GA21 (Zea mays) has a centrally inserted sequence. The mepsps gene has been inserted to give this genetically modified maize plant protection against herbicides with glyphosate. The mepsps gene (mutated-5-enolpyruvyl shikimate-3-phosphate synthase) will form a mEPSPS enzyme. The active substance glyphosate in herbicides targets and binds to the EPSPS enzyme found in bacteria, fungi and plants. The enzyme mEPSPS in GA21 has been changed through mutation of the epsps gene from maize. GA21 with mEPSPS tolerates glyphosate because this mEPSPS lacks a binding site where glyphosate can inhibit it.
GA21 was produced via microprojectile bombardment of maize suspension culture cells. A vector was inserted with components from rice (Oryza sativa) which gives constitutive expression of the formepsps gene in maize. Next to this, a chloroplast transit peptide sequence from sunflower (Helianthus annuus) and a RuBisCo gene sequence from maize have been inserted. This sequence has the function of directing mEPSPS protein to the chloroplast which is the site of aromatic amino acid biosynthesis. Then follows a sequence that codes for the mEPSPS protein itself. The last sequence originates from a bacterium Agrobacterium tumefaciens and is there to terminate further transcription.
This mepsps gene will form mEPSPS enzyme that will make maize GA21 tolerant to herbicides containing glyphosate or sulfosate. GA21 may have lower glyphosate tolerance than, for example, NK603 and other genetically modified maize lines. In contrast, these gene modifications give GA21 enough resistance to glyphosate, so that glyphosate can be used as an active ingredient in herbicides without it killing GA21 at recommended concentrations. There are mEPSPS in almost all cells of GA21, also in pollen.
The mepsps gene originates from maize. The isolated endogenous GA21-epsps gene was modified through site-directed mutagenesis. Site-directed mutagenesis creates a mutation at a defined location, and requires a known template sequence. These maize cells were then grown on a medium with glyphosate. The adult plants that survived glyphosate contained mepsps (mutant epsps gene) which conferred glyphosate tolerance. GA21 should survive being sprayed with herbicides with high concentrations of glyphosate. GA21 has mutated epsps gene from maize created through site-directed mutagenesis to form a glyphosate-resistant variant of 5-enolpyruvulshikimate-3-phosphate synthase (EPSPS). This glyphosate-resistant variant is mEPSPS enzyme formed from mepsps gene which is a mutated version of epsps from maize.
Glyphosate herbicides usually have an adjuvant such as POEA (Polyoxyethylene tallow amine). This is a substance that makes the absorption of glyphosate in cells more efficient. In herbicides such as Roundup, POEA is the adjuvant, which has been shown to be more than a thousand times more toxic than glyphosate in three different human cell lines. The toxicity of POEA has also been shown in studies with various aquatic animals. The commercial herbicides with glyphosate and adjuvant are significantly more toxic than glyphosate alone. In a two-year study of rats exposed to a low dose of glyphosate-based herbicide (Roundup), the rats were found to accumulate serum triglycerides as they aged after one year. It was registered and documented that it created significant changes in the liver's biochemistry, which will lead to changes in the liver's anatomy and pathological changes. Some of these changes were detected within the range between glyphosate permitted in drinking water and glyphosate permitted in foodstuffs.
Mesnage R., Renney G., Séralini G.-E., Ward M., and MN Antoniou. 2017. Multiomics reveal non-alcoholic fatty liver disease in rats following chronic exposure to an ultra-low dose of Roundup herbicide. Sci. Rep. 7, 39328 doi: 10.1038/srep39328.