Joe Cummins, University of Western Ontario, London, Ontario
The herbicide glyphosate is being used extensively in weed control in conjunction with genetically modified (GM) crops. Such widespread use began after 1995 and the application has spread to millions of acres. Before 1995, glyphosate was used in "no till" agriculture to control a broad array of weeds, and it was used extensively in forestry to eliminate broad leafed "weed" trees such as alder or poplar from stands of relatively herbicide-tolerant evergreen trees, the desired "cash crop". The main formulation of glyphosate, "Round-up", was introduced by Monsanto Corporation in 1974 [1].
Glyphosate is a non-selective, broad-spectrum herbicide that kills plants by inhibiting the enzyme, 5-enolpyruvylshikimate-3-phosphate synthetase (EPSPS) critical for the biosynthesis of aromatic amino acids, vitamins, and many secondary metabolites [2]. The shikimate pathway functions in the chloroplasts of green plants. The killing action of the herbicide requires that the plant be growing and exposed to light.
GM crops modified to resist glyphosate exposure are called "Round-up ready" they are modified with two main genes. One gene imparts reduced sensitivity to glyphosate and the other expresses an enzyme that enables the plant to degrade glyphosate. The expression of both genes is directed to the chloroplasts, the site of the herbicide activity, by the addition of coding sequences of a plant-derived transit peptide.
The first gene expresses a bacteria-derived version of the plant enzyme involved in the shikimate biochemical pathway for the production of the aromatic amino acids including phenylalanine, tyrosine and tryptophan. The plant version of this enzyme, ubiquitous in nature, is sensitive to glyphosate - the herbicide disrupts this essential pathway, leading to growth suppression or death of the plant. However, the bacteria-derived version of this enzyme is highly insensitive to glyphosate and fulfils the aromatic amino acid needs of the plant. The second gene, also bacterial, expresses an enzyme that degrades glyphosate. The coding sequence of this gene was altered to enhance the efficiency of glyphosate degradation, compared to the original bacterial version. A plant-derived coding sequence expressing a chloroplast transit peptide was co-introduced with each of the Roundup-ready genes. This peptide facilitates the import of the newly translated enzymes into the chloroplasts, the site of both the shikimate pathway and glyphosate action [3].
The crop genetic modifications are done using biolistic or Agrobacterium transformation. For the bacterial genes to be transcribed in the plant nucleus, a promoter active in plants is used. Presently, in Canada, the promoter and its DNA sequence is withheld based on a bizarre policy designating the information a trade secret [2]. In contrast the United States Department of Agriculture usually identifies both the promoter and the transcription terminator. The most popular promoter, the cauliflower Mosaic Virus (CaMV) promoter, seems to have been used rather infrequently in Round-up ready crops. (It is in some Roundup Ready soya varieties.) The rice-actin promoter or the figwort mosaic virus promoter is in many Round-up ready crops. Often, a plant intron is introduced into the code for the bacterial enzyme to enhance transport of the message from nucleus to cytoplasm. The NOS terminator from the Agrobacterium plasmid is the transcription terminator used most frequently [4,5].
It is worth noting that the prevalent test for GM crops or GM contamination in products is the CaMV promoter [6,7]. The USDA APHIS reviews indicate that promoters such as figwort mosaic virus or rice actin are often employed. Such products will not be detected as GM products or contaminants using the conventional PCR test.
The impacts of glyphosate on humans and other animals have been controversial since the registration of the pesticide. Regulatory agencies worldwide have been hesitant to deal with findings that suggested the pesticide posed an unreasonable threat to humans or to the environment. Rather than dwell on the earlier studies, I shall concentrate on studies that are relatively current.
A recent paper observed that Round-up provoked cell division dysfunction and pointed out that human cancers were associated with defects in cell cycle transitions. The results question the safety of glyphosate and Round-up for human health [8]. Roundup was found to inhibit steroidogenesis by disrupting acute regulatory protein expression [9]. Many environmental toxins have been found to disrupt steroid hormone function leading to sexual dysfunction and reproductive failure. An epidemiological study of the effect of pesticide exposure on spontaneous abortion in Ontario farm populations showed that glyphosate exposure nearly doubled the risk late spontaneous abortion [10].
Round-up exposure caused DNA adducts to appear in mice [11]. Such DNA adducts are associated with gene and chromosome damage leading to cancer. Bovine lymphocytes exposed to glyphosate showed chromosome aberrations indicating that the herbicide was genotoxic [12]. Glyphosate disrupted the enzymatic patterns of pregnant rats and their fetuses [13].
Round-up disrupted the hepatocytes of carp [14]. Glyphosate significantly increased micronuclei (a measure of genotoxicity) in fish [15]. Glyphosate was found to be genotoxic to bull frog tadpoles in Ontario, Canada [16]. Australian frogs were found to be very sensitive to glyphosate exposure [17]. Leopard frog and western chorus frog tadpoles were sensitive to glyphosate exposure [18]. Glyphosate exposure of water snails produced abnormalities that could result in infection in humans [19]. Field dose exposure of earthworms caused at least 50 percent mortality and significant intestinal damage among surviving worms [20]. The earthworm study indicates that glyphosate may heavily damage soil by damaging the earthworm population.
