Conventionally-bred crop varieties may actually pose a greater risk from increased plant toxins than genetically engineered plants. In order to develop varieties with improved disease resistance, plant breeders usually begin by crossing the disease-prone conventional variety with a disease-resistant wild relative of the crop plant. Because the disease resistance in the wild variety may often be caused by higher levels of natural pest-killing toxins, the breeder may be unknowingly selecting varieties with increased levels of a chemical toxic to both the plant pest and to humans. One variety of potatoes developed in the 1970's to be resistant to insects was found to have very high concentrations of glycoalkyloids, a family of chemicals that can cause a potentially lethal suppression of the central nervous system. This potato was never marketed-- primarily because it tasted very bitter!

Plant breeders developed a variety of celery that was highly insect-resistant. Surprisingly, people who handled the variety and then were exposed to strong sunlight developed rashes and burns. It was later discovered that the new variety contained almost eight times the normal quantity of psoralen, a light-activated natural compound known to be toxic and carcinogenic. This celery variety was on the market for several years (including long after it was learned to have high toxin levels).

The FDA does not require any pre-market safety testing of whole-plant foods derived from plant breeding. Although the FDA has the authority to remove foods from the market which are determined to contain unsafe levels of toxins, quantification of those toxins before marketing is a responsibility left entirely to breeders and producers.
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References

Ames, B.N. 1983. Dietary carcinogens and anticarciongens. Science 221:1256-1264.

Ames, B.N. & Gold, L.S. 1990. Chemical carcinogenesis: Too many rodent carcinogens. PNAS 87:7772-7776.

Ames, B.N., Profet, M., Gold, L.S. 1990. Dietary pesticides (99.99% all natural). PNAS 87:7777-7781.

Ames, B.N., Profet, M., Gold, L.S. 1990. Nature's chemicals and synthetic chemicals: Comparative toxicology. PNAS 87:7782-7786.

Wink, M. 1988. Plant breeding: importance of plant secondary metabolites for protection against pathogens and herbivores. Theor Appl Gen. 75:225-233.