Nanotechnology-enabled genetically engineered RNA interference pesticides: socioeconomic context, modes of action and risks

Watch a recording of a presentation delivered by IATP's Dr. Steve Suppan above titled "Nano-enabled RNAi based pesticides: Modes of Action, risks and regulation."

The development of and investment in nanotechnology-enabled pesticides is a new phase in the effort to fight against weed and pest resistance. Some of these pesticides are in an advanced state of product development and a few are near commercialization. Therefore, an introduction to how these products work (Modes of Action) and their risks is timely. (The benefits claimed for these pesticides are widely publicized by their developers and so are mentioned only in passing here.)

Socioeconomic context

This scientific battle against resistance particularly concerns plants that are genetically engineered (GE) to resist proprietary pesticides and to enable continuation of current extensive monocropping practices that incubate resistance.1 In our November 22, 2021 presentation for a webinar co-organized by the Brazilian Research Network on Nanotechnology Society and Environment (Renanosoma), we showed a photo of a weed scientist standing in a soybean field overgrown with weeds nearly as tall as him. The weed scientist referred to the dramatic increase in the number of herbicide-resistant weed types, forecast to begin as early as 2024, as a Pandora’s Box, a reference to an ancient mythical source of all the evils plaguing the world. A creative headline writer characterized the new nano-enabled GE pesticides developed to prevent further U.S. weed resistance plaguing soy, maize and other crops2 as “Breaking Pandora’s Box.”

Of course, the increased scope and efficacy of weed resistance is not limited to the United States. Weed scientists have documented a sharp increase in weed resistance in Brazil correlated with the introduction of a very high percentage of GE maize, cotton and soybean crops. The same researchers have charted a boom in the number of unique cases of herbicide resistant weeds. Seventeen of 51 weed types are resistant to multiple herbicides. The chart characterizes a “pre-glyphosate era” ending in 2005 with a “post-glyphosate era” of herbicide resistant weeds.3 Because of the economic losses associated with crop yield losses and the costs of more numerous applications of herbicides to more hectares of herbicide resistant GE crops, governments, universities, start-up companies in GE research and pesticide manufacturers have invested public funds and private capital to “Break Pandora’s Box.”

Another important incentive for new pesticide products is government granted patents that confer monopoly rights to commercialize the patented products. Start-ups usually sell active ingredient (AI) and/or related technology patents to a pesticide manufacturer for subsequent incorporation into a patented herbicide, insecticide or fungicide. An agribusiness reporter’s analysis of the value of AI patents scheduled to expire in 2020 estimated the worth of the patents at $4 billion4 of an estimated $68 billion global agrichemical market value in 2020.5 A lot of money is at stake, beyond the value of crops losses, in developing a technology that will “Break Pandora’s Box.”

Three technology types to break a Pandora’s Box of increasing resistance

Pesticide product developers have offered three ways to defeat weed resistance to the GE crop and proprietary pesticide technology package: 

1) apply herbicides more toxic than glyphosate to crops engineered to resist those herbicides; 

2) coat existing AIs with patented nanoscale material coatings to better control the pace of the AI release and in theory reduce the rate of weed resistance; 

3) develop new AIs, also nano-enabled, to “silence” the genomic sequences of the weeds or other pests that are vital for their survival. We’ll discuss the first two technologies proposed to “Break Pandora’s Box” very briefly before focusing on the third nano-enabled GE RNA interference pesticides.

One response to the increasing failure of glyphosate to control weeds in GE crops designed for glyphosate use is a revival in the use of more toxic pesticides, such as dicamba, and the development of new GE crops, above all soy and maize, designed to withstand dicamba applications. However, dicamba is a highly volatile herbicide whose drift has damaged hundreds of thousands of U.S. hectares of horticulture plants and trees not genetically engineered for dicamba. Lawsuits to obtain compensation for dicamba damages continue to grow in scale and number.6 The U.S. Environmental Protection Agency has been sued for allowing dicamba to be commercialized despite what was known about the herbicide’s drift damage when used according to manufacturer instructions.7

Weed scientists at the Brazilian Agribusiness Research Corporation (EMBRAPA) have run test plot studies to estimate how much damage would be caused by dicamba to Brazil’s huge soybean crop, including GE soy designed to resist pesticides other than dicamba.8 These scientists conclude, “A 1% dicamba drift in tropical conditions reduces soybean yield by 12%.” This yield reduction, from dicamba sprayed by farm workers on the ground, would increase over time as weeds in soybean fields became resistant to dicamba. (Pesticide toxicity to farmworkers is discussed briefly at the end of this article.)

