Herbicide tolerant plants were created recently to prevent use of environmentally hazardous chemicals on plants. These poisons are not only dangerous to the plants but, also harmful to animals and humans. They are usually made by use of a bacterium-mediated transformation or biolistics. Although simple it is a good example of genetic engineering in commercial use.
How it is made: Before this can be made you must have knowledge on how a effective herbicide works, first, it must be selective; it must biodegrade before presence of any unwanted effects; and lastly it should not hurt your crops. A great example is Glyphosate, it kills by use of enzymes in amino acid biosynthesis. It uses two enzymes in this process, EPSP and GOX.
These active ingredients would then be engineered in the desired plant so, when exposed to the herbicide, the plant would degrade it into harmless substances. In this case glycine and phosphate.
Why it is made: This was made for the obvious reason of economical commercial farming. If it was not done farmers would lose money on crops that die because of the poisons used. Herbicide tolerant plants will then be protected by the poison more safely and, more efficiently Why it should not be done: I do not believe that this type or any other type of genetic engineering should be done. The procedure is to advanced for scientist to delve into at this point in time. This one in particular could cause catastrophic ecological problems if the herbicide tolerant plant cross pollinates with a weed. This would in turn create herbicide tolerant superweeds. Thus, another complication to a growing list of unnatural manmade problems.