Transgenic plants over-expressing insect-specific microRNA acquire insecticidal activity against Helicoverpa armigera: an alternative to Bt-toxin technology

miRNA-mediated insect-resistant transgenic rice poses no risk to a non-target parasitoid, Cotesia chilonis, via direct feeding or through its target host

MicroRNAs (miRNAs) have started to play an important role in pest control, and novel miRNA-based transgenic insect-resistant plants are now emerging. However, an environmental risk assessment of these novel transgenic plants expressing insect miRNAs must be undertaken before promoting their application. Here, transgenic miR-14 rice, which has high resistance to the rice stem borer Chilo suppressalis, was used as an example for evaluation in this study. Taking the tier 1 risk assessment method in Bacillus thuringiensis (Bt) crops as a reference, the effects of the direct exposure of a non-target parasitoid, Cotesia chilonis, to a high concentration of miRNA were evaluated. The results showed that direct feeding with miR-14 at high concentration had no significant effects on the biological parameters of Co. chilonis, whereas when miR-14 was injected into Ch. suppressalis-parasitized larvae, the development duration of Co. chilonis was significantly affected. In combination with the real conditions of the rice paddy field, it could be inferred that transgenic miR-14 rice has no significant negative effects on the important non-target parasitoid, Co. chilonis. These results will provide a foundation for the establishment of a new safety evaluation system for novel RNAi-based transgenic plants.

Insect metamorphosis and chitin metabolism under miRNA regulation: a review with current advances

Insect metamorphosis is a complex developmental process regulated by microRNAs (miRNAs) and hormonal signaling pathways. Key genes driving insect ontogenic changes are precisely modulated by miRNAs, which interact with 20-hydroxyecdysone (20E) and juvenile hormone (JH) to coordinate developmental transitions. Over the past decade, significant progress has been made in understanding miRNA biogenesis, their regulatory roles in gene expression, and their involvement in critical biological processes, including metamorphosis and chitin metabolism. miRNAs are now recognized as essential regulators of chitin metabolism and hormonal signaling, ensuring precise control of insect development. Disrupting the expression of participating genes in hormone signaling pathways through miRNAs leads to aberrant metamorphosis and consequent lethal outcomes, highlighting their potential as targets for pest control. This review summarizes current advances in miRNA-mediated regulation of insect metamorphosis and chitin metabolism, with a focus on their interactions with 20E and JH signaling pathways. By integrating recent findings, we provide insights into the molecular mechanisms underlying miRNA function in developmental transitions and their potential applications in insect pest management strategies. © 2025 Society of Chemical Industry.

The Emerging Applications of Artificial MicroRNA-Mediated Gene Silencing in Plant Biotechnology

Improving crop yield potential is crucial to meet the increasing demands of a rapidly expanding global population in an ever-changing and challenging environment. Therefore, different technological approaches have been proposed over the last decades to accelerate plant breeding. Among them, artificial microRNAs (amiRNAs) represent an innovative tool with remarkable potential to assist plant improvement. MicroRNAs (miRNAs) are a group of endogenous, small (20-24 nucleotides), non-coding RNA molecules that play a crucial role in gene regulation. They are associated with most biological processes of a plant, including reproduction, development, cell differentiation, biotic and abiotic stress responses, metabolism, and plant architecture. In this context, amiRNAs are synthetic molecules engineered to mimic the structure and function of endogenous miRNAs, allowing for the targeted silencing of specific nucleic acids. The current review explores the diverse applications of amiRNAs in plant biology and agriculture, such as the management of infectious agents and pests, the engineering of plant metabolism, and the enhancement of plant resilience to abiotic stress. Moreover, we address future perspectives on plant amiRNA-based gene silencing strategies, highlighting the need for further research to fully comprehend the potential of this technology and to translate its scope toward the widespread adoption of amiRNA-based strategies for plant breeding.

