16.8 Resistance to Aromatic Amino Acid Inhibitors in Crops and Weeds

Glyphosate is a non-selective herbicide, but through the process of biotechnology glyphosate tolerant crops have been developed.

The gene encoding the enzyme that gives plants glyphosate resistance came from a soil bacterium. This version of EPSP synthase has a slightly altered amino acid sequence from that found in plants. The alteration prevents glyphosate from binding, while still allowing the resistant EPSP synthase to function normally, catalyzing amino acid synthesis reactions. Therefore, plants with the bacterial EPSP synthase can be sprayed with the herbicide and even though they absorb glyphosate, they have a resistant enzyme which remains unaffected. These plants continue to produce amino acids needed for growth and development.

Soil bacteria do not have chloroplasts so the bacterial EPSP synthase gene did not contain the DNA sequence that coded for the plant transit peptide. The transit peptide is required for the enzyme to be transported to the chloroplast, the location of the shikimic acid pathway. Through molecular modifications the plant transit peptide sequence was added to bacterial EPSP synthase. This redesigned gene has been used to successfully confer field level tolerance to glyphosate in crops like soybean, corn, sugar beets, and canola. Field level tolerance means that plants containing this genetic modification showed no visual injury symptoms when sprayed with 1 to 2 quarts/acre* of Roundup (trade name for glyphosate).

*This rate is related to research, not a recommended application rate. Always read product labels and follow all rules and regulations for herbicide application.

In the next video clip, Dr. Namuth-Covert describes in general how genetically engineered RoundUp Ready crops were first developed.

Researchers tried to produce glyphosate resistant plants by subjecting plants cells in liquid culture to increasing glyphosate levels. The process produced some level of tolerance; however, when plants were regenerated from these somatic cells, the level of tolerance was not commercially acceptable. This type of selection pressure resulted in selection for cells that contained multiple copies of the EPSP synthase gene, resulting in higher enzyme levels in the chloroplast. The enzyme was still inhibited by glyphosate. In other experiments utilizing EPSP synthase mutants from petunia, an enzyme was discovered that did not bind glyphosate, but it reduced enzymatic activity because of reduced PEP binding. Difficulties encountered trying to develop glyphosate resistant crops using natural selection or mutagensis led to the belief that the natural occurrence of field level resistance to glyphosate would be highly unlikely.

However, after a few years of using the RoundUp Ready crop varieties, there began to be reports of a glyphosate resistant rigid ryegrass (Lolium rigidum) biotype from Australia. The mechanism of resistance is not currently well understood, but it is known that resistance is not attributed to differences in glyphosate absorption, translocation, or metabolism. Then a second resistance event was discovered in a Malaysian goosegrass biotype (Eleusine indica) that was resistant to glyphosate at a rate of 2.3 qt/ac. This particular resistance mechanism has been identified as a missense mutation within the EPSP synthase gene coding sequences. The missense mutation resulted in the substitution of a serine molecule in place of the wild type proline at amino acid 106 in the amino acid sequence. While still considered a rare event, the selection pressure being applied to millions of crop and non-crop acres and billions of individual weeds is bound to uncover more spontaneous mutations that confer glyphosate resistance.  Many (44) cases of weed populations that are now tolerant of glyphosate applications have been documented and researched now globally (http://www.weedscience.org/Summary/SOASummary.aspx ).