GM Crop Database

Maize line Bt11 was developed through a specific genetic modification to be resistant to attack by European corn borer (ECB; Ostrinia nubilalis), a major insect pest of maize in agriculture. The novel variety produced the insecticidal protein, Cry1Ab, derived from Bacillus thuringiensis subsp. kurstaki (B.t.k.) HD-1 strain. Delta-endotoxins, such as the Cry1Ab protein expressed in Bt11, act by selectively binding to specific sites localized on the brush border midgut epithelium of susceptible insect species. Following binding, cation-specific pores are formed that disrupt midgut ion flow and thereby cause paralysis and death. Cry1Ab is insecticidal only to lepidopteran insects, and its specificity of action is directly attributable to the presence of specific binding sites in the target insects. There are no binding sites for delta-endotoxins of B. thuringiensis on the surface of mammalian intestinal cells, therefore, livestock animals and humans are not susceptible to these proteins.

Bt11 was also genetically modified to express the pat gene cloned from the common aerobic soil actinomycete, Streptomyces viridochromogenes strain Tu494, which encodes a phosphinothricin-N-acetyltransferase (PAT) enzyme. 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, the active ingredient in the herbicides Basta, Rely, Finale, and Liberty). Glufosinate ammounium acts by inhibiting the plant enzyme glutamine synthetase, the only enzyme in plants that detoxifies ammonia by incorporating it into glutamine. Inhibition of this enzyme leads to an accumulation of ammonia in the plant tissues, which kills the plant within hours of application. PAT catalyses the acetylation of the herbicide phosphinothricin and thus detoxifies glufosinate ammonium into an inactive compound. The modified maize line is protected from ECB and permits farmers to use phosphinothricin-containing herbicides for weed control in the cultivation of maize.

The BT11 corn line was created through direct DNA transformation of plant protoplasts from the inbred maize line H8540 and regeneration on selective medium. A single plasmid, designated pZO1502, was used in the transformation event and contained a truncated synthetic cry1Ab gene encoding Cry1Ab endotoxin and a synthetic pat gene (to allow transformant selection on glufosinate ammonium). Prior to transformation, the plasmid vector was treated with the restriction endonuclease NotI in order to remove the bla gene from the DNA fragment containing the cry1Ab and pat genes.

Constitutive expression of the cry1Ab gene was controlled by the 35S promoter derived from cauliflower mosaic virus (CaMV) modulated by the IVS6 intron (from maize alcohol dehydrogenase 1S gene), and the 3'-polyadenylation signal of the nopaline synthase (nos) gene from Agrobacterium tumefaciens. The phosphinothricin acetyltransferase (pat) gene was present as a selectable marker enabling identification of the transformed plant cells and to provide field tolerance to glufosinate ammonium. Constitutive expression of the pat gene was under the control of the CaMV 35S promoter, the IVS 2 intron, and NOS 3' terminator.

The plasmid pZO1502 also contained the beta lactamase (bla) gene included as selectable marker to screen transformed bacterial cells. The bla gene codes for a beta-lactamase enzyme that confers resistance to some beta-lactam antibiotics, including the moderate-spectrum penicillin and ampicillin. Bacterial cells that contained the pZ01502 plasmid were selected through their resistance to ampicillin. The bla gene was excised from the plasmid vector prior to transformation of the maize tissue.

Other genetic components incorporated included a nonfunctional lacZ gene, encoding a portion of the enzyme beta-galactosidase; and the pUC origin of replication derived from the plasmid pBR322.

Following the transformation event, plants were regenerated and the pollen of maize plants (Zea mays L.) derived from transformation event BT11 was used to pollinate the female flowers of an inbred maize line. Descendants of the initial crossings were successively back-crossed to evaluate different maize lines carrying the BT11 event. Several hybrid maize varieties have been derived from the Bt11 maize event.

The Inserted DNA
Southern blot experiments confirmed the presence of a single copy the cry1ab and pat genes in BT11 maize lines and the absence of the beta-lactamase encoding bla gene.