Trophic microRNA: Post-transcriptional regulation of target genes and larval development impairment in Plutella xylostella upon precursor and mature microRNA ingestion

MicroRNAs (miRNAs) are post-transcriptional gene regulators. In the miRNA pathway's cytoplasmic part, the miRNA is processed from a hairpin-structured precursor to a double-stranded (ds) mature RNA and ultimately to a single-stranded mature miRNA. In insects, ingesting these two ds forms can regulate the target gene expression; this inspired the trophic miRNA's use as a functional genomics and pest management tool. However, systematic studies enabling comparisons of pre- and mature forms, dosages, administration times and instar-wise effects on target transcripts and phenotypes, which can help develop a miRNA administration method, are unavailable due to the different focuses of the previous investigations. We investigated the impact of trophically delivered Px-let-7 miRNA on the lepidopteran pest Plutella xylostella, to compare the efficacies of its pre- and ds-mature forms. Continuous feeding on the miRNA-supplemented diet suppressed expressions of FTZ-F1 and E74, the target ecdysone pathway genes. Both the pre-let-7 and mature let-7 miRNA forms similarly downregulated the target transcripts in all four larval instars. Pre-let-7 and let-7 ingestions decreased larval mass and instar duration and increased mortality in all instars, exhibiting adverse effects on larval growth and development. miRNA processing Dicer-1 and AGO-1's upregulations upon miRNA ingestion denoted the systemic miRNA spread in larval tissues. The scrambled sequence controls did not affect the target transcripts, suggesting the sequence-specific targeting by the mature miRNA and hairpin cassette's non-involvement in the target downregulation. This work provides a framework for miRNA and target gene function analyses and potentiates the trophic miRNA's utility in pest management.

Promising Biotechnological Applications of the Artificial Derivatives Designed and Constructed from Plant microRNA Genes

MicroRNAs (miRNAs) are small regulatory RNAs that are expressed in a tissue-specific manner during the development of plants and animals. The genes of miRNAs have been found to produce the following two products: (i) primary transcripts of these genes (pri-miRNA) are processed to give rise to mature miRNA, and (ii) in some cases, the pri-miRNA molecules can be translated to form small peptides, named as miPEPs. Gene silencing by artificial microRNAs (amiRNAs) is one of the potential crucial methods for the regulation of desired genes to improve horticultural plants. Likewise, external application of chemically synthesized miPEPs may help plants to resist biotic/abiotic stresses and grow faster. These potent and reliable derivatives of miRNA genes can be applied for improving useful traits in crop plants. This review summarizes the progress in research on the artificial gene derivatives involved in regulating plant development, virus and pest diseases, and abiotic stress resistance pathways. We also briefly discuss the molecular mechanisms of relevant target genes for future research on breeding in plants. In general, this review may be useful to researchers who are implementing amiRNA and miPEP for accelerating breeding programs and developmental studies in crop plants.

Silencing ACE1 Gene with dsRNA of Different Lengths Impairs Larval Development in Leptinotarsa decemlineata

In the search for effective strategies to control the Colorado Potato Beetle, RNA interference technology has emerged as a promising method due to its capacity to suppress genes selectively. Factors such as the target gene and double-stranded RNA (dsRNA) length are critical for optimizing gene silencing efficiency. In this study, we designed and synthesized in vitro dsRNAs of varying lengths targeting the ACE1 gene, which encodes the AChE1 isoform of acetylcholinesterase in the beetle. All tested dsRNA lengths (222 bp, 543 bp, 670 bp, and 870 bp) promoted transcript reduction. The 670 bp dsRNA was the most effective, reducing transcript levels by approximately 40% by day seven, followed by the 543 bp dsRNA. No significant differences were observed between the 222 bp and 870 bp dsRNAs. Furthermore, all of the dsRNA lengths resulted in reduced weight gain and increased mortality in larvae, with the 670 bp dsRNA showing the highest mortality rate, leaving only 63% larval survival, a trend that persisted through day nine. These findings emphasize that dsRNA length is a key factor in the silencing response, underscoring the importance of selecting the optimal length while considering the gene's target, stability, and delivery methods. This study contributes to establishing design criteria for dsRNA, aiding in the development of more effective and sustainable pest management strategies.

Molecular Characterization of miRNAs in Myzus persicae Carrying Brassica Yellows Virus

