Silencing of Mythimna separata chitinase genes via oral delivery of in planta-expressed RNAi effectors from a recombinant plant virus

Effect of short neuropeptide F signaling on larval feeding in Mythimna separata

Mythimna separata is a notorious phytophagous pest which poses serious threats to cereal crops owing to the gluttony of the larvae. Because short neuropeptide F (sNPF) and its receptor sNPFR are involved in a diversity of physiological functions, especially in functions related to feeding in insects, it is a molecular target for pest control. Herein, an sNPF and 2 sNPFRs were identified and cloned from M. separata. Bioinformatics analysis revealed that the sNPF and its receptors had a highly conserved RLRFamide C-terminus and 7 transmembrane domains, respectively. The sNPF and its receptor genes were distributed across larval periods and tissues, but 2 receptors had distinct expression patterns. The starvation-induced assay elucidated that sNPF and sNPFR expression levels were downregulated under food deprivation and recovered with subsequent re-feeding. RNA interference knockdown of sNPF, sNPFR1, and sNPFR2 by injection of double-stranded RNA into larvae not only suppressed food consumption and increased body size and weight, but also led to decrease of glycogen and total lipid contents, and increase of trehalose compared with double-stranded green fluorescent protein injection. Furthermore, molecular docking was performed on the interaction mode between sNPFR protein and its ligand sNPF based on the 3-dimensional models constructed by AlphaFold; the results indicated that both receptors were presumably activated by the mature peptide sNPF-2. These results revealed that sNPF signaling played a considerably vital role in the feeding regulation of M. separata and represents a potential control target for this pest.

Insect pest management in the age of synthetic biology

Arthropod crop pests are responsible for 20% of global annual crop losses, a figure predicted to increase in a changing climate where the ranges of numerous species are projected to expand. At the same time, many insect species are beneficial, acting as pollinators and predators of pest species. For thousands of years, humans have used increasingly sophisticated chemical formulations to control insect pests but, as the scale of agriculture expanded to meet the needs of the global population, concerns about the negative impacts of agricultural practices on biodiversity have grown. While biological solutions, such as biological control agents and pheromones, have previously had relatively minor roles in pest management, biotechnology has opened the door to numerous new approaches for controlling insect pests. In this review, we look at how advances in synthetic biology and biotechnology are providing new options for pest control. We discuss emerging technologies for engineering resistant crops and insect populations and examine advances in biomanufacturing that are enabling the production of new products for pest control.

RNA Interference-Based Forest Protection Products (FPPs) Against Wood-Boring Coleopterans: Hope or Hype?

Forest insects are emerging in large extension in response to ongoing climatic changes, penetrating geographic barriers, utilizing novel hosts, and influencing many hectares of conifer forests worldwide. Current management strategies have been unable to keep pace with forest insect population outbreaks, and therefore novel and aggressive management strategies are urgently required to manage forest insects. RNA interference (RNAi), a Noble Prize-winning discovery, is an emerging approach that can be used for forest protection. The RNAi pathway is triggered by dsRNA molecules, which, in turn, silences genes and disrupts protein function, ultimately causing the death of the targeted insect. RNAi is very effective against pest insects; however, its proficiency varies significantly among insect species, tissues, and genes. The coleopteran forest insects are susceptible to RNAi and can be the initial target, but we lack practical means of delivery, particularly in systems with long-lived, endophagous insects such as the Emerald ash borer, Asian longhorn beetles, and bark beetles. The widespread use of RNAi in forest pest management has major challenges, including its efficiency, target gene selection, dsRNA design, lack of reliable dsRNA delivery methods, non-target and off-target effects, and potential resistance development in wood-boring pest populations. This review focuses on recent innovations in RNAi delivery that can be deployed against forest pests, such as cationic liposome-assisted (lipids), nanoparticle-enabled (polymers or peptides), symbiont-mediated (fungi, bacteria, and viruses), and plant-mediated deliveries (trunk injection, root absorption). Our findings guide future risk analysis of dsRNA-based forest protection products (FPPs) and risk assessment frameworks incorporating sequence complementarity-based analysis for off-target predictions. This review also points out barriers to further developing RNAi for forest pest management and suggests future directions of research that will build the future use of RNAi against wood-boring coleopterans.

