Breakdown of glufosinate in transgenic and non-transgenic maize and oilseed rape

Topic

An altered breakdown pattern is to be expected in transgenic herbicide‑resistant plants compared with conventional plants following application of the complementary herbicide . Using oilseed rape and maize plants with genetically engineered resistance to the herbicide glufosinate (active substance phosphinothricin), the project examined the following questions:

• Is there a difference in the breakdown of glufosinate in transgenic plants compared with conventional plants?

• How are the residues distributed in the various plant parts?

• Do the D and L forms of the active substance differ in their degradability? What difference does it make if the breakdown takes place in cell cultures or in intact plants outdoors?
  (Various molecules come in two types, which differ in their three-dimensional form: the dextrorotatory (D) and levorotatory (L) form. The two types often have a different effect. A 1:1 mixture of the two is called a racemat.)

Summary

In transgenic plants N-acetyl-phosphinothricin is the main conversion product (metabolite) of the herbicide used (glufosinate). Furthermore, the same breakdown products are found in cells of herbicide-resistant plants and conventional plants that are sensitive to the herbicide.

The D form of the active herbicide agent is not broken down by the plants. In the field, bacteria can convert the D form into the L form, which can then be further broken down by the plants.

Experiment description

The experiment is based on the assumption that the set herbicide limits are not exceeded in the cultivation of transgenic oilseed rape and maize and that good professional practice is observed. The investigations are based on appropriate dosage of the herbicide.

Differences in the breakdown of glufosinate were investigated in cell cultures and in plants.

• For the breakdown studies on sterile cell cultures, herbicide-sensitive (conventional) and herbicide-resistant (transgenic) oilseed rape and maize cells were mixed with radioactively marked herbicide and filtered off the culture medium after 3-21 days.

• The breakdown studies on transgenic oilseed rape and maize plants took place under field conditions (in a lattice enclosure). The radioactively marked active agent was applied to the leaves in the spring.

Distribution of residues in the various plant parts

At harvest time, various plant parts (treated and untreated leaves, stems and kernels) were removed and examined.

Differences between the D and L form of the active agent

The cell cultures and the transgenic plants were treated with both forms of the active agent in combination and with each one individually and examined.

Extracts from the plant parts/filtered cells were investigated using a special method. The breakdown products were identified by comparing them with known, radioactively marked reference substances and quantified by means of their radioactivity (radiodetection and scintillation counting).

Results

Differences in the breakdown of glufosinate

In transgenic cells and plants, N-acetyl-phosphinothricin was the main conversion product. It remains stable in transgenic plants.

In addition, small quantities of the same breakdown products (methylphosphinico fatty acids) were found that are found in cells of herbicide-sensitive plants.

Distribution of residues in the various plant parts

Approximately 70-90% of the herbicide applied to the transgenic plants is lost again over the course of the vegetation period through rain and leaf shed. At harvest time, the highest residue levels were found in the treated leaves (4-15% of the quantity applied), the lowest levels were found in maize kernels and rape seeds (0.1 – 0.6%).

Differences between the D and L forms of the active agent

The D form of the active herbicide agent was not converted in either the sensitive or transgenic plant cells. All conversion products came from the L form. The D form is not broken down in intact transgenic plants either. Instead, most of it is washed off by the rain.

95% of the total herbicide residues were water soluble.