The numerous publications showing that glyphosate causes gene damage in animals has been given little concern by those charged with regulating the herbicide. The massive increase in glyphosate use in North America as acreage of GM crops increases to staggering levels does not seem to have stimulated extensive studies to follow up the finding that glyphosate has long term impact on humans and animals. Instead, the studies are ignored or subject to unfair criticism followed by efforts to bury the damaging findings in fatuous claims that the herbicide in benign.
Environment effects of glyphosate are of great significance not only in the vast acreage planted with round-up ready crops, but in the immense forest acreage treated with the herbicide. A number of environmental problems regarding widespread use of glyphosate with Round-up ready crops have come to light in recent years. The discussion that follows will emphasize the environmental problems associated with Round-up ready canola.
The main environmental problem associated with the use of GM crops is believed to be the spread of the genetically modified traits to adjacent crops and into weedy relatives. Canola varieties currently marketed in western Canada include tolerance to glyphosate, glufosinate, bromoxynil, and imidazolinone herbicides. Gene flow between cultivars with different HT traits has already resulted in volunteer canola with multiple resistance in a field situation in western Canada. Canola also has weedy relatives in Canada with which it could form hybrids. For example, birdsrape mustard in
eastern Canada, feral B. rapa canola in western Canada, and wild radish (Raphanus raphanistrum L.) in parts of both regions could be at risk of forming hybrids with herbicide-tolerant B. napus. These hybrids could serve as a bridge for gene transfer to a
subsequent canola crop or else become more difficult to control because of the presence of the herbicide tolerance gene in wild or feral populations [21]. Multiple herbicide resistance to glyphosate, glufosinate and imazethapyr in volunteer canola was detected in western Canada. Pollen flow between volunteer canola plants was shown to be the manner in which the resistance genes stacked to make the super resistant variety.
Sequential crossing of the three individual herbicide tolerant varieties is believed to have led to the formation of the final multiple resistant variety [22]. Last fall, the Canadian government acknowledged that the separation distance (175 meters for production of oil and pressed seed cake for feed) between GM canola and adjacent fields was inadequate because pollen had been observed to spread for at least 800 meters, to produce two pollinations per thousand flowers at 400 meters and 7 pollinations per ten thousand flowers at 800 meter [23].The figure, 7 pollinations per ten thousand flowers, may seem inconsequential until it is realized that each plant from a pollinated seed may produce several hundred seeds. If the herbicide glyphosate is used to kill weeds on a field that field can rapidly be populated with GM canola, even without glyphosate exposure.
Interestingly, the Canadian government had been aware of the problem of extensive pollen flow for some years, before acknowledging it to the public. The observed pollen spread means that essentially no canola grown in western Canada can be claimed to be free of gene modification and production of organic canola may not be possible in western Canada. The Canadian Food Inspection Agency that regulates GM crops in Canada seems engaged in complicity in allowing GM crops to spread throughout the farming area.
A recent study showed that glyphosate resistant alfalfa, soybean and tobacco exposed repeatedly to glyphosate in increasing concentrations caused the gene producing the glyphosate target enzyme to be amplified [24]. Such gene amplification has been observed in human cancer cells exposed to chemotherapy chemicals, creating distinct chromosomal bands in the loci of the resistance gene. It seems likely that all of the Round-up ready crops including corn, canola and cotton will show similar gene amplification which will lead to unbalanced accumulation of glyphosate, unless the gene for glyphosate destruction is amplified along with the target gene. Clearly, an unexpected accumulation would profoundly affect human and animal food uses of the crop.
Glyphosate application on round-up ready crop may have untoward side effects. For example, the nitrogen fixation symbiont in GM and unmodified soy bean is sensitive to glyphosate. Early application of glyphosate led to decreased biomass and nitrogen in the crop, just as growth in soil with available nitrogen and limited soil water led to decreased biomass and seed number following herbicide application [25]. Glyphosate application at elevated temperature (the frequent temperature of 35 C during early summer) to the round-up ready soybean crop led to meristem damage related to increased transport of the herbicide to the meristem [26].
Glyphosate application in conventional weed control led to destruction and local extinction of endangered plant species [27]. In forest ecosystems, bryophytes and lichens were significantly reduced by glyphosate application to control weed trees [28]. Glyphosate treatment of bean seedlings led to short-term increases in dampening-off pathogen in treated soil [29]. Glyphosate application to control invasive species along tidal flats led to unexpected secondary effects. After spraying, the herbicide in sediment declined 88% while in the target perennial grass the herbicide increased 591% and was stored in rhizomes of the grasses [30]. Glyphosate persists in soil and groundwater and was observed in well water in sites adjacent to sprayed areas [31].
It is clear that endangered species are threatened by the increase in glyphosate application related to increases in GM crop production. The herbicide was found to persist and bio-accumulate in some species and was found in drinking water wells neared sprayed areas.
There is a wealth of published scientific studies showing that glyphosate-tolerant crops and massive increase in use of glyphosate in conjunction with genetically modified (GM) crops poses significant threat to human and animal health and to the environment. It seems clear that continued and amplified use of the herbicide will result in loss of endangered species and an extensive remodeling of the environment. And human beings will be poisoned without their knowledge and consent.
Article first published 03/07/02
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