RNAi efficacy is enhanced by chronic dsRNA feeding in pollen beetle

Improving RNA-based crop protection through nanotechnology and insights from cross-kingdom RNA trafficking

Spray-induced gene silencing (SIGS) is a powerful and eco-friendly method for crop protection. Based off the discovery of RNA uptake ability in many fungal pathogens, the application of exogenous RNAs targeting pathogen/pest genes results in gene silencing and infection inhibition. However, SIGS remains hindered by the rapid degradation of RNA in the environment. As extracellular vesicles are used by plants, animals, and microbes in nature to transport RNAs for cross-kingdom/species RNA interference between hosts and microbes/pests, nanovesicles and other nanoparticles have been used to prevent RNA degradation. Efforts examining the effect of nanoparticles on RNA stability and internalization have identified key attributes that can inform better nanocarrier designs for SIGS. Understanding sRNA biogenesis, cross-kingdom/species RNAi, and how plants and pathogens/pests naturally interact are paramount for the design of SIGS strategies. Here, we focus on nanotechnology advancements for the engineering of innovative RNA-based disease control strategies against eukaryotic pathogens and pests.

Effects of Low Doses of a Novel dsRNA-based Biopesticide (Calantha) on the Colorado Potato Beetle

The Colorado potato beetle, Leptinotarsa decemlineata (Coleoptera: Chrysomelidae) is a destructive pest of the cultivated potato, Solanum tuberosum. Members of this species are well-suited to agricultural habitats because of a suite of physiological adaptations and their ability to evolve resistance to multiple insecticides. Recently, a novel double-stranded RNA (dsRNA) insecticide (Calantha, active ingredient ledprona) has been demonstrated as an effective tool to manage Colorado potato beetle populations through RNA interference (RNAi). Previous studies have demonstrated the lethality of the high doses of ledprona but had not assessed possible effects of low doses that may happen due to product degradation in the environment, incomplete spray coverage, and foliage growth. Exposure of fourth instar larvae to low concentrations of ledprona interfered with their pupation. Exposure of adults significantly reduced their mobility after seven days, as well as their fertility. Reproductive effects were stronger in females, especially when exposed before reaching sexual maturity. The observed effects of low doses of ledprona may aid in the overall management of Colorado potato beetles by reducing the size of resident populations, inhibiting beetle movement within and between fields, and reducing the population growth rate.

Influence of diverse storage conditions of double-stranded RNA in vitro on the RNA interference efficiency in vivo insect Tribolium castaneum

BACKGROUND

A significant variation in RNA interference (RNAi) efficiency hinders further functional gene studies and pest control application in many insects. The available double-stranded RNA (dsRNA) molecules introduced into the target cells are regarded as the crucial factor for efficient RNAi response. However, numerous studies have only focused on dsRNA stability in vivo; it is uncertain whether different dsRNA storage conditions in vitro play a role in variable RNAi efficiency among insects.

RESULTS

A marker gene cardinal, which leads to white eyes when knocked-down in the red flour beetle Tribolium castaneum, was used to evaluate the effects of RNAi efficiency under different dsRNA storage conditions. We demonstrated that the dsRNA molecule is very stable under typical cryopreservation temperatures (-80 and -20 °C) within 180 days, and RNAi efficiency shows no significant differences under either low temperature. Unexpectedly, while dsRNA molecules were treated with multiple freeze-thaw cycles up to 50 times between -80/-20 °C and room temperature, we discovered that dsRNA integrity and RNAi efficiency were comparable with fresh dsRNA. Finally, when the stability of dsRNA was further measured under refrigerated storage conditions (4 °C), we surprisingly found that dsRNA is still stable within 180 days and can induce an efficient RNAi response as that of initial dsRNA.

CONCLUSION

Our results indicate that dsRNA is extraordinarily stable under various temperature storage conditions that did not significantly impact RNAi efficiency in vivo insects. © 2022 Society of Chemical Industry.

Current Scenario of Exogenously Induced RNAi for Lepidopteran Agricultural Pest Control: From dsRNA Design to Topical Application

Invasive insects cost the global economy around USD 70 billion per year. Moreover, increasing agricultural insect pests raise concerns about global food security constraining and infestation rising after climate changes. Current agricultural pest management largely relies on plant breeding-with or without transgenes-and chemical pesticides. Both approaches face serious technological obsolescence in the field due to plant resistance breakdown or development of insecticide resistance. The need for new modes of action (MoA) for managing crop health is growing each year, driven by market demands to reduce economic losses and by consumer demand for phytosanitary measures. The disabling of pest genes through sequence-specific expression silencing is a promising tool in the development of environmentally-friendly and safe biopesticides. The specificity conferred by long dsRNA-base solutions helps minimize effects on off-target genes in the insect pest genome and the target gene in non-target organisms (NTOs). In this review, we summarize the status of gene silencing by RNA interference (RNAi) for agricultural control. More specifically, we focus on the engineering, development and application of gene silencing to control Lepidoptera through non-transforming dsRNA technologies. Despite some delivery and stability drawbacks of topical applications, we reviewed works showing convincing proof-of-concept results that point to innovative solutions. Considerations about the regulation of the ongoing research on dsRNA-based pesticides to produce commercialized products for exogenous application are discussed. Academic and industry initiatives have revealed a worthy effort to control Lepidoptera pests with this new mode of action, which provides more sustainable and reliable technologies for field management. New data on the genomics of this taxon may contribute to a future customized target gene portfolio. As a case study, we illustrate how dsRNA and associated methodologies could be applied to control an important lepidopteran coffee pest.

