GM Crop Database

Database Product Description

DK404SR
Host Organism
Zea mays 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.

Product Developer
BASF Inc.

Summary of Regulatory Approvals

Country Food Feed Environment Notes
Canada 1997 1996 1996

Introduction Expand

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.

Summary of Introduced Genetic Elements Expand

Code Name Type Promoter, other Terminator Copies Form
ACCase acetyl-CoA-carboxylase MUT N/A N/A N/A single partially dominant allele No introduced genetic material. Sethoxydim selection.

Characteristics of Zea mays L. (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.

Modification Method Expand

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 Expand

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 Expand

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 Expand

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.

Abstract Collapse

As the development of DK404SR did not employ recombinant DNA technologies, this product was not subject to regulation in any jurisdiction except Canada, where it was regulated as a plant with novel trait under the Seeds Act, a novel feed under the Feeds Act, and as a novel food under the Food and Drug Regulations. Specific environmental, livestock feed, and food safety assessment of this product was only required for Canada.

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, pharmaceuticals and car parts.

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. The original sethoxydim-tolerant mutant line (S2), which was the source of the sethoxydim tolerance trait in hybrid DK404SR, 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 in the presence of mutagenic agents. The somaclonal variant cell line S2 was selected from the resulting mutant cells and subsequently regenerated. This S2 line was backcrossed at least six times with both parental lines of the hybrid DK404SR to transfer the herbicide tolerance trait.

Sethoxydim and other cyclohexanedione herbicides act by inhibiting the enzyme acetyl-CoA-carboxylase (ACCase), 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. Inhibition of this enzyme results in a lethal disruption of lipid biosynthesis. Cyclohexanedione herbicides applied at rates recommended for effective weed control are toxic to conventional maize varieties. The ACCase present in DK404SR contains a mutation within the sethoxydim binding site that reduces the herbicide’s affinity for the enzyme, and thus its inhibitory activity, while retaining the normal catalytic properties of the enzyme. 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 broad-leaved plants and grasses, including annual ryegrass, green foxtail and red fescue.

As the development of DK404SR did not employ recombinant DNA technologies, this product was not subject to regulation in any jurisdiction except Canada, where it was regulated as a plant with novel trait under the Seeds Act, a novel feed under the Feeds Act, and as a novel food under the Food and Drug Regulations. Specific environmental, livestock feed, and food safety assessment of this product was only required for Canada.

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 they did not exhibit weedy characteristics, or negatively affect beneficial or non-target organisms. DK404SR was not expected to impact on threatened or endangered species.

Maize does not have any closely related species growing in the wild in 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 DK404SR. Gene exchange between DK404SR and maize relatives was determined to be negligible in managed ecosystems, with no potential for transfer to wild species in Canada.

A food and livestock feed safety assessment of DK404SR compared the nutritional composition of the modified maize line to that of its unmodified counterparts, revealing no significant differences. The selection of the DK404SR maize hybrid through somaclonal variation had no meaningful effect on the maize plant nutrient levels. It was therefore determined that the use of maize products derived from the DK404SR maize hybrid would have no significant impact on the nutritional quality of the food supply in Canada.

The novel trait in the DK404SR maize hybrid results from modifications of a single maize enzyme, altering the sethoxydim binding site without modifying the metabolic abilities of the ACCase enzyme. Potential toxicity of this enzyme was therefore not a concern. No new genetic material was introduced into the maize genome during the modification process, minimizing potential toxicity and allergenicity concerns.

Links to Further Information Expand


This record was last modified on Friday, March 26, 2010