New hope is on the horizon for potato growers engaged in the ongoing battle against Colorado potato beetle (CPB). Researchers are currently field-testing one of the most effective controls ever developed for the potato’s chief insect villain, and it is entirely chemical-free.
RNA interference (RNAi) is a biological process whereby RNA (ribonucleic acid) molecules activate a protective response against parasite nucleotide sequences by inhibiting their gene expression. In other words, it is the method by which organisms – including pests such as CPB – defend themselves against threats and regulate their own genes. But the same process that is used by a beetle to protect itself can be used to destroy it when it consumes the long double-stranded RNA (dsRNA) in genetically modified plants.
Since 2009, researchers at the Max Planck Institute in Potsdam and Jena, Germany, have been developing genetically modified potato plants to enable their chloroplasts to accumulate dsRNA targeted against essential CPB genes. After feeding on a potato’s leaves – and ingesting the dsRNA – the beetles in the study showed 100 per cent mortality within five days.
Why chloroplasts, rather than plant cell nuclei? In past breeding projects, expression of dsRNA in potato plants’ nuclei has proven inefficient because natural RNAi pathways in nuclei prevented the plant from producing enough long dsRNA. But long dsRNA are free to accumulate in chloroplasts – which have no RNAi mechanism – and the plants are fully protected against CPB.
According to Jiang Zhang, a professor with the College of Life Science at Hubei University in China, and the lead on the project, the technology shows great promise for the future of pest management, but there are no immediate plans for commercialization until all regulatory hurdles have been overcome.
“We encourage more scientists and industry involvement in this field for a better future,” Zhang says. “There is still a long way to go to make it really useful in daily life and be accepted by customers.”
RNAi-based biocontrolRNAi for control of CPB has also gained significant momentum in private research and development. Monsanto and Syngenta have both devoted major investments toward the technology.
In early 2015, Monsanto’s BioDirect technology platform targeted at CPB advanced to Phase 2 – early product development – of the company’s research and development pipeline. The product will have to complete advanced product development and pre-launch before broad commercialization early in the next decade.
While the principle is the same, Monsanto’s product works differently than Max Planck’s modified potato plant: it is sprayed onto the plant’s foliage. Rather than expressing dsRNA in its leaves, dsRNA is applied exogenously to the plant.
“The Colorado potato beetle consumes the leaves of the potato plant where we can focus the BioDirect application, versus needing a plant to produce the dsRNA targeting the pest below-ground where sprays cannot reach,” explains Greg Heck, weed control team lead for Monsanto’s chemistry technology area.
Heck says there are thousands of dsRNA naturally present in host plants that serve a variety of functions, and beetles consume and incorporate dsRNA all the time. “When targeting them for pest control, we seek to supply one additional dsRNA that will turn down a specific gene critical to their ability to feed and grow on the plant.
“Field research conducted on our BioDirect treatment for Colorado potato beetles has already demonstrated some early positive results. This includes reduced Colorado potato beetle larva infestation and plant defoliation in multiple geographies.”
Syngenta’s most advanced RNA-based biocontrol targets CPB in potato – and is also applied via a spray. The company has tested the product in multiple geographies over several years with positive results, says Luc Maertens, Syngenta’s RNAi platform lead based in Belgium. The company hopes to commercialize the product early in the next decade pending continued development and regulatory reviews.
Maertens says the company’s biocontrol is highly selective and starts to work before CPB can cause too much damage.
“The biocontrol is not systemic [in the plant], nor does it work through contact,” he says. “It does not change or have any effect on the DNA of the pest, nor does it involve genetic modification of the plant.”
No technology can work forever, however. Insect resistance to RNAi is a potential risk – one companies and researchers alike are keen to avoid so the technology has maximum benefit and longevity.
Maertens says that as resistance emerges to existing technologies, and the pest spectrum shifts along with climate change and other factors, growers’ needs will change. “Those challenges cannot be answered by only one technology,” he says. “It is imperative to gain insights into probable resistance mechanisms to RNAi triggers in insects, to monitor possible resistance in the field, and to support the use of the technology with appropriate stewardship requirements.”
Of the two methods of RNAi application (genetic modification and spray-on), Zhang believes the former might be better for growers. “Applying dsRNA exogenously is much less cost-effective than expressing dsRNA in the plant itself,” he says. “Spraying may also cause other potential problems in the environment.”
RNAi is not meant to be a silver bullet and should be used as part of a multifaceted pest control strategy. Regardless of the method of application, RNAi may soon be working in a field near you.