Glyphosate resistant crops

A major milestone occurred in 1996 with the commercial introduction of glyphosate resistant soybean [22]. Since then, the glyphosate resistant trait (Roundup Ready , RR) has been introduced into canola, cotton, com, alfalfa, sugar beet, and others. All RR crops thus far contain a glyphosate insensitive EPS PS derived either from the plant or bacteria. 

Kinetic analysis showed that the endogenous maize EPSPS has a Km-PEP of 27 [LM with a Ki of 0.5 pM (Table 6.2.1). Mutations were introduced at the T102I and P106S positions to produce the variant TIPS [26]. The TIPS-EPSPS showed a of 10.6 pM with a K of 58 pM. The double mutant enzyme preserved the EPSPS function while reducing sensitivity to glyphosate. Analysis of single mutations 

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Kleter, G.A., Harris, C.A., Stephenson, G.R. and Unsworth, J. (2008) Comparison of herbicide regimes and the associated potential environmental effects of glyphosate-resistant crops vs. what they replace in Europe. Pestic Manage Sci, 64, 479-488. 

Genetic modification of soybeans, com, canola, and cotton has produced glyphosate-resistant crops that lead to lower pesticide use, yield improvement, and improved water quality (M. Stern). 

Disease Control Benefits of Glyphosate Resistant Crops... 

Throughout western Canada and the central Great Plains of North America, volunteer wheat is becoming a more serious problem (Leeson el al., 2005). This may become a special concern if the volunteer wheat is glyphosate-resistant (Harker el al., 2005). There are also many examples of integration of traits from weeds into crops, and there is some evidence of spread from herbicide-resistant crops into weeds (Gressel, 2002). 

Padgette, S.R., X. Delannay, L.D. Bradshaw, B.H. Wells, and G. Kishore (1995). Development of glyphosate-tolerant crops and perspectives on the potential for weed resistance to glyphosate, p. 154. In De Prado, R., Garcia-Torres, L., and Jorrin, J., eds., International Symposium on Weed and Crop Resistance to Herbicides. Cardoba, Spain Cardobesa de Impresiones Graficas, SL. Abstract, 92. 

The advantage for the farmer is that he needs only one product, instead of several different selective (and more expensive) herbicides. Roundup ready soybeans were launched in 1996 and today 50 percent of the soybean crop in the United States is derived from roundup ready seeds. Other glyphosate-resistant transgenic crops introduced by Monsanto are maize and oil seed rape. Competing companies also developed herbicide-resistant plants or plants genetically modified to be protected against certain pests, but none has achieved a commercial breakthrough, mainly because of political reasons. 

This strategy has led to commercial development of herbicide resistance for glu-fosinate, glyphosate and bromoxynil. Glufosinate and glyphosate resistance will be discussed in detail in later sections of this chapter (see also Chapter 6.2). Bro-moxynil s herbicide activity is due to inhibition of electron transport in photosystem II. Crops engineered with bromoxynil nitrilase metabolize the herbicide to a non-phytotoxic compound [5]. 

This strategy was used to engineer resistance against glyphosate, and imidazoli-nones and sulfonylureas that inhibit acetolactate synthase (ALS), a key enzyme in the biosynthesis of branched chain amino acids. ALS resistant crops have primarily been generated through selection for an herbicide insensitive ALS allele from natural or mutagenized cell or plant populations [3]. 

There are no commercial herbicide resistant crops that function by increased expression of the protein target, although some level of plant resistance has been reported for glyphosate, glufosinate, some DPEs and inhibitors of hydroxy-phenylpyruvate dioxygenase. Similarly, cellular sequestration of the herbicide from the target has been reported with some DPEs, auxins and photosystem I inhibitors, but none have been developed commercially [3]. 

This section includes data for herbicide resistant crops generated by both selection and biotechnology processes. The first commercially available herbicide resistant crop in the United States was imidazolinone resistant corn introduced in 1992. This was followed by glyphosate resistant soybean and canola in 1996. 

There are several other commercially available herbicide resistant crops. The imidazolinone resistant trait is also available in rice, wheat, sunflower, and lentil. Glufosinate resistance is available in rice and glyphosate resistance was recently commercialized in alfalfa in 2005. 

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