Self-defense for plants
A clever strategy to protect plants against herbivores could be used as an attractive alternative to chemical pest control
RNAi and its functions
Whether genes are active or not depends on whether genetic information can be read and translated into proteins. The RNA acts as an interpreter between genetic information (DNA) and proteins. If the interpreter is disabled, a translation into proteins is not possible. The mechanism of RNAi does not only make sure that the own genes in an organism can be silenced; this process also helps plants, fungi and insects to protect themselves against viruses. During viral infection, the pathogens introduce their genetic material in the form of double-stranded RNA (dsRNA) into the cell in order to proliferate there. When the viral RNA is replicated in the cell, it is recognized by the RNAi system and fragmented into small pieces. The cell uses these pieces, the so-called siRNAs (small interfering RNAs), to detect and eliminate foreign RNA.
Nature as a model
Scientists from the Max Planck Institute for Chemical Ecology in Jena and the Max Planck Institute of Molecular Plant Physiology in Potsdam-Golm studied this natural mechanism and exploited it for their experiments. In previous studies, they succeeded in using RNAi to protect potato plants against their worst enemy: the Colorado potato beetle. They produced dsRNA, which targets a vital beetle gene, the actin gene, in the cells of potato plants. These plants literally struck the beetles in their stomachs. The insects lost their appetite and stopped growing, because feeding on double-stranded RNA had resulted in the silencing of the actin gene of the beetles: It could no longer be translated into the corresponding protein. The clou of the research project was to prevent that the plant’s own RNAi system eliminates the introduced dsRNA immediately and thus renders it ineffective against the beetle. Since the chloroplasts of plant cells do not possess an RNAi system, the scientists used chloroplasts for dsRNA production rather than cell nuclei.
Advantages and potentials of the procedure
The procedure based on RNA interference has several positive effects, as David Heckel from the Max Planck Institute for Chemical Ecology and Ralph Bock from the Max Planck Institute for Molecular Plant Physiology, two of the authors, confirm. On the one hand, RNAi helps plants to defend themselves against pests which may strongly reduce the use of pesticides. “If we are targeting the main pest insects with the RNA interference technology, we hope that this will enable us to reduce the overall use of chemical insecticides drastically,” says David Heckel. This could minimize health risks associated with the use of chemicals and protect the environment. Instead of poisoning insects that threaten our crop yields, the method ensures that vital genes in these pests are silenced in a targeted way. Optimally constructed RNA fragments could be targeted against individual insect species. In the past, genetic modifications of plants had raised concerns. These modifications were based on proteins produced by plants to keep insects at bay. “It was often criticized that these substances could cause toxicity or at least allergic reactions in humans,” explained Ralph Bock. “This point would become irrelevant when RNA interference is used, because this method does not result in the production of new proteins, but only the formation of additional RNA.” The authors also see other positive aspects of the procedure, such as the fact that pesticide resistances that insects tend to develop against chemical toxins would no longer play a role. “Moreover, RNA-based pest control would provide protection without further costs because the plant can use this mechanism continuously, without further treatment becoming necessary,” says Ralph Bock. Apart from low application costs and the advantages for the environment, proponents of this method emphasize the flexibility in the search for target genes and the specificity to the individual insect species. While chemical pesticides, such as phosphoric acid esters, attack the nervous system of all insect species, an appropriate RNAi gene only targets a vital trait of the pest insect.
Nevertheless, the RNA interference technology still needs to overcome a few obstacles before it can finally be applied to crops in the field. In many crops, such as wheat or rice, it is not yet possible to engineer the genome of chloroplasts, which would be the most efficient way to produce enough dsRNA fragments to control pests successfully. Moreover, certain insects are able to degrade the additional RNA fragments. This means that the target gene is not completely shut down and pest control is inefficient.
Both Bock and Heckel expect that the RNAi technology will still need six to seven years until application in the field will be possible. “The potato beetle, which originated from Colorado in the USA, has become a worldwide pest now and even reached China,” Heckel says. “The massive spread of such a main pest which has developed resistance to all known insecticides is a persuasive case for the development of transgenic potato varieties. We hope that the advantages of the RNAi technology are sufficiently compelling and help overcome concerns in the public debate on plant biotechnology.“
This research project was supported by the Max Planck Society, the European Research Council, the National Natural Science Foundation of China, the National Key Research and Development Program of China, the Science and Technology Department of Hubei Province of China, and the Recruitment Program of Global Experts.