Bt and Rootworms
Microbiologists have discovered different strains of the bacteria Bacillus thuringiensis (Bt) that have the ability to make insecticidal proteins to many kinds of insect larvae. It is not surprising then that with enough time and effort, strains of Bt were found that made endotoxins specific to western and northern corn rootworm. The commercialization of these discoveries directly impacted how our agronomist thought about controlling rootworm in corn. Again, they were motivated to keep up with the genetics.
Figure 5 shows how the genetic engineers combined the coding regions and promoters for rootworm control. The coding regions were cloned from Bt strains that can kill rootworm larve while the promoters allow for expression at levels needed to kill rootworm larvae. The first rootworm Bt commercialized was Yieldgard rootworm (event Mon286) and it used a coding region encoding the Cry3Bb endotoxin. The genetic engineer was challenged to create events that had higher levels of expression than those achieved with the ECB events. This was because a higher dose of the Cry3Bb is needed to kill rootworm larvae than the dose of Cry1ab needed to kill ECB larvae. Therefore, a modified 35S promoter called A1S4 was developed that combines the 35S promoter with DNA sequences from several sources to direct higher levels of expression. Rootworm larvae will ingest enough of the Cry3Bb protein to die when feeding on the roots of plants without causing damage to the root system. This Cry3Bb protein is also made in a variety of plant tissues including leaves, pollen and seeds. The protein is not toxic to ECB. A lateral flow protein detection test can be used on a leaf or seeds to detect the Cry3Bb protein in a Yieldgard rootworm hybrid.
An alternative strategy for taking advantage of Bacillus thuringiensis to control rootworm is shown in the other transgenes in Fig. 6. The strain of Bt called PS149B1 makes a larger and a smaller protein that together are toxic to rootworm. In the bacteria, the coding regions for these two proteins are a part of the same stretch of DNA behind a single promoter. In plants each coding region needs it’s own promoter so the genetic engineers combined each with the corn ubiquitin promoter. This promoter turns each gene on in the same way so both proteins are made together the cell. Furthermore, they are made at the levels needed to kill the rootworm larvae feeding on root tissues.
There are a wide range of gene modifications a genetic engineer can choose to employ when assembling a transgene designed to improve a crop. This is a critical step in the process of developing a transgenic crop and a step the genetic engineer can impose a great deal of control over. See the video below or use the interactive Transgene Design learning activity to solidify your understanding of this step and some of the differences between the Bt transgenes.