The main concern over transgenic plants with regard to food safety and human health is whether the transgenic plant is likely to pose a greater risk than the non-transgenic variety it is derived from. Food safety risks associated with transgenic plants include the spread of antibiotic resistance, changes in nutrient composition of the plant, and the production of toxic proteins and allergens.
Marker genes are used by genetic engineers to select plants that have been transformed. One of the most widely used marker genes has been the kan-r gene, which encodes an enzyme providing resistance to the antibiotic kanamycin. The use of these marker genes has led to the suggestion that they may be transferred to gut epithelial cells, to gut bacteria and to organisms in the environment.
There is a concern that their use may ultimately enhance the development of bacterial resistance to antibiotics. Although there is no evidence that this occurs, this phenomenon needs to be carefully assessed to rule out any eventual negative health impact.
The scientists have responded and begun to use alternative marker genes. A marker gene such as GFP (green fluorescent protein) can be introduced at no fitness cost, into the host plant along with the agronomically-important gene for the infield monitoring of the expression of the transgene.
For the development of golden rice engineered to synthesize provitamin A, mannose has been used as a selective agent. Positive selection strategies use cytokinins, xylose isomerase gene and phosphomannose isomerase gene for selection of transformed plants.
Recently, gene switches that regulate the expression of transgenes through ecdysone against insecticides have been reported to be useful in reducing the risks associated with transgenic crops. Ligands that are suitable for regulation of biopesticide genes in transgenic crops are the commercially available non-steroidal ecdysone agonists, tebufenozide, methoxyfenozide, halofenozide and chromafenozide.
Most Bt toxins are specific to insects, as they are activated in the alkaline medium of the insect gut. There are no specific receptors for Bt protein in the gastrointestinal tract of mammals, including humans. The Bt proteins are rapidly degraded by stomach juices in vertebrates.
The concentration of Cry proteins in transgenic plants is usually well below 0.1 per cent of the plant’s total protein, and none of the Cry proteins have been demonstrated to be toxic to humans nor have they been implicated to be allergens. The transgenic Bt tomatoes are considered to pose no additional risk to human and animal health as compared to conventional tomatoes.
Similarly, the seed from the Bt- transformed cotton lines is compositionally equivalent to and as nutritious as the seed from the parental lines and other commercial cotton varieties. Both protein and DNA are destroyed during the processing of highly refined foodstuffs such as oils and sugars. This is particularly true for cottonseed oil, which must be heavily refined to remove toxic secondary plant compounds.
Cry1Ab and Cry1Ac become inactive in processed corn and cottonseed meal, but Cry9C is stable when exposed to stimulated gastric digestion and to temperatures at 90°C. The Bt corn containing Cry9C (StarLink®) was, therefore, not permitted for human consumption, although it was allowed for animal consumption.
In a study using Bt maize silage on the performance of dairy cows, it was found that there were no significant differences between Bt and non-Bt maize hybrids in locational performance or ruminal fermentation. The Cry1Ab protein as a component of post-harvest transgenic maize plants dissipates readily and has not been detected in silage prepared from transgenic plants.
There were no differences in the survival and body weight of broilers reared on meshed or pelleted diets prepared with Bt transgenic and non-transgenic maize. On the basis of studies on Bt crops fed to chicken-broilers, chicken-layers, catfish, swine, sheep, lactating dairy cattle, and beef cattle, it was concluded that there are no detrimental effects-on growth, performance, observed health, composition of meat, milk and eggs, etc.
On the basis of extensive studies on the safety issues associated with DNA in animal feed derived from genetically engineered crops, it has been concluded that consumption of milk, meat and eggs produced from animals fed genetically modified crops should be considered as safe as traditional practices.