Improvement of host-induced gene silencing efficiency via polycistronic-tRNA-amiR expression for multiple target genes and characterization of RNAi mechanism in Mythimna separata

Host-induced gene silencing (HIGS) emerged as a new strategy for pest control. However, RNAi efficiency is reported to be low in Lepidoptera, which are composed of many important crop pests. To address this, we generated transgenic plants to develop HIGS effects in a maize pest, Mythimna separata (Lepidoptera, Noctuidae), by targeting chitinase encoding genes. More importantly, we developed an artificial microRNA (amiR) based PTA (polycistronic-tRNA-amiR) system for silencing multiple target genes. Compared with hpRNA (hairpin RNA), transgenic expression of a PTA cassette including an amiR for the gut-specific dsRNA nuclease gene MsREase, resulted in improved knockdown efficiency and caused more pronounced developmental abnormalities in recipient insects. When target gene siRNAs were analysed after HIGS and direct dsRNA/siRNA feeding, common features such as sense polarity and siRNA hotspot regions were observed, however, they differed in siRNA transitivity and major 20-24nt siRNA species. Core RNAi genes were identified in M. separata, and biochemical activities of MsAGO2, MsSID1 and MsDcr2 were confirmed by EMSA (electrophoretic mobility shift assay) and dsRNA cleavage assays, respectively. Taken together, we provide compelling evidence for the existence of the RNAi mechanism in M. separata by analysis of both siRNA signatures and RNAi machinery components, and the PTA system could potentially be useful for future RNAi control of lepidopteran pests.

Strategies for enhancing the efficiency of RNA interference in insects

Low RNA interference (RNAi) efficiency in many insect pests has significantly prevented its widespread application for insect pest management. This article provides a comprehensive review of recent research in developing various strategies for enhancing RNAi efficiency. Our review focuses on the strategies in target gene selection and double-stranded RNA (dsRNA) delivery technologies. For target gene selection, genome-wide or large-scale screening strategies have been used to identify most susceptible target genes for RNAi. Other strategies include the design of dsRNA constructs and manipulate the structure of dsRNA to maximize the RNA efficiency for a target gene. For dsRNA delivery strategies, much recent research has focused on the applications of complexed or encapsulated dsRNA using various reagents, polymers, or peptides to enhance dsRNA stability and cellular uptake. Other dsRNA delivery strategies include genetic engineering of microbes (e.g. fungi, bacteria, and viruses) and plants to produce insect-specific dsRNA. The ingestion of the dsRNA-producing organisms or tissues will have lethal or detrimental effects on the target insect pests. This article also identifies obstacles to further developing RNAi for insect pest management and suggests future avenues of research that will maximize the potential for using RNAi for insect pest management. © 2021 Society of Chemical Industry.

The Use of Engineered Plant Viruses in a Trans-Kingdom Silencing Strategy Against Their Insect Vectors

Plants, plant viruses, and their vectors are co-evolving actors that co-exist and interact in nature. Insects are the most important vectors of plant viruses, serving as both carriers and hosts for the virus. This trans-kingdom interaction can be harnessed for the production of recombinant plant viruses designed to target insect genes via the RNAi machinery. The selection of the adequate viruses is important since they must infect and preferentially replicate in both the host plant and the insect vector. The routes of transmission that determine the extent of the infection inside the insect vary among different plant viruses. In the context of the proposed strategy, plant viruses that are capable of transversing the insect gut-hemocoel barrier and replicating in insect tissues are attractive candidates. Thus, the transmission of such viruses in a persistent and propagative manner is considered as a prerequisite for this strategy to be feasible, a characteristic that is found in viruses from the families Bunyaviridae, Reoviridae, and Rhabdoviridae. In addition, several RNA viruses are known that replicate in both plant and insect tissues via a yet unclarified transmission route. In this review, advances in knowledge of trans-kingdom transmission of plant viruses and future perspectives for their engineering as silencing vectors are thoroughly discussed.