A second response to the “Pandora’s Box” of weed resistance to herbicides has been to coat the AIs of current herbicides with nano-scale materials, usually chemically inert nano-clays or nano-composite materials, to control the release rate of AIs and to enable them to penetrate more efficiently weed plant cells, which macro-scale herbicide droplets cannot do. The result is a higher kill rate while using less volume of AI. Embrapa is also engaged in this research, with scientists claiming that nanotechnology-enabled pesticide products will reduce the volume of pesticides applied and in so doing, cause less damage to the environment adjacent to the nano-pesticide sprayed cropland.9

Lessons learned in nano-coating current AIs have been applied to the development of nano-enabled GE RNA interference (i) pesticides. Oddly, the properties of nanomaterials in this new generation of pesticides have received relatively little risk analysis, compared to that received by the CRISPR Cas9 and other GE techniques used to modify RNAi to target specific weed and insect pests.

First, we’ll briefly discuss risks associated with the application of CRISPR Cas9 applied to plants. We’ll summarize the Mode of Action (MoA) of GE RNAi to turn off or “silence” genes in genomic sequences of weeds and insects that are vital for expressing proteins necessary for the targeted pests’ survival. Among the off-target mutations of the CRISPR Cas9 techniques are those in plants and animals surrounding the crops not engineered to resist the new AI, GE RNAi. We review risks associated with those mutations and other risks, including to farmworkers, enabled by the nano-coatings necessary to stabilize and protect the GE RNAi, making it an applicable pesticide.

The role of CRISPR Cas9 GE technique in nano-enabled RNAi pesticide

The idealized image of CRISPR Cas9 is of the Cas9 “scissors,” an enzyme, cutting a genome sequence, with the cut neatly repaired by an RNA messenger (mRNA) “thread,” guided by the data of a genomic library.10 Messenger RNA is defined as “a single-stranded RNA molecule that is complementary to one of the DNA strands of a gene.”11 (mRNA is used in the manufacture of vaccines, including some anti-SARS CoV-2 vaccines.) RNAi is a double stranded RNA molecule designed to suppress in the targeted genomic sequence genes critical for a pest’s survival. The ideal result of GE techniques is a neatly modified sequence that produces only the genetic mutations and corresponding traits informed by the data library.

In practice, however, the use of CRISPR Cas9 can result in hundreds of unintended mutations when the selection of genomic data to guide the mRNA repair is erroneous.12 As a result, the Cas9 “scissors” cuts in the wrong places in the DNA sequence, and the mRNA makes its repair at an erroneous site. Some of these unintended mutations can express harmful traits, while other traits are beneficial or benign. The genetic engineering of naturally occurring RNAi to silence genes in targeted weeds and other pests can likewise result in unintended mutations in the targeted weeds.

A Friends of the Earth (U.S.) report explains, “Developers of RNAi pesticides aim to exploit naturally existing RNAi pathways in plants, animals, and fungi by manufacturing synthetic interfering RNAs of a particular sequence in order to silence a specific gene or genes.”13 The report’s authors continue: “The interfering RNA and a target messenger RNA bind as they share a sequence that is matching or like each other. The messenger RNA is then cleaved and destroyed. No protein is produced, resulting in ‘interference’ of gene expression, also known as ‘gene silencing.”14 If the sequence deleted is large enough to change the structure of the plant, the RNA interference may be heritable from one generation of the target plant to the next. The RNA interference can also affect non-targeted plants if their gene deleted sequences are similar enough to bind with the sequences of non-target plants.15

Some of the risks associated with the GE risks of RNAi pesticides are as follows: erroneous and on-target gene silencing express unwanted traits; long deletions may result in undesirable heritable traits; altered plant genetic composition may result in allergenicity, increased toxicity, changes in nutritional composition; unwanted immunostimulatory effects can reduce white blood cell count in humans and non-target organisms throughout the food chains.16 These risks do not include those associated with the fate and transport of nano-scale materials that coat RNAi, stabilizing it and preventing its destruction by ultra-violate light rays. RNAi would not be an AI without the protective nanomaterials that enable both the RNAi’s survival and its efficacy in killing pests. Risks specific to these nanomaterials are described in the next section.

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Downloadable Materials

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