Genetic Stability of the Introduced Trait
The stability of the inserted DNA in BT11 maize was demonstrated by a Mendelian inheritance pattern using Southern blot analysis. Segregation analysis of the cry1Ab and pat genes over multiple generations demonstrated that they were closely linked, as they always segregated together. Restriction fragment length polymorphism (RFLP) mapping was used to determine the location of the novel genes in BT11. The single insertion site was mapped to the long arm of chromosome 8.

Expressed Material
The expression levels of both the Cry1Ab protein and the PAT enzyme were determined in the leaves and kernels of transgenic corn. Accounting for extraction efficiencies, the amount of expressed Cry1Ab protein was found to range between 15 – 29 µg/g fresh weight for mature and young leaves, respectively, and 3.7 or 4.76 µg/g fresh tissue for heterozygous or homozygous kernels. The PAT enzyme was detected at levels of 38.6 – 49.4 ng/g fresh weight of leaf tissue, but not in roots, pollen or kernels. However, enzyme activity analysis indicated that PAT was expressed in all tissues throughout the plant. No significant differences were observed between hybrids derived using original elite lines and the selected BT11 line for the agronomic traits of yield, moisture at harvest, root lodging rating, ear height, plant height, heat units to silking or pollen shed. Other than resistance to ECB and tolerance to glufosinate ammonium herbicide, the disease, pest and other agronomic characteristics of BT11 corn were comparable to non-transgenic lines of corn.

Field Testing
The maize event BT11 has been field tested in various lines and hybrids, specifically the hybrids X4334CBR and X4734CBR, in the major maize growing regions of the United States since 1992, in Canada since 1993, and in Europe. Field reports on maize event BT11 compared to non-transgenic counterparts, determined that agronomic characteristics such as vegetative vigour, male and female fertility, time to maturity, seed yield, kernel size, weight and density, were within the normal range of expression currently displayed by commercial maize hybrids. In addition, the level of expression of the PAT enzyme in BT11 maize was found to be at a level high enough to confer tolerance to phosphinothricin herbicides. Overall the field data reports and data on agronomic traits showed that maize event BT11 and lines derived from BT11 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 BT11, pollen dispersal by wind and outcrossing frequency should be no different than for other maize varieties. Gene exchange between BT11 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, or 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 BT11, other than that conferred by resistance to European corn borer. Resistance to European corn borer 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. Zea mays is not invasive and is a weak competitor with very limited seed dispersal.

Secondary and Non-Target Adverse Effects
BT11 maize and the plant-expressed insectidical protein, Cry1Ab, should not have a significant potential to harm organisms beneficial to agricultural ecosystems. The history of use and literature suggest that the bacterial Bt protein is not toxic to humans, other vertebrates, and beneficial insects. Most insecticidal protein toxins from B. thuringiensis subspecies kurstaki, including Cry1Ab, have been shown to be specifically toxic to Lepidopteran larvae on ingestion and appear non-toxic to other species of insects, either directly or through secondary ingestion (predation). Feeding studies conducted under laboratory conditions have produced variable results on the development of some predatory insect species which have fed on prey that have fed on Btk material. Direct feeding studies with pollen from GM maize have shown no effects on honeybee development, lady beetles, insidious flowerbug and green lacewing. Results from feeding studies of young quail fed with modified maize meal in their diet showed no adverse effects.

In summary, it was determined that genetically modified maize event BT11 did not present an increased risk to, or impact on interacting organisms, including humans, with the exception of specific lepidopteran insect species. Furthermore, BT11 was not expected to impact on threatened or endangered lepidopteran species, as none were listed which feed on maize plants in North America.

Impact on Biodiversity
BT11 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 BT11 maize to species in unmanaged environments was insignificant.

Other Considerations
In order to prolong the effectiveness of plant-expressed Bt toxins, and the microbial spray formulations of these same toxins, regulatory authorities in Canada and U.S. have required developers to implement specific Insect Resistant Management (IRM) Programs. These programs are mandatory for all transgenic Bt-expressing plants, including maize event BT11, and require that growers plant a certain percentage of their acreage to non-transgenic varieties in order to reduce the potential for selecting Bt-resistant insect populations. Details on the specific design and requirements of individual IRM programs are published by the relevant regulatory authority.

