Events and management
At first glance the details of gene information in Figure 3 appear to be important only to people who spend their time working with recombinant DNA in test tubes. However, our agronomist has discovered that knowledge of gene design has helped them understand ECB resistance in the Bt hybrids.
For example, answer the following question about the very first Bt hybrids on the market which had the ‘Event 176’ transgenes.
Because this event used a green tissue specific and a pollen specific promoter, the transgenes were only expressed in certain parts of the plant. The ear tissues would not express the gene and make the Cry1a(b) protein so if the tip of the ear grew out of the husk, ECB could feed on that tissue. The agronomist also noted that ECB damage was more prevalent when the event was backcrossed into a hybrid that did not maintain green tissue late into the growing season compared to a longer season hybrid that had this event. The agronomist knew that even though the cells that made up leaf, husk or green stalk tissue still had the transgene with the PEP promoter, the gene was shut off once the plant started shutting down photosynthesis in preparation for drydown.
Transgene design likewise explained why the YieldGard and StarLink events had plant wide, season long expression; why hybrids with these events made the ‘Bt’ proteins in their seeds, and what made the StarLink event different in the eyes of the regulatory agencies. The 35S promoter used in these events allows for expression in all types of metabolically active corn cells. Therefore the Cry1a(b) Bt protein is made in all parts of plant, even the seeds. The StarLink event used this same 35S promoter, but the coding region directed the corn cell to make the Cry9c protein. This ‘Bt’ Cry9c protein was also an effective ECB endotoxin but it attacked a different part of the larvae midgut. When managing an insect pest such as the European Corn Borer, having a second endotoxin protein to kill the pest in a different way can be a valuable tool. However, the unique structure of Cry9c also made it more heat stable and less digestable in a human stomach environment compared to the Cry1a(b) protein. While Cry9c was never demonstrated to be a food allergen, heat stability and slower digestion are properties of other food allergens. Because the transgene was designed to make this protein in the tissues people consume, the regulatory agencies did not approve the product for this use. Our agronomist knew first hand that the difference in the transgene, promoted as an alternative means of controlling ECB, eventually created a management problem that made grain isolation and pollen drift common themes in many of their discussions with clients.