levels, were derived from nonedible plants or nonplant sources, or were engineered in such a way as to increase stability or alter structure substantially, novel exposures might result. The potential for adverse affects depends on the likelihood of increased toxicity or allergenicity of a novel product. The extent to which those properties are altered is partially predictable and testable.

Some inhibitor proteins, such as proteinase inhibitors, are effective defense proteins that are naturally induced by wounding or attack by chewing insects. Proteinase inhibitors are also present in relatively high concentrations in some food plants, such as potato. Animals can suffer adverse effects if foods high in proteinase inhibitors are consumed (Ryan 1990). Some naturally occurring lectins that have pest-protection attributes are also known to be toxic to humans and animals. Foods that are high in proteinase inhibitors and lectins, such as potatoes and beans, are usually cooked, and cooking destroys inhibitor and toxic activity. Depending on the protein, a plant modified to express high concentrations of inhibitors in edible tissues can cause adverse health effects if the plant is consumed raw, and such a risk can be reduced by designing transgenes that are expressed only in nonedible plant parts.

Genes Encoding Enzymes that Alter Secondary Metabolites or Natural Products

The wide variety of plant chemicals with potential pest-protection characteristics suggests that modification, transfer, or overexpression of genes that control natural-product biosynthesis can result in new types of pest-protected plants (sections 2.3.1 and 2.3.2). It is reasonable to predict that manipulation of those pathways can enhance resistance to insects and pathogens. The known toxicity of many protective natural products to nontarget organisms, however, means that such strategies could pose a risk. Furthermore, alteration of enzymes in one pathway might alter flux through other pathways. For example, Fray et al. (1995) demonstrated that constitutive overexpression of phytoene synthase, an enzyme required for carotenoid biosynthesis, in tomato had the unintended consequence of causing a dwarf phenotype, most likely due to decreases in gibberellin hormone and phytol (chlorophyll side chain) biosynthesis. Modulation of pathways for production of pest-protection chemicals could result in new exposures to potentially toxic compounds. That risk might be minimized by engineering transgenes with regulatory control elements that result in localized expression in nonedible tissues and plant parts. The risk might also be lowered through increased understanding of potentially hazardous compounds in commercial crop plants. Up-to-date and easily accessed databases with qualitative and quantitative descriptions of known or suspected toxicants would be particularly valu-

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