Liberty Link

LibertyLink plants contain resistant to the herbicide glufosinate. Glufosinate is an analog of the amino acid glutamate and goes by several commercial names. LibertyLink and RoudupReady plants are the two major types of genetically engineered herbicide resistance in use.

Mechanism of herbicide toxicity

Glufosinate is the active ingredient that inhibits plant growth in herbicides that LibertyLink plants are resistant to. These herbicides can come in a variety of forms, including bialaphos, a tripeptide containing two alanine residues and glufosinate. In the case of bialaphos, the peptide is cleaved to produce the active glufosinate.

Glufosinate is very similar in chemical structure to the amino acid glutamate. Because of this structural similarity, glufosinate is able to act as an inhibitor for the enzyme glutamine synthetase, for which glutamate is a substrate. The inhibition of glutamine synthase (GS) results in disruption of the glycolate pathway. Metabolic flux through the pathway is reduced and intracellular levels of pathway intermediates accumulate. One of these intermediates, glyoxylate, is a a strong inhibitor of the key photosynthetic enzyme ribulose bisphosphate carboxylase. Without the activity of this enzyme, plants are unable to fix carbon and rapidly die.

Glufosinate is an effective herbicide against most grasses and broadleaf weeds. Many crop species are also sensitive to glufosinate.

Glufosinate cannot travel far through plant cells and must be absorbed through the leaves. For this reason, the herbicide is applied directly to the leaves.

Mechanism of resistance

Two types of soil bacteria, Streptomyces hygroscopicus and Streptomyces viridochromogenes, produce bialaphos, a tripeptide containing two alanine residues and glufosinate. The bacteria presumably produce this compound as an antifungal agent. To avoid the toxic effects of the bialaphos themselves, the bacteria also produce enzymes that acetylate glufosinate, making it unable to inhibit glutamine synthetase. The genes that code for these enzymes are known as the bar and pat genes. By transforming plants with these genes, scientists were able to create plants that can acetylate and therefore render innocuous the intracellular glufosinate.

Safety concerns

Deacetylation of acetylated glufosinate in the gut

A safety concern that has been mentioned claims that the acetylated glufosinate may be deacetylated in the human gut and regain its toxicity. While humans do not contain the deacetylase enzyme necessary to catalyze this reaction, it is possible that some of the bacteria in the human gut do or that the acidic environment in the stomach could favor deacetylation. Glufosinate can be toxic to mammals in large amounts.

Bibliography
1. Beriault, J. N., Horsman, G. P., & Devine, M. D. (1999). Phloem transport of D,L-glufosinate and acetyl-L-glufosinate in glufosinate-resistant and -susceptible brassica napus. Plant Physiology, 121(2), 619-628.
2. Thompson, C. J., Movva, N. R., Tizard, R., Crameri, R., Davies, J. E., Lauwereys, M., et al. (1987). Characterization of the herbicide-resistance gene bar from streptomyces hygroscopicus. The EMBO Journal, 6(9), 2519-2523.
3. Tsai, C., Wang, C., & Wang, C. (2006). Physiological characteristics of glufosinate resistance in rice. Weed Science, 54, 634-640.
4. Satoh, A., Murakami T., Takebe, H., Imai, S., & Seto, H., (1993). Industrial Development of Bialaphos, a Herbicide from the Metabolites of Streptomyces hygroscopicus SF1293. Actinomycetologica, 7, 128-132.
5. Knezevic, S. Z. (2006). Herbicide tolerant crops: 10 years later. Maydica, 52, 245-250.
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