The potential bonanza of rewards flowing from RNA interference (RNAi) technology, or gene silencing, has agricultural life science companies rushing to invest. Transgenic science takes a gene with a specific trait from one living thing and splices it into the genetic code of another organism to get it to express that same trait. RNA interference, though, turns off selected genes already in the organism, stopping expression of that genetic trait.
The spider mite is less than one millimetre in size but might be the peskiest pest on the planet.
The tiny arthropod preys upon dozens of commercial crops, including soybeans, cucumbers, apples, peppers and corn, feeding on the underside of leaves and causing yield losses. It is also a common pest in commercial greenhouses and gardens.
Spider mites are particularly pesky because they eat more than 1,100 plant species and can reproduce up to 10 generations per year during the growing season.
A female two-spotted spider mite can lay 200 eggs over its 21- to 28-day life, which can become adults in five to seven days in hot, dry conditions.
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The population explosion and speedy turnover of generations, when combined with natural mutations, can rapidly lead to mites with insecticide resistance.
As of 2012, there were 389 recorded cases of two-spotted spider mites developing resistance to a pesticide, according to Western University in London, Ont. The mites can overcome a new pesticide in two to four years, so spending millions to develop a new insecticide has little value.
The spider mite’s resilience and its global impact on field and greenhouse crops made it the perfect test case for biologists and geneticists at Western University, who are studying what could be a revolutionary control method for agricultural pests.
Scientists at Western are leading an international team of researchers from Spain, Belgium, France and the United States in a project called Genomics in Agricultural Pest Management. They are using RNA interference (RNAi), a genetic tool to shut down specific genes, to kill the mites or slow population growth.
Two American biologists, Craig Mello and Andrew Fire, received the Nobel Prize in Medicine in 2006 for their discovery of RNA interference. Scientists had assumed for years that RNA molecules were simply messengers that carried genetic information from DNA.
In the 1980s, biologists learned it’s possible to use other RNA molecules to obstruct the work of messenger RNA and prevent the formation of proteins. Based on that discovery, scientists assumed they could block the production of faulty proteins and shut down the over-expression of genes related to diseases such as cancer.
Molecular biologists didn’t understand the process until Mello and Fire’s revolutionary work on a roundworm, C. Elegans, in the 1990s.
In a 1998 paper published in Nature, Mello and Fire said it was possible to silence gene expression in C. Elegans by inserting double stranded RNA into the one mm long roundworm.
They concluded that by interfering with the messenger RNA, it was possible to suppress the expression of a specific gene and the formation of a specific protein in the organism.
Mello and Fire’s discovery opened up an entirely new field of molecular biology, where scientists can silence gene expression as needed.
In the 15 years since, most RNAi research dollars have been directed at human health and potential therapeutic treatments.
Applying the technology to human therapeutics has proved trickier than anticipated, but agricultural applications appear to be less complicated, said Doug Macron, who reports on the gene silencing industry for GenomeWeb, a science information service.
Molecular biologists and entomologists around the globe have been trying to apply RNA interference to pest management for the last several years.
One of them is Vlad Zhurov, a research associate in Western’s biology department and a member of the Genomics in Agricultural Pest Management team led by Western biologist Miodrag Grbic.
“By introduction of the double stranded (RNA) molecules into the organism (spider mite), which actually match the sequence of the gene that this organism possesses, you can knock down the native organism gene,” Zhurov said.
“In pest control, we need to know a sequence of a certain gene that we want to knock down in the pest.”
Grbic and his team have a good sense of which spider mite genes to target because in 2011, they published the mite’s genomic sequence in Nature.
The scientists are now working with Arabidopsis Thaliana, the small flowering plant better known as thale cress and often used as a model plant in biology.
The concept is to insert a sequence of double stranded RNA into the plant that matches a target gene in the spider mite. The mites feed on the plant and consume the RNA. Once inside the spider mite’s cells, the synthetic RNA interferes with gene expression and the formation of critical proteins.
“It’s not a 100 percent eradication of every mite…. In a way, you create a sickly spider mite,” Zhurov said.
“Usually only a few mites arrive to a given location and they have to establish a colony. If we weaken them sufficiently so that they won’t be able to establish this colony … not killing them is just fine.”
Experimental results indicate that RNAi can control spider mite colonies.
“Recently, we were able to show the reduction of survival of spider mites developing on the Arabidopsis (plant),” he noted.
“So now we will be looking at the fecundity of spider mites to see whether RNAi reduces … how many eggs they deposit, how many progeny they develop.”
Pierre Meulien, president of Genome Canada, one of the funders of the spider mite project, said the research is particularly exciting because the technology is directed at a specific gene in a specific pest.
“These things are so specific that you can target, very specifically, the unique pest you want to kill…. It’s not like a general pesticide,” he said.
“The precision would be exquisite because of the uniqueness of the (RNA) sequence.”
Research also suggests that the molecule spreads quickly through the organism when an insect ingests double stranded RNA.
“It seems that when these molecules get into the digestive system of a (pest), then those molecules remain active,” Meulien said.
“This is big news because no matter what plant you want to protect … you have a way of genetically manipulating the plant so it expresses a large amount of this stuff. Then all you have to do is wait for it (the insect) to (feed) on the leaves.”
Marcé Lorenzen, a professor of entomology at North Carolina State University, said that isn’t always correct, it depends on the insect and the targeted gene. Oral RNAi, in which insects feed on plants and ingest double stranded RNA, is highly effective in certain cases.
“I call them the sucking and piercing little insects, all the insects that can actually pierce the leaf and suck. Aphids and things like that, oral RNAi works incredibly well,” Lorenzen said.
“The double stranded RNA move from cell to cell and make it systemic.”
Lorenzen, who is attempting to use RNAi to control the red flour beetle, which feeds on stored grain, said it doesn’t spread readily throughout the cells of all pests. A higher dose of double stranded RNA may be required to affect certain species, including the red flour beetle.
Lorenzen and other scientists are exploring other ways to deliver RNAi, such as a biological spray.
“My lab is trying to express double stranded RNAs in micro-organisms (possibly bacteria) … hopefully for a low cost, eco-friendly bio-pesticide,” she said.
“If you can (deliver) a high enough dose (to the pest)… you can get it to work.”
As noted in a research and development document, Monsanto is looking at two approaches to apply RNAi to pest management: BioDirect, which features bio-pesticides that contain double stranded RNA, and another technology that targets the western corn rootworm. A sequence of rootworm RNA is inserted into the corn plant, which represents a transgenic process.
“We have to have a transgenic plant, which will be … expressing double stranded RNA construct at every stage of plant development and in every tissue,” said Zhurov, referring to the spider mite project.
RNAi technology for pest management remains in the laboratory phase. If Western University scientists can prove it works, they will have to partner with seed companies to transfer the technology to a commercial crop.
Zhurov said the regulatory process for the transgenic technology could prove challenging, given the vigorous public debate over GMOs.
“RNAi technology, in general, it’s a proven and very efficient technique, which is used in research. In many cases RNAi should be fairly easily scalable to the industrial level,” he said.
“(But) we have to overcome this public perception that trangenesis is bad.”
Lorenzen said it may be premature to say the regulatory process will be difficult, but biologists will have to spend a significant amount of time explaining the technology to the public and policy makers.
“I don’t know how difficult it will be, but I think the regulatory community will have to be educated.”
Eric Jan, a University of British Columbia RNAi expert, agreed it’s challenging to clearly describe the biological process because he struggles to talk to his family about his work.
“When I try to explain this to my parents or my wife, they understand DNA and they understand proteins. But it’s the RNA part that’s confusing to them.”
