Transgenic crops can be generated with the use of recombinant DNA techniques which alter the crop’s genetic makeup by manipulating the genome—either by introduction, deletion, substitution, or silencing of an individual gene or group of genes of interest. The functionality of transgenes has expanded with time. In the beginning,only traits that exhibited complete dominance, free of the interaction from the native plant genome or the environment, were targeted. For such traits, only one copy of the trait introduced into one of the inbred parents was required. Fortunately, some of these dominant traits such as insect resistance and herbicide tolerance provided solution to major production hurdles encountered by farmers in producing major crops of food and fiber.
Cultural practice modification offered by early transgenic varieties not only enhanced economic return to the farmer because of higher crop yields but also had multiplier effects of soil erosion prevention from reduced tillage,and reduction in environmental pollution from the residual herbicides and insecticides. Now, the scope of transgenic technology has expanded to include quantitative traits such as stress tolerance and yield improvement that necessitate the interaction of the introgressed genes with native genes engaged in the metabolic pathway for the phenotypic trait expression where environment may also influence considerably the final phenotypic expression.Stacking of introgressed genes in hybrids to create value combination of traits is also receiving considerable attention.
Genetic engineering is usually resorted when improvement through conventional breeding and mutagenesis have been exhausted. Such situations arise if a desired trait is not present in the crop germplasm,the trait has proven difficult or very time consuming to improve through conventional breeding, or there is a need to remove or switch off particular genes. Commercialization of first genetically engineered crop started back in 1996 and since then it has reached new heights in its application and wide adaptability to various sectors of modern agriculture. Since 1996 to 2013 there has been tremendous increase in the acreage of genetically engineered crops. Between 1996 and 2013 there has been more than 100 fold increase in the acreage of genetically engineered crops . The ability of genetic engineering to incorporate foreign traits into plants was first exhibited in 1970s.
Although approved earlier for limited sale, it was not until 1996 that the Monsanto Company got the approval to commercially market European corn borer resistant corn , Colorado potato beetle resistant potato , cotton bollworm complex resistant cotton , and a non-selective, broad spectrum herbicide glyphosate tolerant soybean Merr. The genes for all insecticidal proteins, modifiedcry1Ab in corn , modified cry1Ac in cotton , and modified cry3Ab in potato derived from commonly found soil bacteria—Bacillus thuringiensis.Glyphosate tolerant soybean was developed by incorporating abacterium glyphosate resistant EPSP synthase gene . Each generation represents unique thrust areas for developing transgenic crops and has contributed to the present pool of commercially adopted transgenic crops .Modern agriculture has been quick to adopt the commercial first generation transgenic crops expressing herbicide tolerance and insect resistance since these effects were clearly visible in the crop production systems. The second generation transgenic crops were designed for product quality characteristics but they did not lived up to expectations, and no commercial crop with these specific characteristics is presently in the market. However, the first approved transgenic food, Calgene’s Flavr Savr tomato which reached the market in 1994, succeeded in delaying softening of the ripe fruit after harvesting but was a complete commercial failure and was withdrawn from the market in 1997 out of safety concerns.
The third generation of transgenic crops has been engineered for use as biofactories or living reactors in the production of pharmaceuticals and industrial chemicals and is often referred as “molecular farming”. The early and most cost-reward producing use of GE has been in the development of insecticide and pesticide resistance in field crops. A great deal of interest has currently been shown in incorporating tolerance to environmentals tresses in crop cultivars in order to stabilize the yield under fluctuating environmental conditions. In addition, as enhanced nutritive value of crop has gathered much interest to combat malnutrition in developing countries and to meet the food preference of naturalists, several transgenic cultivars with fortified nutritive values have been released. Some degree of success has also been accomplished in developing crops with chemical constituent of industrial value and the use of plants as hosts for pharmaceutical products. The transgenes currently employed in weed management confer resistance to crops allowing use of effective broad-spectrum herbicides. The gene characterizing bacterial enzyme conferring tolerance to glyphosate has been most widely used, however, transgenes conferring tolerance to bromoxynil,glufosinate,and sulfonylurea are also registered .