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[SANET-MG] Dicamba ready crops



4 June 2007
Professor Joe Cummins
Dicamba ready GM crops
It is growing ever more evident that genetically modified (GM) individual herbicide resistant crops are not sustainable. Herbicide resistant weeds soon appear rendering the GM crop obsolete. Herbicide resistant weeds are increasing rapidly (1,2).Herbicide tolerant crops may themselves become “volunteer” weeds and Monsanto Company recognized the problem they had created and patented herbicide mixtures to combat tolerant weeds and volunteer crops (3). Herbicide tolerant weeds arise either by mutation or by gene flow from the transgenic crops(4).The herbicide resistance gene transferred from GM crop to a weedy relative has been observed to transfer that gene to other weed species (5). It has grown clear that the useful life of the original GM herbicide resistant crops are rapidly nearing their end. Crops resistant to the phenoxy herbicide Dicamba, have been developed as a replacement for the outmoded resistance to the herbicides glyphosate, glufosinate or bromoxynil. Dicamba is produced by BASF chemical corporation and marketed as Banvel , the dimethylamine salt or as Vanquish the diglycolamine salt (6). “Inert” ingredients in the formulations include 3,6-dichlorosalysilic acid, dimethylnitosamine, ethylene glycol and 2,7-dichlorodibenzo-p-dioxin. Dicamba is an old herbicide which served alongside “agent orange” in Vietnam (7). That old polluting phenoxy herbicide was mysteriously born again as an environmentally friendly chemical through the magic of public relations. Dicamba degradation was characterized in the bacterium Psuedomonas maltophilia which can utilize the herbicide as a sole carbon source. Three separate enzymes, an oxygenase, a ferredoxin and a reductase act sequentially to convert Dicamba to its first degradation product 3,6-dichlorsalyicylic acid (8). The primary patent for Dicamba degrading organisms including transgenic crops was assigned to the University of Nebraska. The genes for the Dicamba oxygenase, ferridoxin and reductase were used to transform crop plants. No selectable markers were required because the transformants could be identified by the fluorescence of 3,6-dichlorsalyicylic acid (9). Success has been reported in transforming the soybean with only the dicamba monooxygenase gene driven by a promoter from peanut chlorotic streak virus and enhancer from the tobacco etch virus , a chloroplast transit peptide gene from pea for localization in the chloroplast and a transcription terminator from pea. In this transformation, localizing the Dicamba monooxygenase to the chloroplast, the ferridixin and reductase genes were not required to maintain active Dicamba degradation (10) Between 2004 and 2007 over forty field test releases of Dicamba resistant corn, cotton or soybean were permitted in the United Sates. These field tests were mainly undertaken by Monsanto Corporation accompanied by one test each from the University of Kansas and BASF corporation. Several field test releases included crops stacked with transgenes for glyphosate résistance or for yield or oil composition. With the exception of the University of Kansas release the donors of the Dicamba resistance transgenes were designated confidential business information (CBI) but CBI constructs are unlikely to be much different from the Psuedomonas maltophilia genes used in the Kansas release(11). However, the use of CBI designations in field releases where the public may be exposed to toxic products is both unwise and unwarranted. Environmental impact studies do not appear to be available for the numerous field tests releases of either Dicamba field tests nor of the stacked Dicamba tests and the locations of the tests are withheld from those who may be exposed. Since the use of Dicamba may greatly increase ,the toxicity of the herbicide is worth reviewing. Both pure Dicamba and Banvel proved to increase sister chromatid exchange in human lymphocytes. The study concluded that Dicamba is a DNA damage agent and potentially hazardous to humans (12). Dicamba proved genotoxic in mutation tests using transgenic Arabidopsis thaliana plants (13). Dicamba proved to be a peroxisome proliferator in rats , such activity is associated with liver cancer in rodents (14). Dicamba is not benevolent and its increased deposition in the environment is unwise. Dicamba is relatively persistent in the environment and it is slowly degraded in turf grass (15). In Saskatchewan Dicamba was found to leach into the South Saskatchewan river , a source of drinking water for urban populations(16) High levels of Dicamba were observed in rivers draining tow prairie watersheds , Manitoba streams draining agricultural lands were elevated during application period in May and June (17). Predictably the overuse of herbicides in GM crops has resulted in the GM crops becoming obsolescent as weeds grow herbicide tolerant. Introducing GM crops resistant to an old polluting phenoxy herbicide seems like a bad idea turning an environmental disaster into a real catastrophe. Returning to sustainable agriculture seems to be the only way to deal with the resistant weeds and the “volunteer” GM crops that show the herbicide resistant GM crops cannot cope with natural processes leading to resistant weeds.
