Database Product Description
- Host Organism
- Zea mays (Maize)
- Trade Name
- Bt Xtra™
- Resistance to European corn borer (Ostrinia nubilalis); phosphinothricin (PPT) herbicide tolerance, specifically glufosinate ammonium.
- Trait Introduction
- Microparticle bombardment of plant cells or tissue
- Proposed Use
Production for human consumption and livestock feed.
- Product Developer
- Dekalb Genetics Corporation
Summary of Regulatory Approvals
Summary of Introduced Genetic Elements Expand
Characteristics of Zea mays (Maize) Expand
Donor Organism Characteristics Expand
Modification Method Expand
Characteristics of the Modification Expand
Environmental Safety Considerations Expand
Food and/or Feed Safety Considerations Expand
Maize (Zea mays L.), or corn, is grown primarily for its kernel, which is largely refined into products used in a wide range of food, medical, and industrial goods.
Only a small amount of whole maize kernel is consumed by humans. Maize oil is extracted from the germ of the maize kernel and maize is also a raw material in the manufacture of starch. A complex refining process converts the majority of this starch into sweeteners, syrups and fermentation products, including ethanol. Refined maize products, sweeteners, starch, and oil are abundant in processed foods such as breakfast cereals, dairy goods, and chewing gum.
In the United States and Canada maize is typically used as animal feed, with roughly 70% of the crop fed to livestock, although an increasing amount is being used forthe production of ethanol. The entire maize plant, the kernels, and several refined products such as glutens and steep liquor, are used in animal feeds. Silage made from the whole maize plant makes up 10-12% of the annual corn acreage, and is a major ruminant feedstuff. Livestock that feed on maize include cattle, pigs, poultry, sheep, goats, fish and companion animals.
Industrial uses for maize products include recycled paper, paints, cosmetics, car parts and pharmaceuticals.
The European corn borer (ECB), Ostrinia nubilalis, is the most damaging insect pest of maize in the United States and Canada; losses resulting from ECB damage and control costs exceed $1 billion each year. An average of one ECB cavity per maize stalk across an entire field can reduce yield by as much as 5% when caused by first generation larvae, and 2.5% when caused by second generation larvae, with annual yield losses estimated at 5 to 10 %.
Despite consistent losses to ECB, chemical insecticides are utilized on a relatively small acreage (less than 20%). Historically, this reluctance stems from the difficulties in identifying and managing ECB in maize crops: ECB larval damage is hidden, heavy infestations are unpredictable, insecticides are costly, timing of insecticide application is difficult and multiple applications may be required to guarantee ECB control.
Weeds are also a major production problem in maize cultivation. Even a light infestation of weeds can reduce yields by 10 to 15%; severe infestations can reduce yields by 50% or more. Typically, weeds are managed using a combination of cultural (e.g., seed bed preparation, clean seed, variety selection) and chemical controls. Depending on the production area and the prevalent weed species, herbicides may be incorporated into the soil before planting (pre-plant), applied after planting but before emergence (pre-emergence), or applied after the maize plants emerge (post-emergence). Ideally, for maize production, weeds should be controlled for the full season. However, the most critical period for weed control is usually about six to eight weeks after crop emergence, during the 4th to 10th leaf stages. This critical period in the life cycle of maize must be kept weed free in order to prevent yield loss.
Maize line DBT418 was genetically modified to contain two novel genes, cry1Ac and bar, for insect and herbicide tolerance respectively. Both genes were introduced into a maize line by particle acceleration (biolistic) transformation.
The transgenic maize line DBT418 was developed to resist ECB by producing its own insecticide. This event was genetically engineered by introducing the cry1Ac gene, isolated from the common soil bacterium Bacillus thuringiensis (Bt), into the maize line AT824. The cry1Ac gene produces the insect control protein Cry1Ac, a delta-endotoxin. Cry proteins, of which Cry1Ac is only one, act by selectively binding to specific sites localized on the lining of the midgut of susceptible insect species. Following binding, pores are formed that disrupt midgut ion flow, causing gut paralysis and eventual death due to bacterial sepsis. Cry1Ac is lethal only when eaten by the larvae of lepidopteran insects (moths and butterflies), 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 the delta-endotoxins of B. thuringiensis on the surface of mammalian intestinal cells, therefore, livestock animals and humans are not susceptible to these proteins.
