Elegance and Effectiveness: Biotech as Insect Assassin
Insects. You can swat them, you can spray them, you can repel them, and now, thanks to Bryony Bonning’s lab, you can create a peptide sequence that will bind to their gut and deliver a toxin to kill them.
Bonning, a Professor in Iowa State University’s Department of Entomology, is developing new technologies to control insect pests. This work includes the use of insect viruses to either deliver toxins or to silence the RNA in a given pest.
Recent research has taken a divergent turn. Toxins derived from the Bacillus thuringiensis (Bt) bacterium are widely used to suppress insect populations in transgenic plants, especially corn and cotton. Bt toxins work very well against beetles and caterpillars, which eat the plant tissue, but Bt toxins have not been successful against sap-sucking insects. Because the sap-sucking insects simply pierce leaves to extract sap and do not ingest the leaves on which Bt would be found, there has been no evolutionary selection for toxins that kill sap-sucking insects. These insects have not, therefore, been as affected by the presence of Bt toxins in plants. In the past, growers have planted Bt cotton thinking they would have a very low pest load. When they didn’t spray for pests, the sap-sucking insect populations bloomed.
Aphids are among the most destructive sap-sucking insect pests on cultivated plants in temperate regions. They transmit about half of all the insect-borne plant viruses. With funding from the U. S. Department of Agriculture, Bonning’s lab was examining the molecular interactions between the plant virus and the aphid vector of the virus. The plant virus is ingested by the aphid, binds to a receptor in the gut and then moves into the aphid’s body. Dr. Sijun Liu, an Assistant Scientist in the Department of Entomology, isolated a peptide that competed with the virus for binding in the gut. They envisioned a transgenic plant that would produce the peptide which would prevent or reduce the amount of virus taken up by the aphid.
“When we presented it to industry, they said, ‘We really just want the aphids dead,’” Bonning laughed.
So the researchers considered other ways to capitalize on what they had discovered. What if they attached the gut-binding peptide to a Bt toxin providing the toxin with an anchor? Would the toxin then kill the aphid?
The Bt toxin Cyt2Aa, which is toxic to mosquito larvae, was selected for modification with the aphid gut binding peptide. Of 12 Cyt2Aa constructs to which the aphid gut binding peptide had been added, five retained toxicity to mosquito larvae. These five constructs were then tested against pea aphids and showed significantly improved toxicity relative to the wild type Cyt2Aa. They then tried the constructs with the green peach aphid, which feeds on hundreds of plant species across many families of plants, and the technique worked, but slightly less effectively than for the pea aphid, for which the peptide sequence had been designed. Bonning postulates that the same approach can be applied for use against other pest insects following similar isolation of peptides that bind to the gut epithelium of the targeted pest.
Reviewers of the manuscript reporting these findings requested the scientists identify the protein bound by the pea aphid gut binding peptide. Dr. Nanasaheb Chougule who led the Cyt2Aa modification project, showed that the peptide bound to the membrane associated enzyme aminopeptidase N. In addition, Luke Linz, a graduate student working in Bonning’s lab, produced some elegant work to prove that aminopeptidase N is the receptor for the plant virus. This is the first plant virus receptor identified in an insect.
“We recognized the competition between the virus and the pea aphid gut binding peptide for the receptor before we knew what the receptor was,” Bonning says.
Their work is published in the Proceedings of the National Academy of Sciences.
What’s on the horizon for Bonning, the Department of Entomology and Iowa State University? CAMTech, of course! CAMtech is the Center for Arthropod Management Technologies, a National Science Foundation (NSF)-supported Industry/University Cooperative Research Center (I/UCRC). Iowa State University is taking the lead in its partnership with the University of Kentucky (UK) for this project. CAMtech will be headquartered on campus in Science II and will be supported by seven industry members.
The move to develop the center was initiated in 2012 on receipt of an NSF I/UCRC Planning Grant. The planning meeting was held last September at Reiman Gardens between ISU and UK researchers and pest-control industry representatives. The researchers presented brief research proposals and the industry representatives provided immediate feedback, negative, positive or indifferent. They also shared their wish lists for future projects. The researchers then had a set of vetted proposals that were then sent out for ranking by the prospective industry members. The five top-ranked proposals went into the ISU proposal for full funding of the NSF center in March of 2013.
The competition for NSF-funded centers is fierce, and the point of attrition is securing commitment to membership from industry members.
When Bonning first read the description of the process to establish an NSF I/UCRC, she could see that it was a great deal of work for relatively little immediate payback. But she could also see from the I/UCRC for the Center for Non-Destructive Evaluation on the ISU campus, that the real value of the NSF centers is raising awareness of expertise at the university, which brings more research money into the university both from industry and elsewhere. In short, it potentially brings a spiral of increasing excellence to Iowa State University.
The first industry advisor meeting will be in October 2013, with research beginning in January 2014.