GM Crop Database

Database Product Description
 
DK404SR
Host Organism
Zea mays L. L. (Maize)
 
Trait
Cyclohexanone herbicide tolerance, specifically sethoxydim.
 
Trait Introduction
Selection of somaclonal variants from embryo cultures.
 
Proposed Use
Production of Z. mays for human consumption (wet mill or dry mill or seed oil), and meal and silage for livestock feed. These materials will not be grown outside the normal production area for corn.
 
Company Information
BASF Inc.
Research and Development Center
26 Davis Drive
Research Triangle Park
NC  USA
 
 
Summary of Regulatory Approvals
 
Country Environment Food and/or Feed Food Feed Marketing
Canada 1996 1997 1996  
Click on the country name for country-specific contact and regulatory information.
Introduction
 
The maize hybrid DK404SR was developed to allow the use of sethoxydim, the active ingredient in the herbicide Poast? for the control of annual and perennial grasses in maize crops. DK404SR originated from a somaclonal variant selected from embryo tissue and expresses a modified version of acetyl-CoA-carboxylase (ACCase) enzyme.

ACCase is a key enzyme in the fatty acid biosynthesis pathway, necessary for the synthesis and maintenance of membranes and for the incorporation of fatty acids into triacylglycerides. The enzyme is inhibited by cyclohexanedione herbicides, such as sethoxydim, resulting in a lethal disruption of lipid biosynthesis. The mutant ACCase gene encodes for a modified version of acetyl-CoA-carboxylase enzyme in which the sethoxydim binding site is altered such that the ACCase enzyme is not inhibited by the sethoxydim herbicide, yet still retains its normal catalytic properties in fatty acid synthesis. The modified maize plants also demonstrated some level of cross-tolerance to the related herbicides, aryloxyphenoxypropionates, including haloxyfop (not registered in Canada), fluazifop (Venture) and quizalofop (Assure). These herbicides all have the same mode of action through ACCase inhibition. Natural tolerance to sethoxydim herbicide is found in several broadleaved plants and grasses and includes annual ryegrass, green foxtail and red fescue.

General Description
 
1. Sethoxydim Tolerance
Sethoxydim is a cyclohexanone herbicide registered in Canada for the control of annual and perennial grasses. Unmodified corn is not tolerant to sethoxydim, while broadleaved plants and some grasses such as annual ryegrass, green foxtail and red fescue are naturally tolerant to it. It is active against the enzyme acetyl-CoA-carboxylase (ACCase) of susceptible plants.

The enzyme ACCase catalyzes an important step in the biosynthesis of fatty acids necessary for membrane synthesis and maintenance, and for incorporation into triacylglycerides. Herbicide induced inhibition results in a lethal disruption of lipid biosynthesis.

Dekalb Canada Inc. selected a corn line, S2, that produces an altered ACCase enzyme that is not inhibited by the herbicide while retaining its normal catalytic properties. The mutation also resulted in some level of cross-tolerance to the related herbicides haloxyfop (not registered in Canada), fluazifop (Venture) and quizalofop (Assure). These herbicides all have the same mode of action through ACCase inhibition.

ACCase activities of unmodified corn and homozygous seedlings derived from S2 were shown to be similar in the absence of herbicide. A hundred fold higher sethoxidym concentration was required to inhibit half of the Poast compatible corn's ACCase activity, compared to the concentrations required to inhibit half of the unmodified corn's ACCase activity.

2. Development Method
Somatic embryos surviving on sethoxydim enriched media were selected, and the somaclonal variant cell line S2 was then selected.

The original S2 line was backcrossed with both parental lines of the hybrid at least six times.

3. Stable Integration into the Plant's Genome
Herbicide tolerance was consistently displayed by D404SR plants that were at least six generations away from the original variant.

The regenerated S2 plants were heterozygous for the mutant allele. The sethoxydim tolerance trait was shown to be inherited as a single partially dominant allele.

4. Nutrition
Nutritional composition for the modified and unmodified corn lines was compared. The selection of DK412SR and DK404SR corn lines through somaclonal variation has had no meaningful effect on the corn plant nutrient levels. The use of corn products derived from DK412SR and DK404SR corn lines would therefore have no significant impact on the nutritional quality of the Canadian food supply.
Reference: Canadian Food Inspection Agency, Plant Biotechnology Office; Office of Food Biotechnology, Health Canada

Summary of Introduced Genetic Elements
 
Code Name Type Promoter, other Terminator Copies Form
ACCase acetyl-CoA-carboxylase  (Z. mays) MUT N/A
N/A
N/A single partially dominant allele No introduced genetic material. Sethoxydim selection.

Characteristics of Zea mays L. (Maize)
 
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.

Modification Method
 
The maize hybrid DK404SR was developed from sethoxydim tolerant inbred lines. The original sethoxydim tolerant mutant line (S2) was selected as a somaclonal variant from maize embryo tissue grown under sethoxydim selective pressure. The process involved growing somatic embryos on sethoxydim enriched media. From the somatic embryos that survived, the somaclonal variant cell line S2 was selected and subsequently regenerated. This S2 line was backcrossed at least six times with both parental lines of the hybrid DK404SR to transfer the sethoxydim tolerant trait.

