Interestingly, the Lenape (also known as breeding line no. B5141-6) potato has been found to express some useful attributes, such as high solids content. Thus Lenape continues to be used successfully as a parent in conventional breeding programs, providing its genes to new commercial potato varieties, such as Atlantic and Denali. Progeny with genes providing the high solids content are selected or maintained, and the genes responsible for high solanine content in the tubers are selected against or rejected. Additionally, Lenape has been transformed with a genetic construct containing the solanidine glucose-adenosine diphosphate glucosyltransferase (SGT) transgene in the antisense direction, which is designed to interfere with the solanine biosynthesis (Moehs et al., 1997). In field trials, several transgenic Lenape-derived lines expressed substantially less solanine than the parent Lenape, apparently due to antisense expression, an rDNA method for “turning off” an undesirable gene (McCue et al., 2003).
Several breeding programs are developing potatoes derived from conventional crosses between the ordinary potato Solanum tuberosum and relatives of other species, such as S. acaule (Kozukue et al., 1999) or S. chacoense (Sanford et al., 1998; Zimnoch-Guzowska et al., 2000). These are conventional breeding programs in which genes from two different species are exchanged. The intent of these conventional breeding programs is to generate potato varieties with new beneficial features, while minimizing deleterious traits from the foreign species.
Breeders typically monitor levels of toxins in plants that are known to naturally contain them, even though such monitoring is only voluntary in the United States. However, an unexpected and unintended problem may result when combining different species because thousands of genes would be interacting, not just one or two genetic elements of interest. For example, hybrids of S. tuberosum and S. brevidens produced not only the usual glycoalkaloids, but also the toxin demissidine, which is not produced in either parent (Laurila et al., 1996). This singular result shows that non-genetic engineering breeding methods can have unintended effects and generate potentially hazardous new products.
Any time genes are mutated or combined, as occurs in almost all breeding methods, the possibility of producing a new, potentially hazardous substance exists. Conceivably, similar outcomes could result from using rDNA to transfer specific genes from S. brevidans to S. tuberosum, giving rise to hybrids expressing the novel toxin demissidine. In either case, the hazard lies with the presence of the toxin, and not with the method of breeding. Genetic engineering could also be used to transfer only the beneficial genes from S. brevidans, leaving behind the genes responsible for the novel toxin.
Another example of a toxic compound from traditional crops is psoralens in celery (see Box 3-2). Celery naturally produces these irritant chemicals that deter insects from feeding on the plant and also confer protection from some diseases (Beier and Oertli, 1983). Celery plants with an elevated expression of psoralens will suffer less damage from disease and insect predation and have more aesthetic appeal to consumers, who tend to reject insect- or disease-damaged produce.