Glyphosate is an inhibitor of 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), a well-known enzyme of the shikimate pathway for aromatic amino acid biosynthesis present in plants, fungi and bacteria. Plants, including weeds, exposed to glyphosate are unable to produce aromatic amino acids and hence die.
To produce Roundup Ready® canola, two genes were introduced into the canola genome:
- the cp4 epsps gene, derived from the common soil bacterium Agrobacterium strain CP4, which encodes for the production of the CP4 EPSPS enzyme
- the gox gene from Ochrobactrum anthropi strain LBAA, which encodes for the production of the enzyme glyphosate oxidase (GOX).
Both gene products are expressed within in the plant and together are responsible for tolerance to glyphosate (refer to figure below). Because CP4 EPSPS has a naturally high tolerance to inhibition by glyphosate, Roundup Ready canola plants continue to produce aromatic amino acids even after treatment with glyphosate. In addition, the GOX protein catalyzes the breakdown of glyphosate into glyoxylic acid and aminomethylphosphonic acid (AMPA).
The disarmed Agrobacterium tumefaciens plant transformation delivery system was used to produce Roundup Ready canola line GT73 (White, 1989; Howard et al., 1990). This system is well documented to transfer and stably integrate T-DNA into a plant nuclear genome (White, 1989; Howard et al., 1990).
Only the DNA required for glyphosate-tolerance was transferred and inserted at a single locus in the canola genome. A single chromosomal copy of the DNA has been stably inherited across multiple generations of Roundup Ready canola plants. Moreover, the consistent commercial performance of Roundup Ready canola further supports the stability of the inserted DNA and functioning of the CP4 EPSPS and GOX proteins.
White, F. F. 1989. Vectors for Gene Transfer in Higher Plants. In: Plant Biotechnology. S. Kung and C.J. Arntzen, editors. Butterworths, Boston. pp. 3-34.