DD1996-13: Determination of Environmental Safety of BASF Canada Inc.'s Sethoxydim Tolerant Corn (Zea mays L.) Hybrid DK404SR

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Supplement to Decision Document DD96-13

Issued: 1996-05

This Decision Document has been prepared to explain the regulatory decision reached under the guidelines Dir94-08 Assessment Criteria for Determining Environmental Safety of Plants with Novel Traits and its companion document Dir94-11 The Biology of Zea mays L. (Corn/Maize).

The Canadian Food Inspection Agency (CFIA), specifically the Plant Biosafety Office of the Plant Health and Production Division, has evaluated information submitted by BASF Canada Inc. regarding a corn hybrid referred to as DK404SR in the present document. These plants were modified to express tolerance to the herbicide sethoxydim. CFIA has determined that these plants with novel traits should not pose concerns with respect to environmental safety.

Unconfined release into the environment of the corn hybrid DK404SR is therefore authorized. Also, any line derived from the same original somaclonal variant may be considered substantially equivalent to DK404SR, provided no interspecific crosses are performed, provided the intended use is similar, and provided it is known that these plants do not display any additional novel traits.

Please note that, while determining the environmental safety of plants with novel traits is a critical step in the commercialization of these plant types, other requirements still need to be addressed, such as for the evaluation of feed safety (CFIA) and food safety (Health Canada), for Variety Registration (CFIA) and for sethoxydim registration for use on corn (Pest Management Regulatory Agency, Health Canada).

Table of Contents

  1. Brief Identification of the Plant with Novel Traits (PNT)
  2. Background Information
  3. Description of the Novel Trait
    1. Sethoxydim Tolerance
    2. Development Method
    3. Stable Integration into the Plant's Genome
  4. Assessment Criteria for Environmental Safety
    1. Potential of the PNT to Become a Weed of Agriculture or Be Invasive of Natural Habitats
    2. Potential for Gene Flow to Wild Relatives Whose Hybrid Offspring May Become More Weedy or More Invasive
    3. Altered Plant Pest Potential
    4. Potential Impact on Non-Target Organisms
    5. Potential Impact on Biodiversity
  5. Regulatory Decision

I. Brief Identification of the Plant with Novel Traits (PNT)

Designation(s) of the PNT: Corn hybrid DK404SR

Applicant: BASF Canada Inc.

Plant Species: Corn (Zea mays L.)

Novel Traits: Sethoxydim herbicide tolerance

Trait Introduction Method: Selection of a somaclonal variant from embryo culture

Proposed Use of PNT's: Cultivation as hybrid grain corn for livestock feed and human consumption. These materials will not be grown outside the usual production area for corn in Canada.

II. Background Information

BASF has developed a corn hybrid tolerant to sethoxydim, the active ingredient of the herbicide Poast, currently registered in Canada as a post-emergence herbicide for the control of various grassy weeds in broadleaf crop plants such as canola or soybeans. This corn hybrid DK404SR, also referred to as Poast compatible corn in the present document, exhibits no significant injury when treated with Poast at normal field application rates, and will allow the use of Poast as a post-emergence herbicide on corn, thus providing an alternative for weed control in hybrid corn production.

The original sethoxydim tolerant mutant line was developed by Dekalb Canada Inc., through somaclonal variation of embryo tissue grown under sethoxydim selective pressure. The herbicide tolerant phenotype results from the presence of a gene coding for a modified version of acetyl-CoA-carboxylase, a key enzyme in the fatty acid biosynthesis pathway, such that this enzyme is no longer inactivated by sethoxydim.

These lines have been tested in Canada under confined conditions since 1993 in Ontario.

BASF Canada Inc. has provided data on the identity of the corn hybrid DK404SR, a description of the modification method and breeding history, data on the role and the activity levels of the modified acetyl-CoA-carboxylase, and on the stability of trait expression. References to scientific publications were also included.

Agronomic characteristics such as vegetative vigour, male and female fertility, time to maturity, seed yield, kernel size, weight and density, were compared to those of unmodified corn counterparts.

The Plant Biosafety Office of the Plant Health and Production Division, CFIA, has reviewed the above information, in light of the following assessment criteria for determining environmental safety of plants with novel traits, as described in the regulatory directive Dir94-08:

  • potential of the PNT's to become weeds of agriculture or to be invasive of natural habitats,
  • potential for gene-flow to wild relatives whose hybrid offspring may become more weedy or more invasive,
  • potential for the PNT's to become plant pests,
  • potential impact of the PNT's or their gene products on non-target species, including humans, and
  • potential impact on biodiversity.

III. Description of the Novel Trait

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.

