogenes pat

S. viridochromogenes is ubiquitous in the soil. It exhibits very slight antimicrobial activity, is inhibited by streptomycin, and there have been no reports of adverse affects on humans, animals, or plants.

Modification Method Expand

The T25 lines were derived by chemically mediated transformation of cultured protoplasts obtained from a embryogenic corn line cultures with purified DNA containing the pat gene isolated from S. viridochromogenes. Transformed cell colonies were selected and regenerated on medium containing glufosinate ammonium. The primary transformant T25, was then backcrossed with both commercial public inbred lines and proprietary inbred lines of the yellow dent corn type.

The pat gene introduced in the transformant T25 was modified to optimize its expression in plants without altering the amino acid sequence of the PAT enzyme.

Characteristics of the Modification Expand

The Introduced DNA

Molecular analyses showed that maize line T25 contains 1 copy of the pat gene. Further analyses demonstrated that T25 possessed an incomplete sequence of the antibiotic resistance encoding bla gene. Northern blot analyses showed that maize lines derived from T25 did not produce mRNA from the disrupted bla gene fragment, nor did they produce beta-lactamase, the enzyme encoded by the bla gene.

Genetic Stability of the Introduced Trait

The integration of the transferred DNA, into event T25, was demonstrated to be stable by segregation analysis and Southern blots. The data provided demonstrated the integration of one copy of the pat gene in transformation events T25. Commercial maize lines containing the transformation event are several generations removed from the original transformation event, further demonstrating the stable inheritance of the herbicide resistant trait.

Expressed Material

The PAT protein was expected to be expressed in all tissues of the plant, since the pat gene cassette contains the CaMV 35S promoter (P35S from cauliflower mosaic virus). Analysis of PAT protein expression was carried out by ELISA using plants grown at different geographic locations. Expression in grain ranged from below the limit of quantification to 430 ng/g dry weight. The PAT protein levels in the matrices (dry milling) were approximately 46 ng/g in hulls, 45 ng/g in grits, 17.2 ng/g in combined meal, 12.9 ng/g in flour and 276 ng/g in germ. In crude and refined oil from maize T25 grains, the PAT protein was not detectable. No PAT protein was detectable in starch produced from event T25 grain.

Environmental Safety Considerations Expand

Field Testing

The maize event T25 has been field tested in the major maize growing regions of the United States since 1992 and in Canada since 1993. T25 has been evaluated extensively in the laboratory, greenhouse, and field experiments. Field reports on T25, compared to non-transgenic counterparts, determined that agronomic characteristics such as yield, plant height, over-wintering capacity, flowering period, and disease susceptibility were within the normal range of expression currently displayed by commercial maize hybrids. Overall the field data reports and data on agronomic traits showed that maize event T25 and lines derived from these event, have no potential to pose a plant pest risk.

Outcrossing

Since pollen production and viability were unchanged by the genetic modification resulting in maize line T25, pollen dispersal by wind       and outcrossing frequency should be no different  than for other maize varieties. Gene exchange between maize lines of T25 and other cultivated maize  varieties will be similar to that which occurs naturally between cultivated  maize varieties  at the present time. In Canada and the United States, where there are few plant species closely- related to maize in the wild, the risk of gene flow to other species appears remote. Cultivated maize, Zea mays L. subsp. mays, is sexually compatible with other members of the genus Zea, and to a much lesser degree with members of the genus Tripsacum.

Weediness Potential

No competitive advantage was conferred to maize line T25, other than that conferred by resistance to glufosinate ammonium herbicides. Resistance to glufosinate ammonium will not, in itself, render maize weedy or invasive of natural habitats since none of the reproductive or growth characteristics were modified.

Cultivated maize is unlikely to establish in non-cropped habitats and there have been no reports of maize surviving as a weed. In agriculture, maize volunteers are not uncommon but are easily controlled by mechanical means or by using herbicides, other than glufosinate ammonium. Zea mays is not invasive and is a weak competitor with very limited seed dispersal.

Secondary and Non-Target Adverse Effects

It was determined that because genetically modified maize event T25 did not have a significant adverse impact on organisms beneficial to plants or agriculture or on nontarget organisms, this events was not expected to impact threatened or endangered species. The PAT enzyme responsible for glufosinate ammonium tolerance has very specific enzymatic activity, does not possess proteolytic or heat stability typical of toxic compounds, and does not affect the metabolism of the plant. Other crops such as sugarbeet, oilseed rape, chicory, soybean, cotton and rice, have been modified by recombinant DNA techniques to express the PAT enzyme with no apparent effect on the agronomic performance of succeeding crops. The expression level of PAT in T25 is comparable to other transformed species, and therefore it was concluded that no significant residual effects from T25 is expected.

Impact on Biodiversity

T25 has no novel phenotypic characteristics that would extend its use beyond the current geographic range of maize production. Since the risk of outcrossing with wild relatives in Canada and the United States is remote, it was determined that the risk of transferring genetic traits from T25 maize lines to species in unmanaged environments was insignificant. It was determined that the overall relative impact on plant biodiversity was neutral, as was the impact on animal and microbe biodiversity since the introduced PAT enzyme was not expected to alter the plant's metabolism and as such, novel compounds would not be produced.

Food and/or Feed Safety Considerations Expand

Dietary Exposure

Maize line from T25 were intended mainly for use in animal feed. The major human food uses for maize are extensively processed starch and oil fractions prepared by wet or dry milling procedures and include products such as corn syrup and corn oil, bran, grits, meal and flour. There was no PAT protein detected in starch or oil and in most of the maize products. An intake of very small amounts of PAT protein is possible through the consumption of dry milled meal and grits, and flour.

Nutritional Data

Compositional analyses of maize grain from T25 derived lines and current commercial maize varieties were compared for compositional and nutritional parameters. Grain of field maize grown in two seasons was analyzed for proximates and dietary fibre, as well as for free and total amino acids, free and total fatty acids, vitamins (B1, B2, B6, niacin, pantothenic acid, folic acid, tocopherols), and for the anti-nutrient phytic acid.  No biologically relevant differences were identified in the compositional characteristics of grain produced from maize T25 compared with its conventional counterpart, and the T25 grain composition falls within the range of non-GM conventional varieties, except for the expression of the PAT protein.

Allergenicity

The PAT enzyme expressed in T25-derived maize lines does not possess characteristics typical of known protein allergens and is extremely unlikely to be allergenic. There were no regions of homology when the sequences of the introduced protein were compared to the amino acid sequences of known protein allergens. There was no evidence found of post-translational modifications of the PAT protein. Unlike known protein allergens, the PAT protein was rapidly degraded by acid and/or enzymatic hydrolysis when exposed to simulated gastric fluids.  In vitro digestibility studies, under simulated mammalian gastric conditions, demonstrated that the PAT enzyme was inactivated within minutes and was rapidly degraded. No adverse effects have been reported to be associated with this enzyme.

Links to Further Information Expand

Brazil Ministry of Science and Technology-MCT National Technical Biosafety Commission-CTNBio CFIA Chinese Agriculture Department Columbian Ministry of Health and Social Protection EU Scientific Committee on Plants European Food Safety Authority (EFSA) European Union Commission FSANZ Health Canada Japan Ministry of Health, Labor and Welfare Malaysian National Biosafety Board (NBB) US FDA USDA

This record was last modified on Monday, October 31, 2016