BT11 maize plants are not likely to eliminate the use of chemical insecticides which are traditionally applied to about 25 to 35% of the total maize acreage planted. BT11 maize may positively impact current agricultural practices used for insect control by 1) offering an alternative method for control of European corn borer (and potentially other Cry1Ab-susceptible pests of maize); 2) reducing the use of insecticides to control European corn borer and the resulting potential adverse effects of such insecticides on beneficial insects, farm worker safety, and ground water contamination; and 3) offering a new tool for managing insects that have become resistant to other insecticides currently used or expressed in maize, including other Bt-based insecticides.

Dietary Exposure
BT11 maize was 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 products include corn syrup and corn oil, neither containing protein. Human exposure to the modified protein from whole grain corn in the diet was considered to be very low due both to its low abundance in the protein fraction of the grain and to the proportionately low percentage of protein in the kernel, compared with the major starch component. The level of the Cry1Ab protein in maize kernels, the only part of the plant used for human food, was very low - less than 5 ng/g fresh weight or 5 parts per billion. The dietary exposure will be lower than that experienced through eating products sprayed with Bt-based insecticides. Furthermore, the processing steps for maize would be expected to remove and/or destroy the Cry1Ab protein. Thus the level of Cry1Ab protein present in processed products derived from BT11 maize would be extremely low. Overall, the dietary exposure of people to insect resistant hybrids of BT11 maize was anticipated to be the same, or lower, as for other lines of commercially available field maize.

Nutritional Data
Detailed compositional analyses were carried out to establish the nutritional adequacy of BT11 maize, and to look for any unintended effects by comparing it to non-modified control lines. The nutritional composition of the ECB resistant maize hybrids was analysed at six locations for grain and silage. For the grain analysis, protein, oil, starch and fibre content of the ECB resistant maize lines were shown to be substantially equivalent to the untransformed controls. Silage samples from the ECB resistant maize hybrids had significantly higher levels of ADF and NDF than samples from the control lines, however the values were well within the normal range for corn silage. All other nutrients tested (protein, calcium, magnesium, phosphorus and potassium) were shown to be equivalent to their respective controls.

Proximate analysis (protein, fat, fibre) was performed using Near Infrared Reflectance (NIR) technology. The method was conducted with appropriate calibration and statistical analysis, to show equivalence between the NIR technique and traditional chemistry. Sample falling outside the normal range using NIR were analysed using AOAC approved, traditional chemical methods. The proximate analysis of the ECB resistant maize hybrids gave values well within the published range for traditional maize cultivars.

Animal feeding studies demonstrated that were few significant differences in results between BT11 maize and non-transformed maize. Four separate studies were made with cattle. The first, a short-term experiment with dairy cattle was designed to study any carry over of the Cry1Ab and PAT proteins into milk. Neither protein could be detected in the milk of any animals fed whole-crop BT11 maize. The second longer-term trial with dairy cattle compared performance of animals fed BT11 maize or its conventional counterpart. There was no effect on dry matter intake, milk production, milk composition or a number of rumen parameters relating to feed utilization.

Laying hens were also fed a diet for 14 days incorporating 64% maize meal from BT11 maize. The diet with transgenic maize contained 240 - 263 ng/g Cry1Ab and 59 - 74 ng/g PAT. The control diet was slightly contaminated with transgenic kernels, accounting for 18 ng/g Cry1Ab. No significant differences were observed for feed intake, bodyweight, egg production, and egg weight. Egg whites, yolks and tissue samples (breast, thigh, liver) were analysed for Cry1Ab and PAT at the end of the experimental period. Neither protein was found in these tissues above the detection limits of 6 ng/g for CRY1Ab and 30 ng/g for PAT.