microRNAs (miRNAs) influence many biological processes at the post-transcriptional level. However, the molecular characterization of miRNAs in the Myzus persicae response to Brassica yellows virus (BrYV) stress remains unclear. In this study, we present the results of miRNA profiling in Myzus persicae under two different treatments: treatment one (raised on turnip plants), and treatment two (raised on Arabidopsis thaliana). A total of 72 known and 113 novel mature miRNAs were identified in both non-viruliferous and viruliferous aphids, under treatment one. In treatment two, 72 known and 112 novel mature miRNAs were identified in BrYV-free aphids; meanwhile, 71 known and 115 novel miRNAs were identified in BrYV-carrying aphids. Moreover, eight upregulated and four downregulated miRNAs were identified in viruliferous aphids under treatment two, whereas only two miRNAs were differentially expressed under treatment one. These results indicated the relative BrYV level could influence miRNA expression in aphids. KEGG enrichment analysis showed the predicted genes targeted by differentially expressed miRNAs were primarily involved in Peroxisome, neuroactive ligand-receptor interaction, and metabolism of xenobiotics by cytochrome P450 pathways. Taken together, these findings reveal the effect of BrYV on miRNAs in Myzus persicae and provide key clues for further studies on the molecular mechanisms of BrYV transmission via aphids.

Research Progress on miRNAs and Artificial miRNAs in Insect and Disease Resistance and Breeding in Plants

MicroRNAs (miRNAs) are small, non-coding RNAs that are expressed in a tissue- and temporal-specific manner during development. They have been found to be highly conserved during the evolution of different species. miRNAs regulate the expression of several genes in various organisms, with some regulating the expression of multiple genes with similar or completely unrelated functions. Frequent disease and insect pest infestations severely limit agricultural development. Thus, cultivating resistant crops via miRNA-directed gene regulation in plants, insects, and pathogens is an important aspect of modern breeding practices. To strengthen the application of miRNAs in sustainable agriculture, plant endogenous or exogenous miRNAs have been used for plant breeding. Consequently, the development of biological pesticides based on miRNAs has become an important avenue for future pest control methods. However, selecting the appropriate miRNA according to the desired target traits in the target organism is key to successfully using this technology for pest control. This review summarizes the progress in research on miRNAs in plants and other species involved in regulating plant disease and pest resistance pathways. We also discuss the molecular mechanisms of relevant target genes to provide new ideas for future research on pest and disease resistance and breeding in plants.

Plant-derived artificial miRNA effectively reduced the proliferation of aphid (Aphidoidea) through spray-induced gene silencing

BACKGROUND

Aphids (Hemiptera: Aphididae) are notorious sap-sucking insects that rampantly threaten agricultural production worldwide. Current management against aphids in the field heavily relies on chemical pesticides, which makes economical and eco-friendly methods urgently needed. Spray-induced gene silencing (SIGS) offers a powerful and precise approach to pest management. However, the high costs and instability of double-stranded RNA (dsRNA) regulators applied for downstream RNA interference (RNAi) still limit this strategy. It remains uncertain if RNAi regulators applied in SIGS could extend to small RNA (sRNA), especially miRNA.

RESULTS

We chose two sRNA sequences, miR-9b and miR-VgR, whose corresponding targets ABCG4 and VgR are both essential for aphid growth and development. The efficacy of these sequences was initially verified by chemically synthetic single-stranded RNA (syn-ssRNA). Through spray treatment, we observed a significantly decreased survival number and increased abnormality rate of green peach aphids fed on the host under laboratory conditions. Based on our previous study, we generated transgenic plants expressing artificial miR-9b (amiR-9b) and miR-VgR (amiR-VgR). Remarkably, plant-derived amiRNA exerted potent and long-lasting inhibitory efficacy with merely one percent concentration of chemical synthetics. Notably, the simultaneous application of amiR-9b and amiR-VgR exhibited superior inhibitory efficacy.

CONCLUSION

We explored the potential use of sRNA-based biopesticide through SIGS while investigating the dosage requirements. To optimize this strategy, the utilization of plant-derived amiRNA was proposed. The results suggested that attributed to stability and durability, deploying amiRNA in pest management is a potential and promising solution for the field application. © 2024 Society of Chemical Industry.

microRNAs: Key Regulators in Plant Responses to Abiotic and Biotic Stresses via Endogenous and Cross-Kingdom Mechanisms

