Syndicate content

Full Crop Protection from Colorado Potato Beetle Achieved by Generating High Concentrations of Beetle-Gene-Specific dsRNA in Host Plant Chloroplasts for Powerful RNA Interference Effect

Colorado potato beetles are a dreaded pest of potatoes all over the world. Because they do not have natural enemies in most potato-producing regions, farmers try to control them with pesticides. However, this strategy is often ineffective because the pest has developed resistances against nearly all insecticides. Now, scientists from the Max Planck Institutes of Molecular Plant Physiology in Potsdam-Golm and Chemical Ecology in Jena have shown that potato plants can be protected from herbivory using RNA interference (RNAi). The scientists genetically modified plants to enable their chloroplasts to accumulate double-stranded RNAs (dsRNAs) targeted against essential beetle genes. The results will be published in the February 27, 2015 iissue of Science. The article title is “Full Crop Protection from an Insect Pest by Expression of Long Double-Stranded RNAs in Plastids.” RNA interference (RNAi) is a type of gene regulation that occurs naturally in eukaryotes. In plants, fungi, and insects it also is used for protection against certain viruses. During infection, many viral pathogens transfer their genetic information into the host cells as double-stranded RNA (dsRNA). Replication of viral RNA leads to high amounts of dsRNA which is recognized by the host's RNAi system and chopped up into smaller RNA fragments, called siRNAs (small interfering RNAs). The cell then uses siRNAs to detect and destroy the foreign RNA. But the RNAi mechanism can also be exploited to knock down any desired gene, by tailoring dsRNA to target the gene's messenger RNA (mRNA). When the targeted mRNA is destroyed, synthesis of the encoded protein will be diminished or blocked completely. Targeting an essential gene of a crop pest can turn dsRNA into a precise and potent insecticide.
Some crop plants have recently been engineered by modifying their nuclear genomes to produce dsRNA against certain insects. “This never resulted in full protection from herbivory", says Dr. Ralph Bock of the Max Planck Institute of Molecular Plant Physiology, “because the plant's own RNAi system prevents the accumulation of sufficient amounts of dsRNA. We wanted to circumvent this problem by producing dsRNA in the chloroplasts instead." These organelles, which perform photosynthesis in green plants, are descendants of formerly free-living cyanobacteria, which are prokaryotes that lack an RNAi system. Presuming that chloroplasts would accumulate high amounts of dsRNA, the scientists in Dr.Bock's group decided to generate so-called “transplastomic” plants. In such plants, the chloroplast genome, instead of the nuclear genome, is the target of genetic modification.

To test this system on a real insect pest, the scientists chose the Colorado potato beetle. This little striped beetle was introduced into Europe accidentally at the end of the 19th century. Nowadays, it is a worldwide pest and can cause massive damage in agriculture. Besides potato leaves, the adult beetle and its larvae also feed on other nightshade crops, like tomato, bell pepper, and tobacco. The pest is difficult to control because of the widespread occurrence of insecticide resistance.

“By using chloroplast transformation, we generated potato plants that accumulate high amounts of long stable dsRNAs targeting essential genes in the beetle,” says Dr. Bock.
The efficacy of the dsRNAs as an insecticide was tested at the Max Planck Institute for Chemical Ecology in Jena. Larvae were fed on detached potato leaves and the mortality was monitored for nine days. The leaves were taken from transplastomic dsRNA plants, conventional transgenic dsRNA plants with a modified nuclear genome, and unmodified plants. For comparison, dsRNAs targeting two different genes were tested.

“Transplastomic leaves producing dsRNA against the actin gene caused a mortality rate of 100% after five days of feeding,” says Dr. Sher Afzal Khan from Jena. The actin gene encodes a structural protein that is essential for cell integrity. In contrast, plants with a modified nuclear genome expressed much less dsRNA and only slightly slowed down the beetles' growth.

These recent results show that changing the target of transformation from the nuclear genome to the chloroplast genome overcomes the major hurdle towards exploiting RNAi for crop protection. As many insect pests increasingly develop resistances against chemical pesticides and Bt toxins, RNAi represents a promising strategy for pest control. This technology allows for precise protection without chemicals and without production of foreign proteins in the plant.

[Press release] [Science abstract]