Toxicity
The potential for acute toxicity of the Cry1Ab and PAT proteins was assessed by evaluating physical and chemical characteristics of the proteins and also by acute oral toxicity in mice. Data were generated that demonstrated that the active Cry1Ab protein product of the inserted cry1Ab gene was equivalent to the protein contained in microbial Bt spray formulations that have been safely used in agriculture for more than 40 years. Studies on the Cry1Ab protein expressed in plants and the resulting proteolytic fragments were compared to the bacterial proteins and shown to be of similar molecular weight, amino acid sequence, immunological reactivity and susceptibility to trypsin digestion. The protein was not glycosylated and showed similar bioactivity and host range specificity to the native protein. The biological activity of the trypsinized Cry1Ab protein from the modified maize was the same as that of the native Cry1Ab on ECB, with a LC50 of 0.46-0.51 µg/ml (f.w.) and an EC50 of 0.060-0.067 µg/g (f.w.). After one week incubation in soil, 99% of the bioactivity of Cry1Ab in transgenic leaf tissue was lost due to aerobic degradation.

The potential acute oral toxicity of Cry1Ab was assessed in mice. High oral dose feeding studies were conducted on mice fed Cry1Ab protein, or its truncated form, at up to 4000 mg/kg bodyweight without toxic effects. These results were consistent with other studies on the acute toxicity of Cry1Ab in mice and in rabbits and did not demonstrate any potential mammalian toxicity from Cry1Ab protein.

The toxicity of PAT protein was assessed using similar animal studies. Results from acute oral toxicity testing in mice did not indicate any toxic effects. A comparison of the amino acid sequence of the PAT protein to a database of known toxins demonstrated that it does not share any significant similarity with any known protein toxins. The sequences were obtained from the GenBank, EMBL, Swissprot and PIR databases. Furthermore, the PAT enzyme has very specific enzymatic activity and does not possess proteolytic or heat stability, and does not affect plant metabolism. Acetyltransferases are common enzymes in bacteria, plants and animals and no toxic or allergic effects were expected.

Allergenicity
The potential for the novel proteins Cry1Ab and PAT to be allergenic was investigated using a number of criteria, including amino acid sequence homology with known allergens, history of use and common physicochemical properties of allergens, including the sensitivity to digestion by digestive enzymes.

The deduced amino acid sequences of the introduced proteins were compared to 219 known allergens present in public domain databases (GenBank, EMBL, Swissprot, PIR) and no homologies were detected. Unlike known protein allergens, studies have shown that Cry1Ab proteins were rapidly inactivated when subjected to simulated mammalian gastric fluids. Similarly, the PAT protein was found to be rapidly digested in conditions that mimic human digestion. Also, PAT was found to be present at very low levels, if at all, in maize kernels.

Herbicide Residue Analysis
The principal residue identified in transgenic BT11 maize plants after post-emergence use of glufosinate ammonium was N-acetyl-glufosinate with lesser quantities of glufosinate and 3-[hydroxy(methyl)phosphinoyl]propionic acid (MPP) which was also found in non-transgenic plants. In maize grain, which exhibits much lower residues than the other plant parts, the principal residue identified was MPP with lesser amounts of N-acetyl-glufosinate. About 80 field trials in Europe were conducted with different application rates and residues in harvested grain for each metabolite was below 0.05 mg/kg. In green maize, forage and fodder, higher residues can occur. The glufosinate-derived residues do not concentrate in any maize processed fraction, which are relevant food or feed items. These include flour, starch, grits and oil. Residues are not detectable in crude and refined oil. In ruminant and poultry feeding studies no detectable residues were found in meat, milk or eggs at the dose calculated to represent the highest residues in livestock feed under Good Agricultural Practices and taking into account the potential use of glufosinate herbicide in several tolerant crops. It was concluded, on the basis of the available data, that residues of glufosinate ammonium and its metabolites, N-acetyl-glufosinate and 3-[hydroxy(methyl)phosphinoyl]propionic acid (MPP) expressed as glufosinate free acid equivalents, were below 0.1 mg/kg maize grain. In food of animal origin from livestock animal fed with feedstuffs after application of glufosinate herbicide in tolerant maize no residues above the limit of determination are to be expected.