References
1. Owen,M and Zelaya,I. Herbicide-resistant crops and weed resistance to herbicides Pest Manag. Sci. 2005,61,301-11 2. Ho,M-W and Cummins, J. Roundup ready sudden death, superweeds and allergens Science in Society 2004,21, 24 3. Flint,J,Probst,N and Gubbiga,N. Tank mixtires and premixtures for weed control programs that include postmergence applications of glyphosate plus germinicides in glyphosate tolerant soybeans 2001 United States patent 6,239,072 4. Ellerstrand,N. Current knowledge of gene flow in plants: implications for transgene flow Phil. Trans.R.Soc. Lond. 2003.358.1163-70 5. Zekaya,I,Owen,M and VanGessel,M. Transfer of glyphosate resistance: evidence of hybridization in Conyza American Journal of Botany 2007,94,660-73 6. United States. Department of. Agriculture. Forest Service. Pacific Northwest Region. Dicamba Herbicide Information Profile February 1999 www.fs.fed.us/r6/nr/fid/pubsweb/dicamba_99.pdf 7. Sutton,P. The History of Agent Orange Use in Vietnam an Historical Overview from the Veteran's Perspective 2002 Hanoi, Vietnam www.hatfieldgroup.com/files/A%20%20HISTORY%20OF%20AGENT%20ORANGE%20USE.pdf 8. Wang X, Li B, Herman PL and Weeks DP. A Three-Component Enzyme System Catalyzes the O Demethylation of the Herbicide Dicamba in Pseudomonas maltophilia DI-6. Appl Environ Microbiol. 1997 Apr;63(4):1623-26. 9. Weeks,D,Wang,X and Herman.P. Methods and materials for making and using transgenic dicamba degrading organisms 2006 United States Patent 7,022,896 10. Behrens MR, Mutlu N, Chakraborty S, Dumitru R, Jiang WZ, Lavallee BJ, Herman PL, Clemente TE and Weeks DP. Dicamba resistance: enlarging and preserving biotechnology-based weed management strategies. Science. 2007 May 25;316(5828):1185-8. 11. Field Test Release Applications in the U.S.June 2007 http://www.isb.vt.edu/cfdocs/fieldtests1.cfm 12. Gonzalez NV, Soloneski S and Larramendy ML. Genotoxicity analysis of the phenoxy herbicide dicamba in mammalian cells in vitro. Toxicol In Vitro. 2006 Dec;20(8):1481-7 13. Filkowski J, Besplug J, Burke P, Kovalchuk I and Kovalchuk O. Genotoxicity of 2,4-D and dicamba revealed by transgenic Arabidopsis thaliana plants harboring recombination and point mutation markers. Mutat Res. 2003 Dec 9;542(1-2):23-32. 14. Espandiari P, Thomas VA, Glauert HP, O'Brien M, Noonan D and Robertson LW. The herbicide dicamba (2-methoxy-3,6-dichlorobenzoic acid) is a peroxisome proliferator in rats. Fundam Appl Toxicol. 1995 Jun;26(1):85-90. 15. Roy JW, Hall JC, Parkin GW, Wagner-Riddle C and Clegg BS. Seasonal leaching and biodegradation of dicamba in turfgrass. J Environ Qual. 2001 Jul-Aug;30(4):1360-70. 16. Cessna AJ, Elliott JA, Tollefson L and Nicholaichuk W. Herbicide and nutrient transport from an irrigation district into the South Saskatchewan River. J Environ Qual. 2001 Sep-Oct;30(5):1796-807 17. Muir DC and Grift NP. Herbicide levels in rivers draining two prairie agricultural watersheds (1984) J Environ Sci Health B. 1987 Jun;22(3):259-84.

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