Database Product Description
- Host Organism
- Zea mays (Maize)
- Trade Name
- 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 livestock feed and products for industrial applications.
- Product Developer
- Aventis CropScience
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 for the 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 CBH-351 was genetically modified to contain two novel genes, cry9C 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 CBH-351 was developed to resist ECB by producing its own insecticide. This event was genetically engineered by introducing the cry9C gene, isolated from the common soil bacterium Bacillus thuringiensis (Bt), into a maize line by particle acceleration (biolistic) transformation. The cry9C gene produces the insect control protein Cry9C, a delta-endotoxin. Cry proteins, of which Cry9C 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. Cry9C 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 cry9c gene, CBH-351 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 cry9c gene. CBH-351 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 CBH-351 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.
Maize line CBH-351 was tested in field trials in the United States and Canada, beginning in 1995. Data collected from these trials demonstrated that CBH-351 was not different from conventional maize varieties. Agronomic characteristics and kernel traits were within the expected range of expression reported for commercial maize hybrids. Maize line CBH-351 was comparable to conventional maize lines and did not exhibit weedy characteristics, or negatively affect beneficial or nontarget organisms. CBH-351 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 CBH-351. Gene exchange between CBH-351 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.
Kernel composition, quality and other characteristics of CBH-351 maize were not significantly different from non-transgenic maize.
CBH-351 maize has been approved in the United States only for livestock feed use. The safety for use in animal feed was supported by the lack of toxicity of bacterial-expressed Cry9C protein in high oral dose feeding studies with laboratory animals and the low potential for toxicity of plant-expressed Cry9C protein demonstrated by a lack of amino acid sequence homology with known protein toxins.
The allergenic potential of the Cry9C protein was assessed by examining the amino acid sequence homology between the Cry9C protein and known protein allergens. A database search determined that there were no significant similarities in the amino acid sequences for the Cry9C protein and known protein allergens. A pepsin digestibility study determined that the Cry9C protein was stable to digestion for up to 4 hours and was heat stable when incubated at 90C for 10 minutes.
The Environmental Protection Agency (EPA) ruled, in the pesticide assessment of Cry9C in maize line CBH-351, that the use of maize line CBH-351 be restricted to animal feed only. The EPA also established that it was acceptable to find the Cry9C protein in meat, poultry, milk, or eggs derived from animals fed CBH-351 maize feed. Following authorization of CBH-351 for use in animal feed, the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) Scientific Advisory Panel (SAP) was tasked with reviewing scientific information concerning the allergenic potential of the Cry9C protein. In the SAP Report (December 2000) it was concluded that there was a medium likelihood that the Cry9C protein was a potential allergen.
Links to Further Information Expand
This record was last modified on Friday, March 26, 2010