From the potato farms of Prince Edward Island to the cornfields of Iowa, there is a never-ending struggle between farmers and insects. Farmers apply chemical pesticides to protect their crops, which drive the evolution of more insecticide-resistant pests. This, in turn, forces farmers to use insecticides more frequently and at higher doses, which then selects for even more resistant insects. And so on and so on.

In an effort to gain the upper hand, researchers are turning to transgenic plants as a way to increase crop yield while reducing pesticide use. For example, some species of corn, cotton, and potato plants have been engineered to produce a bacterial toxin called Bt that is lethal to insects. Insects that eat leaves from Bt-producing plants ingest the toxin and are killed. But Bt isn’t effective against all agricultural pests and resistance has already been documented in some insects.

A promising area of transgenic plant research is focused on the use of RNA interference, or RNAi, to control insect pests. For any gene to be expressed, the DNA must first be read and converted into RNA. The RNA message is then decoded to produce a protein. Think of your cell as a house and the DNA as the master building plan for that house. Every time you need to make a repair, the general contractor consults the building plan and sends a message to the tradesperson to make the component that is needed. The RNA is the message that your cell uses to produce the parts needed to keep everything running smoothly. In RNAi, the RNA message is intercepted and the proper parts are not made. When the RNA message is for an essential cellular component, blocking the message can lead to cell death and if enough cells are affected, the death of the entire organism.

For the past few years, scientists have been trying to co-opt RNAi in transgenic plants as an effective and safe way to kill insect pests. Transgenic plants were created that produced interfering messages targeted against specific insect genes. When insects ate these transgenic plants, the interfering messages would be taken up into the insect cells and block the expression of their target genes. However, the success of this approach has been limited, which brings us to…

The problem

In their previous attempts at using RNAi, scientists created transgenic plants expressing interfering messages from their cells’ nucleus. But before they could find their targets, these interfering messages were being chopped up into shorter messages by molecular scissors found within plant cells. As it turns out, long messages are much more efficient than shorter messages at blocking their target genes. While the insects that preyed on these transgenic plants grew more slowly, they did not die and the plants were not completely protected.

To get around this problem, a team of scientists led by Ralph Bock at the Max Planck Institute in Germany engineered a transgenic potato plant that expresses the interfering messages in their chloroplasts. Chloroplasts are individual compartments within a plant cell that house the machinery required for photosynthesis. Importantly for this study, chloroplasts do not contain the molecular scissors present in the rest of the cell, thereby ensuring that the interfering messages produced would be protected from being chopped. Could these longer interfering messages do a better job of blocking insect genes and protecting the plant?

The experiment

The scientists decided to test their idea using the Colorado potato beetle (CPB) as the target pest. The CPB is a notorious insect pest of potatoes, tomatoes, eggplants and other related plants. It feeds primarily on plant leaves and has no natural predators. Currently, chemical pesticides are the main method of controlling the CPB but within the last few decades, the bug has developed resistance to all the major types of insecticides.

For the interfering message, the scientists decided to target an essential protein called actin. Actin serves as the framework for a cell, providing structural support for it to grow and divide. The researchers created two types of transgenic plants: one where the actin-interfering message would be expressed in the plant cell’s nucleus and later chopped up, and one where the message would be expressed in the plant cell’s chloroplasts and protected from being cut.

To determine the effectiveness of each transgenic plant in killing CPB, the scientists measured survival in CPB larvae fed leaves from either normal potato plants, nuclear transgenic plants or chloroplast transgenic plants. The results were dramatic. All the larvae fed on the chloroplast transgenic potato leaves died after five days. Among larvae that ate either normal potato leaves or nuclear transgenic potato leaves, mortality was between 10% and 20% after nine days. The larvae that ate the chloroplast transgenic leaves also became sicker earlier, meaning that they ate less and greater portions of the leaves were preserved. By expressing long actin-interfering messages in chloroplasts, the scientists had engineered a supremely more effective method of killing insect pests.

The implications

In solving the problem of producing long interfering messages in plants, the scientists in this study made a huge leap forward in the development of RNAi as a viable pest control mechanism. The appeal of RNAi lies in its specificity. In theory, you could design your interfering message so that it targets a specific gene in a specific insect species. This would avoid the non-targeted effects of conventional pesticides by preventing the indiscriminate killing non-harmful insect species. Because RNAi targets essential insect genes and relies on an important aspect of insect physiology, it will be much harder for resistance to develop against these interfering messages.

This study paves the way for a new generation of environmentally safe, RNAi-based pest control technologies that will help farmers improve crop yields and move away from chemical pesticides. But for now, more work is needed to determine whether the chloroplast strategy will be effective in other plants and against other pests.

Reference:
Zhang, J., Khan, S., Hasse, C., Ruf, S., Heckel, D., & Bock, R. (2015). Full crop protection from an insect pest by expression of long double-stranded RNAs in plastids Science, 347 (6225), 991-994 DOI: 10.1126/science.1261680