IV. Assessment Criteria for Environmental Safety

1. Potential of the PNT to Become a Weed of Agriculture or Be Invasive of Natural Habitats

The biology of corn (Zea mays), described in Dir94-11, shows that unmodified plants of this species are not invasive of unmanaged habitats in Canada. Corn does not possess the potential to become weedy due to traits such as lack of seed dormancy, the non-shattering aspect of corn cobs, and poor competitive ability of seedlings. According to the information provided by BASF Canada Inc., Poast compatible corn derived from S2 was determined not to be different from their counterparts in this respect. No competitive advantage was conferred to D404SR corn hybrids, other than tolerance to sethoxydim and cross-tolerance, to various degrees, to certain related herbicides of the same class.

Tolerance to these herbicides will not, in itself, render corn weedy or invasive of natural habitats since none of the reproductive or growth characteristics were modified. All commercial broadleaved crop kinds are naturally tolerant to sethoxydim and there is no correlation with invasiveness characteristics.

Corn is an open-pollinated species and DK404SR could cross-pollinate with other corn hybrids. The resulting progeny could acquire the herbicide tolerance gene. Progeny from self-pollination of DK404SR will also be sethoxydim tolerant. Heterozygous progeny express herbicide tolerance, although at a lower level than homozygotes. Sethoxydim tolerant corn volunteers will thus appear in subsequent crops. Corn volunteers are commonly found in soybean fields when soybeans are cultivated in the year following corn. While these volunteers could be managed by mechanical means and other available herbicides, the use of sethoxydim and, to a lesser extent, of the related herbicides fluazifop and quizalofop to control volunteer corn in soybean may be compromised.

The above considerations led CFIA to conclude that D404SR hybrids have no altered weed or invasiveness potential compared to currently commercialized corn hybrids. Please note however that there is growing concern, with the adoption of several herbicide tolerant corn varieties, that various corn volunteers with different novel herbicide tolerances could become difficult to control in crop rotation cycles. Therefore, agricultural extension personnel, in both the private and public sectors, should promote careful management practices for growers who use these herbicide tolerant crops, to minimize the development of a corn volunteer problem.

2. Potential for Gene Flow to Wild Relatives Whose Hybrid Offspring May Become More Weedy or More Invasive

The biology of corn, as described in Dir94-11, indicates that there are no wild relatives in Canada that can hybridized with Zea mays.

CFIA therefore concludes that gene flow from D404SR hybrids to corn relatives is not possible in Canada.

3. Altered Plant Pest Potential

The intended effect of the novel trait is unrelated to plant pest potential, and corn is not a plant pest in Canada (Dir94-11). In addition, agronomic characteristics of the modified corn hybrids such as: plant size and vigour, growth, male and female fertility, time to maturity, flowering period, and seed yield, were shown to be within the range of values displayed by currently commercialized corn hybrids, and indicate that the growing habit of corn was not inadvertently altered.

CFIA has therefore determined that D404SR corn hybrids do not display any altered pest potential.

4. Potential Impact on Non-Target Organisms

The novel trait results from modifications of a single corn enzyme, thus altering the sethoxydim binding site without modification of metabolic abilities. Potential toxicity of this enzyme is therefore not a concern. Agronomic characteristics showed that the corn hybrid D404SR is substantially equivalent to unmodified counterparts.

CFIA has therefore determined that the unconfined release of these corn hybrids will not result in altered impacts on interacting organisms, including humans, compared with currently commercialized counterparts.

5. Potential Impact on Biodiversity

Hybrid D404SR has no novel phenotypic characteristics which would extend its use beyond the current geographic range of corn production in Canada. Since corn does not outcross to wild relatives in Canada, there will be no transfer of novel traits to unmanaged environments.

CFIA has therefore concluded that the potential impact on biodiversity of these corn hybrids is equivalent to that of currently commercialized corn varieties.

V. Regulatory Decision

Based on the review of data and information submitted by BASF Canada Inc., and through comparisons of corn hybrids D404SR with unmodified corn counterparts, the Plant Biosafety Office of the Plant Health and Production Division, CFIA, has concluded that the novel gene and its corresponding trait do not confer to these corn hybrids any characteristic that would result in intended or unintended environmental effects following unconfined release.

If at any time, BASF Canada Inc. becomes aware of any new information regarding risk to the environment, or risk to animal or human health, that could result from release of these materials in Canada, or elsewhere, BASF Canada Inc. must immediately provide such information to CFIA. On the basis of such new information, CFIA may re-evaluate the potential impact of the release and re-evaluate its decision.

Unconfined release into the environment of the corn hybrid DK404SR is therefore authorized. Also, any line derived from the same original somaclonal variant may be considered substantially equivalent to DK404SR, provided no interspecific crosses are performed, provided the intended use is similar, and provided it is known that these plants do not display any additional novel traits.

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