Dramatic shifts in global climate have intensified abiotic and biotic stress faced by plants. Plant microRNAs (miRNAs)-20-24 nucleotide non-coding RNA molecules-form a key regulatory system of plant gene expression; playing crucial roles in plant growth; development; and defense against abiotic and biotic stress. Moreover, they participate in cross-kingdom communication. This communication encompasses interactions with other plants, microorganisms, and insect species, collectively exerting a profound influence on the agronomic traits of crops. This article comprehensively reviews the biosynthesis of plant miRNAs and explores their impact on plant growth, development, and stress resistance through endogenous, non-transboundary mechanisms. Furthermore, this review delves into the cross-kingdom regulatory effects of plant miRNAs on plants, microorganisms, and pests. It proceeds to specifically discuss the design and modification strategies for artificial miRNAs (amiRNAs), as well as the protection and transport of miRNAs by exosome-like nanovesicles (ELNVs), expanding the potential applications of plant miRNAs in crop breeding. Finally, the current limitations associated with harnessing plant miRNAs are addressed, and the utilization of synthetic biology is proposed to facilitate the heterologous expression and large-scale production of miRNAs. This novel approach suggests a plant-based solution to address future biosafety concerns in agriculture.

Epigenetic studies in insects and the valproic acid perspective

Abstract Valproic acid in association with sodium valproate (VPA) is an important anticonvulsant drug used for decades to treat neurological disorders. VPA also acts as an epigenetic modulator by inhibiting histone deacetylases, permitting histone acetylation, affecting the DNA and histone methylation status and gene expression, and inducing chromatin remodeling. Insects represent an important animal model for studies in several areas of science. Their high phenotypic plasticity makes them alternative models for epigenetic studies. This brief review emphasizes recent reports on insect epigenetics and the contribution of studies on the VPA action in insects, including effects on epigenetic markers, extending the pharmacological understanding of the potential of this drug, and demonstrating the usefulness of insects as an alternative animal model to drug studies.

Neutral Dietary Effects of Two MicroRNAs, Csu-Novel-260 and Csu-Mir-14, on the Non-Target Arthropod Folsomia candida

RNA interference (RNAi) that is triggered by small or short RNAs has shown enormous potential in the development of pest control strategies. Two microRNAs (miRNAs), Csu-novel-260 and Csu-miR-14, were used in insect-resistant genetically engineered (IRGE) rice lines to confer resistance to Chilo suppressalis. However, a risk assessment of RNAi-based products is essential to determine the safety of a biopesticide or IRGE crop for commercialization. The non-target organism Folsomia candida, which plays an important ecological role as a soil decomposer in agricultural ecosystems, was used to assess the risk of miRNAs Csu-novel-260 and Csu-miR-14. In this study, a dietary miRNA toxicity assay system was established in F. candida. The expression levels of target genes, survival rate, fecundity and body size were investigated to evaluate the effects of the miRNAs on F. candida under the worst-case scenario. The results showed that the dietary miRNA toxicity assay system could be used for risk assessment of miRNA in F. candida. The target genes of miRNAs were influenced by miRNA at some time points. However, no significant differences were observed in the life-table parameters in F. candida fed with a diet containing miRNAs. The dietary effects of two miRNAs on F. candida are neutral.

A novel tasi RNA-based micro RNA-induced gene silencing strategy to tackle multiple pests and pathogens in cotton (Gossypium hirsutum L.)

This study demonstrates the combinatorial management of multiple pests through a trans-acting siRNA (tasiRNA)-based micro RNA-induced gene silencing (MIGS) strategy. Transgenic cotton events demonstrated improved efficacy against cotton leaf curl disease, cotton leaf hopper and root-knot nematode. Cotton (Gossypium hirsutum L.), an important commercial crop grown worldwide is confronted by several pests and pathogens, thus reiterating interventions for their management. In this study, we report, the utility of a novel Arabidopsis miRNA173-directed trans-acting siRNA (tasiRNA)-based micro RNA-induced gene silencing (MIGS) strategy for the simultaneous management of cotton leaf curl disease (CLCuD), cotton leaf hopper (CLH; Amrasca biguttula biguttula) and root-knot nematode (RKN, Meloidogyne incognita). Cotton transgenics were developed with the MIGS construct targeting a total of 7 genes by an apical meristem-targeted in planta transformation strategy. Stable transgenics were selected using stringent selection pressure, molecular characterization and stress-specific bio-efficacy studies. We identified 8 superior events with 50-100% resistance against CLCuD, while reduction in the root-knot nematode multiplication factor in the range of 35-75% confirmed resistance to RKN. These transgenic cotton events were also detrimental to the growth and development of CLH, as only 43.3-62.5% of nymphs could survive. Based on the corroborating evidences obtained by all the bioefficacy analyses, 3 events viz., L-75-1, E-27-11, E-27-7 were found to be consistent in tackling the target pests. To the best of our knowledge, this report is the first of its kind demonstrating the possibility of combinatorial management of pests/diseases in cotton using MIGS approach. These identified events demonstrate immense utility of the strategy towards combinatorial stress management in cotton improvement programs.