Progress and prospects of arthropod chitin pathways and structures as targets for pest management

Chitin is a structural component of the arthropod cuticular exoskeleton and the peritrophic matrix of the gut, which play crucial roles in growth and development. In the past few decades, our understanding of the composition, biosynthesis, assembly, degradation, and regulation of chitinous structures has increased. Many chemicals have been developed that target chitin biosynthesis (benzoyphenyl ureas, etoxazole), chitin degradation (allosamidin, psammaplin), and chitin regulation (benzoyl hydrazines), thus resulting in molting deformities and lethality. In addition, proteins that disrupt chitin structures, such as lectins, proteases, and chitinases have been utilized to halt feeding and induce mortality. Chitin-degrading enzymes, such as chitinases are also useful for improving the efficacy of bio-insecticides. Transgenic plants, baculoviruses, fungi, and bacteria have been engineered to express chitinases from a variety of organisms for control of arthropod pests. In addition, RNA interference targeting genes involved in chitin pathways and structures are now being investigated for the development of environmentally friendly pest management strategies. This review describes the chemicals and proteins used to target chitin structures and enzymes for arthropod pest management, as well as pest management strategies based upon these compounds, such as plant-incorporated-protectants and recombinant entomopathogens. Recent advances in RNA interference-based pest management, and how this technology can be used to target chitin pathways and structures are also discussed.

Chitinous Structures as Potential Targets for Insect Pest Control

Chitinous structures are physiologically fundamental in insects. They form the insect exoskeleton, play important roles in physiological systems and provide physical, chemical and biological protections in insects. As critically important structures in insects, chitinous structures are attractive target sites for the development of new insect-pest-control strategies. Chitinous structures in insects are complex and their formation and maintenance are dynamically regulated with the growth and development of insects. In the past few decades, studies on insect chitinous structures have shed lights on the physiological functions, compositions, structural formation, and regulation of the chitinous structures. Current understanding of the chitinous structures has indicated opportunities for exploring new target sites for insect control. Mechanisms to disrupt chitinous structures in insects have been studied and strategies for the potential development of new means of insect control by targeting chitinous structures have been proposed and are practically to be explored.

Screening potential reference genes for quantitative real-time PCR analysis in the oriental armyworm, Mythimna separata

The oriental armyworm, Mythimna separata, is a major insect pest in China and other Asian countries. Unfortunately, suitable reference genes for quantitative real-time PCR (qRT-PCR) have not been previously identified in M. separata for normalizing target gene expression. In this study, we evaluated the expression stability of eight candidate genes (18S, ACT, EF1-α, GAPDH, RPS7, RPS13, RPL32 and TUB) in M. separata using the comparative ΔCt method, BestKeeper, Normfinder geNorm and ReFinder, a comprehensive software platform. The results indicated that the appropriate reference gene varied depending on the experimental conditions. We found that ACTIN, EF1-α and TUB were optimal for different developmental stages; TUB, RPS13 and EF1-α showed the most stable expresssion in different tissues; RPS13 and 18S were the best reference genes for monitoring expression under high temperature conditions; TUB, RPS13 and RPS7 exhibited the most stable expression under larval-crowding conditions; RPS7, EF1-α, RPL32 and GAPDH were the best for pesticide exposure experiments. This study provides tools for reliable normalization of qRT-PCR data and forms a foundation for functional studies of target gene expression in M. separata.

Viral Delivery of dsRNA for Control of Insect Agricultural Pests and Vectors of Human Disease: Prospects and Challenges

RNAi is applied as a new and safe method for pest control in agriculture but efficiency and specificity of delivery of dsRNA trigger remains a critical issue. Various agents have been proposed to augment dsRNA delivery, such as engineered micro-organisms and synthetic nanoparticles, but the use of viruses has received relatively little attention. Here we present a critical view of the potential of the use of recombinant viruses for efficient and specific delivery of dsRNA. First of all, it requires the availability of plasmid-based reverse genetics systems for virus production, of which an overview is presented. For RNA viruses, their application seems to be straightforward since dsRNA is produced as an intermediate molecule during viral replication, but DNA viruses also have potential through the production of RNA hairpins after transcription. However, application of recombinant virus for dsRNA delivery may not be straightforward in many cases, since viruses can encode RNAi suppressors, and virus-induced silencing effects can be determined by the properties of the encoded RNAi suppressor. An alternative is virus-like particles that retain the efficiency and specificity determinants of natural virions but have encapsidated non-replicating RNA. Finally, the use of viruses raises important safety issues which need to be addressed before application can proceed.