© ISTOCK.COM/TAWNINTAEWRNA interference (RNAi)—the process by which small interfering RNAs (siRNAs) bind to and cleave complementary mRNA sequences, inhibiting their translation into proteins—is not new to agriculture. In fact, as a naturally occurring biological process, RNAi was mediating plant metabolism, growth, and pathogen defense long before humans began cultivating crops for their own benefit. But in the last 15 years, RNAi’s role in agriculture has grown as researchers have developed greater understanding of the mechanisms underlying the phenomenon and employed it to improve pathogen resistance, nutrition, and yield of crop plants. RNAi-enhanced crops have been approved for cultivation by regulatory agencies in the United States, Europe, Canada, Australia, New Zealand, and Brazil, and some of these crops—for example, papaya—have already reached our plates.
RNAi is a particularly potent tool for fighting common crop pathogens. By simply integrating virus- or bacteria-derived DNA sequences into the plant genome, pathogen-targeting siRNAs can be produced, triggering the endogenous RNAi mechanisms to target and degrade homologous sequences produced by invading pathogens. Commercial cultivation of virus-resistant papaya and extensive field testing of virus-resistant plum (under high disease pressure) since 1996 have shown that the pathogen-derived RNAi technology can deliver very effective and durable resistance. More recently, this strategy has produced virus-resistant common beans, fungal-resistant bananas, nematode-resistant soybeans, and insect-resistant corn. To date, RNAi has proven more cost-effective and environmentally friendly than the use of pesticides to control pathogens, and RNAi-fortified crops have the potential to impact food security and economic development. Recent regulatory approval in Brazil for cultivation of a common bean modified by RNAi for resistance to bean golden mosaic virus is cause for optimism. In Africa researchers are employing RNAi to combat viral diseases of the tropical root crop cassava (Manihot esculenta), a staple of African diets.
Agricultural researchers are also using RNAi-based technology to develop nutritionally enhanced crops. For example, RNAi was used to downregulate the omega-6 fatty acid desaturase gene, resulting in increased levels of monounsaturated (oleic) fatty acids in soybean seeds. The high–oleic oil soybeans are beneficial for human health and industrial oil production. Other nutritionally enhanced products in development include tomatoes with increased carotenoids, high-amylose and reduced-gluten wheat, and oranges with high levels of beta-carotene.
Despite widespread debate on public acceptance of genetically modified organisms, several unique features support the safety profile of RNAi-enhanced products, including the ubiquitous nature of siRNAs in plants; the history of safe use and consumption of naturally occurring and transgene-derived RNAi crops; high species specificity that minimizes off-target effects; and lack of toxicity and allergenicity, resulting from the fact that no transgenic protein is produced by such plants. Clearly, RNAi holds tremendous potential for producing healthier crop plants with enhanced nutritional value.
Narender Nehra is the director of regulatory affairs at the Donald Danforth Plant Science Center’s Institute for International Crop Improvement (IICI), where Nigel Taylor is a senior research scientist. Mark Halsey, director of product development; Titus Alicai, program leader for root crops research in Uganda; and Douglas Miano, regulatory lead in Kenya, also contributed to this article.