Editor's choice: Evaluating the potential for adverse interactions within genetically engineered breeding stacks.

Plant breeding has a long history of developing varieties with desirable traits in response to the needs of both growers and consumers. Although the bases for most of these traits are not known genetically or biochemically, conventional breeding combines these multiple traits to create new hybrids and stable varieties that are safe and not generally subject to safety assessment. With the advent of genetic engineering, a tool for incorporating additional traits has become available to plant breeders. The safe application of genetic engineering to food and feed crops is widely acknowledged as a useful tool in addressing global agricultural challenges, including population growth and climate change. As used here, the term genetically engineered (GE) stack refers to a plant in which two or more transgenic events (i.e. single-locus insertions) that have been separately assessed for safety have been combined by conventional breeding (Table I). In recent years, increasing numbers of GE stacks have been planted, the first of which offered combinations of insect and herbicide tolerance genes to combat a wider range of pests and weeds than covered by the single events (Que et al., 2010; James, 2011). Two main questions arise when considering the food and feed safety of GE stacks: (1) does incorporation of more than one event increase genomic instability, and (2) can potential interactions between the products of the combined events impact safety? A related paper considered the stacking of events in light of the plasticity of plant genomes and concluded that enhanced genetic instability from a transgene or from common sequences in two or more transgenes is remote (Weber et al., 2012). This paper addresses the second question of potential interactions between events and their products combined in a stack, reviews the basic principles of plant breeding and its history of safe use, and extends these principles to the feed and food safety of events combined through the same processes used in conventional breeding of non-GE plants. Potential environmental impacts are outside the scope of food and feed safety. The new varieties developed through modern biotechnology are identified by a number of terms, including genetically modified (GM), GE, transgenic, biotech, recombinant, and plants with novel traits. The term GE is used here as defined by Weber et al. (2012). For these reasons, the term GE is preferred over the term GM. There are many methods encompassed by the general term conventional breeding, including wide crosses and selection, mutagenesis, and somaclonal variation. When the parental species are not closely related, the cross may be facilitated by embryo rescue, somatic hybridization, or x-ray-induced translocations. The term interaction, as used in this paper, refers to an effect, such as a new or modified metabolic activity, resulting from a combination of transgenes. An example of an interaction is protein-protein binding resulting in a novel effect only seen with a specific combination of proteins, for instance, protein cofactors or subunits for the same enzymatic complex or subcellular metabolic binding reaction. Examples can also include a direct metabolic interaction that would inhibit or activate components in a metabolic pathway shared by the proteins newly combined in the GE stack or components of independent metabolic pathways that indirectly interact by way of a common metabolite. Thus, interactions within GE stacks generally refer to the metabolic or physiochemical interplay between the products of transgenes or between the product of one transgene and the second gene, rather than between the two genes themselves.