Resistance to RNA interference by plant-derived double-stranded RNAs but not plant-derived short interfering RNAs in Helicoverpa armigera

Plant-mediated RNA interference (RNAi) has emerged as a promising technology for pest control through expression of double-stranded RNAs (dsRNAs) targeted against essential insect genes. However, little is known about the underlying molecular mechanisms and whether long dsRNA or short interfering RNAs (siRNAs) are the effective triggers of the RNAi response. Here we generated transplastomic and nuclear transgenic tobacco plants expressing dsRNA against the Helicoverpa armigera ATPaseH gene. We showed that expression of long dsRNA of HaATPaseH was at least three orders of magnitude higher in transplastomic plants than in transgenic plants. HaATPaseH-derived siRNAs are absent from transplastomic plants, while they are abundant in transgenic plants. Feeding transgenic plants to H. armigera larvae reduced gene expression of HaATPaseH and delayed growth. Surprisingly, no effect of transplastomic plants on insect growth was observed, despite efficient dsRNA expression in plastids. Furthermore, we found that dsRNA ingested by H. armigera feeding on transplastomic plants was rapidly degraded in the intestinal fluid. In contrast, siRNAs are relatively stable in the digestive system. These results suggest that plant-derived siRNAs may be more effective triggers of RNAi in Lepidoptera than dsRNAs, which will aid the optimization of the strategies for plant-mediated RNAi to pest control.

Recent advances and potential applications of cross-kingdom movement of miRNAs in modulating plant's disease response

In the recent past, cross-kingdom movement of miRNAs, small (20–25 bases), and endogenous regulatory RNA molecules has emerged as one of the major research areas to understand the potential implications in modulating the plant’s biotic stress response. The current review discussed the recent developments in the mechanism of cross-kingdom movement (long and short distance) and critical cross-talk between host’s miRNAs in regulating gene function in bacteria, fungi, viruses, insects, and nematodes, and vice-versa during host-pathogen interaction and their potential implications in crop protection. Moreover, cross-kingdom movement during symbiotic interaction, the emerging role of plant’s miRNAs in modulating animal’s gene function, and feasibility of spray-induced gene silencing (SIGS) in combating biotic stresses in plants are also critically evaluated. The current review article analysed the horizontal transfer of miRNAs among plants, animals, and microbes that regulates gene expression in the host or pathogenic organisms, contributing to crop protection. Further, it highlighted the challenges and opportunities to harness the full potential of this emerging approach to mitigate biotic stress efficiently.

Trans-kingdom expression of an insect endogenous microRNA in rice enhances resistance to striped stem borer Chilo suppressalis

BACKGROUND

The striped stem borer (SSB), Chilo suppressalis Walker, is a major pest of rice worldwide. Breeding of transgenic rice expressing Bacillus thuringiensis (Bt) toxins is a powerful strategy to control SSB. However, pests may evolve certain resistance to Bt toxins in transgenic plants. Hence, new controlling strategies must be continuously developed.

RESULTS

We successfully generated SSB-resistant rice (csu-53) expressing the artificial microRNA (amiRNA) of SSB endogenous miRNA (csu-novel-miR53) through the RNAi-based technology. Feeding assays demonstrated that csu-53 rice inhibited larval growth, delayed pupation time, and reduced pupal weight and eclosion rate of SSB larva. In a 10-day feeding experiment, the miRNA mimic of csu-novel-miR53 also suppressed larval growth and more importantly increased larval mortality. Transcriptome analysis identified 28 differentially expressed unigenes (DEGs) in the midgut between SSB larvae fed on csu-53 rice and the wild type. One DEG (DN90065_c0_g12) validated by qRT-PCR had a predicted target site of csu-novel-miR53. In addition, in vitro double-stranded RNA synthesis and further feeding assay proved that DN90065_c0_g12 is most likely the target of csu-novel-miR53.

CONCLUSION

amiRNA-mediated strategy can be applied to the development of insect-resistant crops, and the novel amiRNA csu-novel-miR53 of SSB has important application potential in developing SSB resistant rice. © 2021 Society of Chemical Industry.

