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.
Figure 14: EPSP synthase is directed to the chloroplast.
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 (Fig 14). 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).
Researchers had 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 also had reduced enzymatic activity because of reduced PEP binding. Difficulties encountered trying to develop glyphosate resistant crops using natural selection or mutagenesis led to the belief that the natural occurrence of field level resistance to glyphosate would be highly unlikely.
Very recently there have been 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. A second resistance event has been discovered in a Malaysian goosegrass biotype (Eleusine indica) that is 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.
A website, weedscience.org, has been established and is maintained by a consortium of public and private weed researchers to document and catalog the latest information on herbicide resistant weeds world wide. The website is funded and supported by the Herbicide Resistance Action Committee (HRAC), the North American Herbicide Resistance Action Committee (NAHRAC), and the Weed Science Society of America (WSSA).