Herbicides, resistance

Degradation or Transformation. Degradation or transformation of a herbicide by soil microbes or by abiotic means has a significant influence not only on the herbicide s fate in the environment but also on the compound s efficacy. Herbicides that are readily degraded by soil microbes or other means may have a reduced environmental impact but may not be efficacious. Consider the phenomenon of herbicide-resistant soils. In these cases, repeated application of a given herbicide has led to a microbial population with an enhanced ability to degrade that herbicide (252,253). This results in a decrease or total loss of the ability of the herbicide to control the weed species in question in a cost-effective manner. 

Chemical, cultural, and mechanical weed control practices have been relatively successful ia reducing yield losses from weeds (448). However, herbicide-resistant weed populations, soil erosion, pesticide persistence ia the environment, and other problems associated with technologies used (ca 1993) to control weeds have raised concerns for the long-term efficacy and sustainability of herbicide-dependent crop production practices (449). These concerns, coupled with ever-increasing demands for food and fiber, contribute to the need for innovative weed management strategies (450). 

DNA construct will often contain an effect gene and a selectable marker gene (such as antibiotic or herbicide resistance), both of which are bracketed by promoter and terminator sequences. A plasmid vector carries this cassette of genetic information into the plant genome by one of the above methods. 

In fields where some weeds were cleared using herbicides, other, more herbicide-resistant, species have appeared, such as common horsetail, coltsfoot, foxtail, wild oats, false wheat, etc. As a result of herbicide use, scratchweed, which cannot be destroyed by any herbicide, is making inroads into cereal crops, and chamomile has taken over rapeseed [6]. Using herbicides on rice fields caused the spread of wild, pesticide-resistant, low-yield forms of red-grain rice. 

Friesen, L.F., Nelson, A.G. and Van Acker, R.C. (2003). Evidence of contamination of pedigreed canola (Brassica napus) seedlots in western Canada with genetically engineered herbicide resistance traits . Agron J, 95, 1342-1347. 

Hall, L., Topinka, K., Huffman, J., Davis, L. and Good, A. (2000). Pollen flow between herbicide-resistant Brassica napus is the cause of multiple-resistant B. napus volunteers. Weed Sci, 48, 688-694. 

Legere, A. (2005). Risks and consequences of gene flow from herbicide-resistant crops canola (Brassica napus L) as a case study , Pest Manag Sci., 61, 292-300. 

Rieger, M.A., Lamond, M., Preston, C., Powles, S.B. and Roush, R.T. (2002). Pollen-mediated movement of herbicide resistance between commercial canola fields, Science, 296, 2386-2388. 

Snow, A.A., Andersen, B. and Jorgensen, R.B. (1999). Costs of transgenic herbicide resistance introgressed from Brassica napus into weedy B. rapa. Mol. Ecol., 8, 605-615. 

Brazier M, Cole DJ, Edwards R (2002) O-Glucosyltransferase activities toward phenolic natural products and xenobiotics in wheat and herbicide-resistant and herbicide-susceptible black-grass (Alopecurus myosuroides). Phytochemistry 59 149-156... 

Herbicide market, 13 285-286 Herbicide-resistant crops (HRCs),... 

Stalker, D.M., McBride, K.E., and MalyJ, L.D. Herbicide resistance in transgenic plants expressing a bacterial detoxification gene. Science (Washington, DC), 242(4877) 419-423, 1988. 

Herbicide-resistant and pesticide-resistant crops should avoid soil erosion and limit the spread of synthetic herbicides and pesticides. Modified crops may also provide heat-stable monounsaturated oleic acid, avoiding the problem of heat-unstable poljainsaturated fetty acids that give unhealthy trans-fatty acids as side products of industrial hydrogenation (Mazur 1999). On the longer term, biotechnology may also provide renewable fuel and raw chemicals that may replace petroleum. 

The EPA is involved in the regulation process if the transgenic plant expresses a pest- or herbicide-resistant engineered trait in addition to a biopharmaceutical. 

In an attempt to help farmers prevent the onset of resistance in the weeds on their land the Herbicide Resistance Action Committee (HRAC) has produced a table of herbicide classes by mode of action (anon. 1996). This table is designed to tell the grower which compounds... 

From Herbicide Resistance Action Committee Classification of Herbicides by Mode of Action. WSSA - Weed Science Society of America. 

Herbicide resistance is becoming a problem in many situations. The rush of compounds that interfere with branched chain amino acid biosynthesis has meant that a very high area of land has been treated with compounds that have the same mode of action. The application... 

Padgette, S. R., Re, D. B., Barry, G. F. el al. (1994). New weed control opportunities development of soybeans with a Roundup Ready gene. In Herbicide-resistant Crops Agricultural, Economics, Environmental, Regulatory, and Technologycal Aspects, ed. S. O. Duke. Boca Raton, FL CRC Press. 

Brimner, T.A., Gallivan, G.J. and Stephenson, G.R. (2005) Influence of herbicide-resistant canola on the environmental impact of weed management. Pestic Manage Sci, 61, 47—... 

Zeng, L. and Baird, W. V. (1997). Genetic basis of dinitroaniline herbicide resistance in a highly resistant biotype of goosegrass (Eleusine indica). J. Hered. 88, 427-432. 

Transgenic plants containing a nitrilase specific for the herbicide bromoxynil (= 3,5-dibromo-4-hydroxybenzonitrile)have also been developed [93] the Cal-gene company transformed tobacco plants with the bacterial Klebsiella ozaenae gene encoding nitrilase [94] that detoxifies the herbicide by hydrolysis (conversion of bromoxynil to 3,5-dibromo-4-hydroxybenzoic acid), resulting in the establishment of the herbicide-resistant transgenic plants. 

---