Integrating RNAi Technology in Smallholder Farming: Accelerating Sustainable Development Goals

Approximately 84% of farms globally are <2 hectares; these and other smallholder farms collectively produce over one third of humanity's food. However, smallholder farms, particularly in developing countries, encounter difficulties in both production and profits due to their vulnerabilities. Sustainable intensification—increasing crop yield without significantly greater resource use—must be globally adopted in smallholder farming to achieve various Sustainable Development Goals (SDGs) endorsed by the United Nations (UN). While traditional techniques for conservation agriculture must be maintained and further promoted, new technologies will undoubtedly play a major role in achieving high yields in a sustainable and environmentally safe manner. RNA interference (RNAi) technology, particularly the use of transgenic RNAi cultivars and/or sprayable double-stranded RNA (dsRNA) pesticides, could accelerate progress in reaching these goals due to dsRNA's nucleotide sequence-specific mode of action against eukaryotic and viral pests. This sequence-specificity allows silencing of specific genetic targets in focal pest species of interest, potentially resulting in negligible effects on non-target organisms inhabiting the agroecosystem. It is our perspective that recent progress in RNAi technology, together with the UN's endorsement of SDGs that promote support in- and for developing countries, should facilitate an integrated approach to sustainable intensification of smallholder farms, whereby RNAi technology is used in combination with traditional techniques for sustainable intensification. However, the development of such approaches in developing countries will require developed countries to adhere to currently-defined socioeconomic SDGs.

Uniting RNAi Technology and Conservation Biocontrol to Promote Global Food Security and Agrobiodiversity

Habitat loss and fragmentation, and the effects of pesticides, contribute to biodiversity losses and unsustainable food production. Given the United Nation’s (UN’s) declaration of this decade as the UN Decade on Ecosystem Restoration, we advocate combining conservation biocontrol-enhancing practices with the use of RNA interference (RNAi) pesticide technology, the latter demonstrating remarkable target-specificity via double-stranded (ds)RNA’s sequence-specific mode of action. This specificity makes dsRNA a biosafe candidate for integration into the global conservation initiative. Our interdisciplinary perspective conforms to the UN’s declaration, and is facilitated by the Earth BioGenome Project, an effort valuable to RNAi development given its utility in providing whole-genome sequences, allowing identification of genetic targets in crop pests, and potentially relevant sequences in non-target organisms. Interdisciplinary studies bringing together biocontrol-enhancing techniques and RNAi are needed, and should be examined for various crop‒pest systems to address this global problem.

Highly Variable Dietary RNAi Sensitivity Among Coleoptera

Many herbivorous beetles (Order Coleoptera) contribute to serious losses in crop yields and forest trees, and plant biotechnology solutions are being developed with the hope of limiting these losses. Due to the unprecedented target-specificity of double-stranded RNA (dsRNA), and its utility in inducing RNA interference (RNAi) when consumed by target pest species, dsRNA-based plant biotechnology approaches represent the cutting edge of current pesticide research and development. We review dietary RNAi studies in coleopterans and discuss prospects and future directions regarding RNAi-based management of coleopteran plant pests. Herein, we also provide a balanced overview of existing studies in order to provide an accurate re-assessment of dietary RNAi sensitivity in coleopterans, despite the limitations to the existing body of scientific literature. We further discuss impediments to our understanding of RNAi sensitivity in this important insect order and identify critical future directions for research in this area, with an emphasis on using plant biotechnology approaches.

RNAi Crop Protection Advances

RNAi technology is a versatile, effective, safe, and eco-friendly alternative for crop protection. There is plenty of evidence of its use through host-induced gene silencing (HIGS) and emerging evidence that spray-induced gene silencing (SIGS) techniques can work as well to control viruses, bacteria, fungi, insects, and nematodes. For SIGS, its most significant challenge is achieving stability and avoiding premature degradation of RNAi in the environment or during its absorption by the target organism. One alternative is encapsulation in liposomes, virus-like particles, polyplex nanoparticles, and bioclay, which can be obtained through the recombinant production of RNAi in vectors, transgenesis, and micro/nanoencapsulation. The materials must be safe, biodegradable, and stable in multiple chemical environments, favoring the controlled release of RNAi. Most of the current research on encapsulated RNAi focuses primarily on oral delivery to control insects by silencing essential genes. The regulation of RNAi technology focuses on risk assessment using different approaches; however, this technology has positive economic, environmental, and human health implications for its use in agriculture. The emergence of alternatives combining RNAi gene silencing with the induction of resistance in crops by elicitation and metabolic control is expected, as well as multiple silencing and biotechnological optimization of its large-scale production.

Lab-to-Field Transition of RNA Spray Applications - How Far Are We?

The drastic loss of biodiversity has alarmed the public and raised sociopolitical demand for chemical pesticide-free plant production, which is now treated by governments worldwide as a top priority. Given this global challenge, RNAi-based technologies are rapidly evolving as a promising substitute to conventional chemical pesticides. Primarily, genetically modified (GM) crops expressing double-stranded (ds)RNA-mediating gene silencing of foreign transcripts have been developed. However, since the cultivation of GM RNAi crops is viewed negatively in numerous countries, GM-free exogenous RNA spray applications attract tremendous scientific and political interest. The sudden rise in demand for pesticide alternatives has boosted research on sprayable RNA biopesticides, generating significant technological developments and advancing the potential for field applications in the near future. Here we review the latest advances that could pave the way for a quick lab-to-field transition for RNA sprays, which, as safe, selective, broadly applicable, and cost-effective biopesticides, represent an innovation in sustainable crop production. Given these latest advances, we further discuss technological limitations, knowledge gaps in the research, safety concerns and regulatory requirements that need to be considered and addressed before RNA sprays can become a reliable and realistic agricultural approach.