Biogenesis and Mechanisms of miRNA-Mediated Gene Regulation

Endogenous RNAs that control post-transcriptional gene expression are microRNAs (miRNAs). These small regulatory molecules play a crucial role in certain biological processes and their expression is often strictly regulated. They are small 21-24 nucleotide molecules that act as major regulators of gene expression at the post-transcriptional level. One of the mechanisms by which miRNAs control the gene expression is to interact the interaction of the seeds with the 3'-end and more seldom the 5'-end of mRNA transcribed by the target genes. miRNAs have been identified as important cytoplasmic regulators of gene expression. miRNAs function as post-transcriptional regulators of their messenger RNA (mRNA) targets by mRNA degradation and/or translational repression. It is becoming evident, however, that miRNAs have nuclear functions as well. About the cell type, the physiological state of the body, and various external factors, the following explanation will summarize the complex multilevel regulation of miRNA expression. A better understanding of the molecular mechanisms controlling miRNA expression will provide clarification of the variations in the expression of protein-coding genes. This article is protected by copyright. All rights reserved.

Guiding RNAi Design Through Characterization of Endogenous Small RNA Pathways

RNA interference (RNAi) is a common eukaryotic gene regulation process driven by small RNA effectors. Mechanisms that govern regulatory small noncoding RNA behavior have been extensively described in only a handful of organisms, which suggests that the most effective RNAi approach in many organisms, such as insect pests, remains to be determined. Taking advantage of advances in high-throughput sequencing, characterization of small RNA molecules can be achieved through bioinformatic approaches without the need for genetic experiments. This chapter describes pipelines for characterizing three main classes of small RNAs (microRNAs, small-interfering RNAs, and piwi-associated RNAs) using computationally determined small RNA biogenesis signatures. Obtaining information regarding the abundance of different small RNA classes through these pipelines will lead to a better-informed RNAi strategy, thereby identifying the most efficacious approach for RNAi.

Establishment and Application of Ligation Reaction-Based Method for Quantifying MicroR-156b

Microribonucleic acids (miRNAs) play significant roles in the regulation of biological processes and in responses to biotic or abiotic environmental stresses. Therefore, it is necessary to quantitatively detect miRNAs to understand these complicated biological regulation mechanisms. This study established an ultrasensitive and highly specific method for the quantitative detection of miRNAs using simple operations on the ground of the ligation reaction of ribonucleotide-modified deoxyribonucleic acid (DNA) probes. This method avoids the complex design of conventional reverse transcription. In the developed assay, the target miRNA miR156b was able to directly hybridize the two ribonucleotide-modified DNA probes, and amplification with universal primers was achieved following the ligation reaction. As a result, the target miRNA could be sensitively measured even at a detection limit as low as 0.0001 amol, and differences of only a single base could be detected between miR156 family members. Moreover, the proposed quantitative method demonstrated satisfactory results for overexpression-based genetically modified (GM) soybean. Ligation-based quantitative polymerase chain reaction (PCR) therefore has potential in investigating the biological functions of miRNAs, as well as in supervising activities regarding GM products or organisms.

Overexpression of an Osa-miR162a Derivative in Rice Confers Cross-Kingdom RNA Interference-Mediated Brown Planthopper Resistance without Perturbing Host Development

Rice is a main food crop for more than half of the global population. The brown planthopper (BPH, Nilaparvata lugens) is one of the most destructive insect pests of rice. Currently, repeated overuse of chemical insecticides represents a common practice in agriculture for BPH control, which can induce insect tolerance and provoke environmental concerns. This situation calls for innovative and widely applicable strategies for rice protection against BPH. Here we report that the rice osa-miR162a can mediate cross-kingdom RNA interference (RNAi) by targeting the NlTOR (Target of rapamycin) gene of BPH that regulates the reproduction process. Through artificial diet or injection, osa-miR162a mimics repressed the NlTOR expression and impaired the oviposition of BPH adults. Consistently, overproduced osa-miR162a in transgenic rice plants compromised the fecundity of BPH adults fed with these plants, but meanwhile perturbed root and grain development. To circumvent this issue, we generated osa-miR162a-m1, a sequence-optimized osa-miR162a, by decreasing base complementarity to rice endogenous target genes while increasing base complementarity to NlTOR. Transgenic overexpression of osa-miR162a-m1 conferred rice resistance to BPH without detectable developmental penalty. This work reveals the first cross-kingdom RNAi mechanism in rice-BPH interactions and inspires a potentially useful approach for improving rice resistance to BPH. We also introduce an effective strategy to uncouple unwanted host developmental perturbation from desirable cross-kingdom RNAi benefits for overexpressed plant miRNAs.