In addition to the cry1Ac gene, DBT418 was developed to allow for the use of glufosinate ammonium, the active ingredient in phosphinothricin herbicides (Basta®, Rely®, Liberty®, and Finale®), as a weed control option, and as a breeding tool for selecting plants containing the cry1Ac gene. DBT418 contains the bar gene isolated from a common soil actinomycete, Streptomyces hygroscopicus. This gene allows for the production of the enzyme phosphinothricin N-acetyltransferase (PAT) which confers tolerance to glufosinate.
The PAT enzyme in maize line DBT418 converts L-phosphinothricin (PPT), the active ingredient in glufosinate ammonium, to an inactive form, thereby conferring resistance to the herbicide. In the absence of PAT, application of glufosinate leads to reduced production of the amino acid glutamine and increased ammonia levels in the plant tissues, resulting in the death of the plant. The PAT enzyme is not known to have any toxic properties.
Expression of the Cry1Ac protein and PAT occurred in most, but not all, tissues of the maize plant with levels highest in leaves, and lower levels in roots, prop roots, stalk, tassel, cob, husk, and kernels. No Cry1Ac or PAT proteins were detected in silk or pollen.
Maize line DBT418 was tested in field trials in the United States, beginning in 1993, and in Canada, beginning in 1995. Data collected from these trials demonstrated that DBT418 was not different from conventional maize varieties. Agronomic characteristics such as seedling vigor and yield were within the expected range of expression reported for commercial maize hybrids. The transgenic line DBT418 provided significant protection from feeding damage caused by the European corn borer (Ostrinia nubilalis) and Southwestern corn borer (Diatraea grandiosella). Maize line DBT418 was comparable to conventional maize lines in all other respects and did not exhibit weedy characteristics, or negatively affect beneficial or nontarget organisms. DBT418 was not expected to impact on threatened or endangered species.
Maize does not have any closely related species growing in the wild in the continental United States or Canada. Cultivated maize can naturally cross with annual teosinte (Zea mays ssp. mexicana) when grown in close proximity, however, these wild maize relatives are native to Central America and are not naturalized in North America. Additionally, reproductive and growth characteristics were unchanged in maize line DBT418. Gene exchange between DBT418 and maize relatives was determined to be negligible in managed ecosystems, with no potential for transfer to wild species in Canada and the United States.
Regulatory authorities in Canada and the United States have mandatory requirements for developers of Bt maize to implement specific Insect Resistant Management (IRM) Programs. The potential for ECB populations to develop tolerance or become resistant to the Bt toxin is expected to increase as more maize acreage is planted with Bt hybrids. These IRM programs are designed to reduce the potential development of Bt-resistant insect populations, as well as prolonging the effectiveness of plant-expressed Bt toxins, and the microbial Bt spray formulations of these same toxins.
The food and livestock feed safety of maize line DBT418 was established based on several standard criteria. As part of the safety assessment, the nutritional composition of the grain and forage of DBT418 were analyzed in order to determine protein, oil, fibre, ash, moisture, and starch content, and were found to be equivalent to conventional maize. The grain’s amino acid composition was also analyzed and found to be identical to that of conventional maize hybrids.
The potential for toxicity and allergenicity of the DBT418-expressed Cry1Ac and PAT proteins was demonstrated by examining their physiochemical characteristics and amino acid sequence homologies to known protein toxins and allergens. Stability to acid and/or proteolytic digestion, heat stability, glycosylation, and molecular weight were determined and digestibility and acute oral toxicity studies were also conducted. The Cry1Ac protein has a history of safe use, as demonstrated by its use in microbial Bt spray formulations in agriculture for more than 30 years with no evidence of adverse effects. Similarly, PAT has a very specific enzymatic activity and does not possess proteolytic or heat stability, and does not affect plant metabolism. Acetyltransferases such as PAT are common enzymes in bacteria, plants and animals and no toxic or allergic effects were expected. These facts, combined with the lack of amino acid sequence homologies between Cry1Ac and PAT proteins, and known allergens and toxins, and the rapid degradation of the novel proteins in acidic gastric fluids, was sufficient to provide with reasonable certainty a lack of toxicity and allergenic potential.
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This record was last modified on Monday, September 2, 2013