Plants contain hundreds of pathogen race-specific pest-protective genes that are often referred to as R genes (see section 2.3). Most of these genes are evolutionarily conserved in structure and, most likely, in function. R genes for protection against pathogens are routinely transferred between plants by conventional breeding. There are no known toxic or nontarget effects of R gene products aside from their role in triggering localized and systemic defense responses in the presence of specific pathogens. Transfer of race-specific R-gene from a nonedible plant species to an edible species would result in new exposure of consumers and nontarget species to a specific R-gene product, although not to a new class of proteins. Because plants expressing R genes are not thought to mount a defense response unless encountered by pathogens, the chances of unintended health effects due to pleiotropic effects are remote. Compared with R-gene transfer by conventional breeding, introduction of an R gene via a transgenic method should result in fewer unintended effects, because of the lack of introduction of non-R gene DNA into the new variety.
Familiarity with the structure and function of race-nonspecific and other types of pathogen-protective genes is less than that with the major class of race-specific R genes, although their history of use in conventional breeding suggests that few hazards exist. Most of these pathogen-protective genes are probably conserved among different species of plants. It is unlikely that exposures to new classes of genes or gene products will result from transfer of these genes between plants, particularly if the source is an edible plant. However a protective gene may encode a protein that increases the concentration of one or more plant compounds with potential nontarget toxicity, thus leading to a potential hazard.
A number of genes that code for degradative or hydrolytic functions, such as glucanases and chitinases, with pest-protective activities can be induced by infection (section 2.3.1). These types of enzymes are ubiquitous in plants and are common constituents of foods. Transfer of such genes from one edible plant to another is unlikely to cause a problem. However, if these genes were expressed at relatively high constitutive