Characteristics of the Modification
 
The Introduced DNA
The novel herbicide tolerant trait introduced into the maize hybrid DK404SR was achieved through a somaclonal mutation within the maize genome. There was no new genetic material introduced into the maize hybrid DK404SR as a result of the modification.

Genetic Stability of the Introduced Trait
Expression of the herbicide tolerance trait was consistently displayed by DK404SR plants that were at least six generations away from the original S2 line.

The sethoxydim tolerance trait was inherited as a single partially dominant allele, as determined from the original mutant S2 line in which regenerated S2 plants were heterozygous for the mutant allele. The heterozygous progeny expressed herbicide tolerance at lower levels compared to homozygous plants.

Expressed Material
The activity of the ACCase enzyme in unmodified maize and homozygous seedlings derived from the S2 line was shown to be similar in the absence of sethoxydim herbicide. Application of sethoxydim herbicide inhibited the activity of ACCase by 50% in modified maize plants when applied at a concentration 100 fold higher than the sethoxydim concentration required to achieve a similar level of inhibition of ACCase activity in unmodified maize plants.

Environmental Safety Considerations
 
Field Testing
Field testing of DK404SR maize hybrids demonstrated that agronomic characteristics such as: plant size and vigour, growth, male and female fertility, time to maturity, flowering period, and seed yield, were within the range of values displayed by currently commercialized hybrids. It was determined that the growing habit of the modified maize plants was not inadvertently altered and that DK404SR maize hybrids did not display any altered pest potential.

Outcrossing
Since pollen production and viability were unchanged by the genetic modification resulting in maize hybrid DK404SR, pollen dispersal by wind and outcrossing frequency should be no different than for other maize hybrids. Gene exchange between DK404SR and other cultivated maize hybrids will be similar to that which occurs naturally between cultivated maize varieties at the present time. In Canada, where there are no plant species closely-related to maize in the wild, the risk of gene flow to other species is 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. However, none of the sexually compatible relatives of maize in Canada are considered to be weeds in Canada, and it is therefore unlikely that introgression of the modified ACCase gene would provide a selective advantage to these populations as they would not be routinely subject to herbicide treatments.

Weediness Potential
No competitive advantage was conferred to maize line DK404SR, other than that conferred by resistance to sethoxydim herbicide. Resistance to Poast herbicide 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 or by using herbicides that are not based on sethoxydim as appropriate. Zea mays is not invasive and is a weak competitor with very limited seed dispersal.

Secondary and Non-Target Adverse Effects
The novel trait present in the maize hybrid DK404SR was a mutation within a single corn enzyme, that altered the sethoxydim binding site without modification of metabolic abilities. Therefore, the potential toxicity of the ACCase enzyme was not a concern. It was determined that the maize hybrid DK404SR would not result in altered impacts on interacting organisms, including humans, compared with currently commercialized counterparts.

Impact on Biodiversity
Maize hybrid DK404SR 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 is remote, it was determined that the risk of transferring genetic traits from DK404SR to species in unmanaged environments was insignificant.

Food and/or Feed Safety Considerations
 
Dietary Exposure
Maize hybrid DK404SR is a yellow dent type used primarily for animal feeding. However, such field corn may be dry- or wet-milled into various processed maize products, such as fructose maize syrups, starch, oil, grits and flour for human food use. The human food uses of grain from DK404SR maize hybrid are not expected to be different from the uses of unmodified maize hybrids. As such, the dietary exposure of consumers to grain from DK404SR maize hybrids will not be different from that for other commercially available field corn varieties.

Nutritional Data
Nutritional composition for the modified and unmodified maize was compared. The selection of the DK404SR maize hybrid through somaclonal variation had no meaningful effect on the corn plant nutrient levels. The use of maize products derived from DK404SR maize hybrid would therefore have no significant impact on the nutritional quality of the food supply.

Toxicity
The novel trait in DK404SR maize hybrid results from modifications of a single maize enzyme, thus altering the sethoxydim binding site without modifying the metabolic abilities of ACCase enzyme. Potential toxicity of this enzyme was therefore not a concern.

Links to Further Information
 
Canadian Food Inspection Agency, Plant Biotechnology Office[PDF Size: 141727 bytes]
Decision Document 96-13: Determination of Environmental Safety of BASF Canada Inc.'s Sethoxydim Tolerant Corn (Zea mays L.) Hybrid DK404SR
Canadian Food Inspection Agency, Plant Biotechnology Office[PDF Size: 54708 bytes]
Supplement to Decision Document DD96-13: Determination of Livestock Feed Safety of BASF Canada Inc.'s Sethoxydim Tolerant Corn (Zea mays L.) Hybrid DK404SR
Office of Food Biotechnology, Health Canada[PDF Size: 8241 bytes]
NOVEL FOOD INFORMATION - FOOD BIOTECHNOLOGY SETHOXYDIM TOLERANT CORN (DK412SR AND DK404SR)


THIS RECORD WAS LAST MODIFIED ON SUNDAY, JULY 22, 2001
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