Spatio-Temporal Profiling of Metarhizium anisopliae-Responsive microRNAs Involved in Modulation of Plutella xylostella Immunity and Development

Metarhizium anisopliae, a ubiquitous pathogenic fungus, regulates a wide array of the insect pest population. The fungus has been employed to control Plutella xylostella, an insecticide-resistant destructive lepidopteran pest, which causes substantial economic losses in crops worldwide. Integration of modern gene-silencing technologies in pest control strategies has become more crucial to counter pesticide-resistant insects. MicroRNAs (miRNA) play essential roles in the various biological process via post-transcriptional gene regulation. In the present study, RNA-seq analysis of control (CK36h, CK72h) and fungal-infected (T36h, T72h) midguts was performed to reveal underlying molecular mechanisms occurring in larval midgut at different time courses. We aimed at exploring M. anisopliae-responsive miRNAs and their target genes involved in development and immunity. After data filtration, a combined set of 170 miRNAs were identified from all libraries. Interestingly, miR-281, miR-263, miR-1, miR-6094 and miR-8 were listed among the most abundantly expressed conserved miRNAs. Furthermore, we experimentally studied the role of differentially expressed miR-11912-5p in regulating corresponding target trypsin-like serine proteinase (Px_TLSP). The luciferase assay (in vitro) revealed that miRNA-11912-5p significantly downregulated its target gene, suggesting it might play a crucial role in defense mechanism of P. xylostella against M.+ anisopliae infection. We used synthetic miRNA mimic/inhibitor (in vivo), to overexpress/silence miRNA, which showed harmful effects on larval duration, survival and adult fecundity. Additionally, fungal application in the presence of mimics revealed enhanced sensitivity of P. xylostella to infection. Our finding provides an insight into the relatively obscure molecular mechanisms involved in insect midgut during the fungal infection.

RNA interference and crop protection against biotic stresses

RNA interference (RNAi) is a universal phenomenon of RNA silencing or gene silencing with broader implications in important physiological and developmental processes of most eukaryotes, including plants. Small RNA (sRNA) are the critical drivers of the RNAi machinery that ensures down-regulation of the target genes in a homology-dependent manner and includes small-interfering RNAs (siRNAs) and micro RNAs (miRNAs). Plant researchers across the globe have exploited the powerful technique of RNAi to execute targeted suppression of desired genes in important crop plants, with an intent to improve crop protection against pathogens and pests for sustainable crop production. Biotic stresses cause severe losses to the agricultural productivity leading to food insecurity for future generations. RNAi has majorly contributed towards the development of designer crops that are resilient towards the various biotic stresses such as viruses, bacteria, fungi, insect pests, and nematodes. This review summarizes the recent progress made in the RNAi-mediated strategies against these biotic stresses, along with new insights on the future directions in research involving RNAi for crop protection.

The overexpression of insect endogenous microRNA in transgenic rice inhibits the pupation of Chilo suppressalis and Cnaphalocrocis medinalis

BACKGROUND

Chilo suppressalis and Cnaphalocrocis medinalis are destructive rice pests co-occurring in major rice-growing areas in China. RNA interference (RNAi)-based insect-resistant genetically engineered (IRGE) crops provide a promising approach for pest management by suppressing gene expression or translation. A microRNA (miRNA)-mediated IRGE rice line expressing endogenous Chilo suppressalis miRNA Csu-novel-260, showing significant resistance against Chilo suppressalis, provides an attractive control strategy for Chilo suppressalis by suppressing the expression of the disembodied (dib) gene expression. However, whether this transgenic line also shows the resistance against Cnaphalocrocis medinalis remains unknown.

RESULTS

A spatiotemporal expression analysis of Csu-novel-260 in the transgenic rice line was performed by quantitative reverse transcription polymerase chain reaction (qRT-PCR) to determine the paddy field pest exposure dose. In diet feeding assays, a chemically synthesized Csu-novel-260 agomir at 200 fmol g^-1 significantly inhibited Chilo suppressalis pupation. However, larval development, survival and pupal weight were not significantly affected. Additionally, the transgenic line significantly affected Cnaphalocrocis medinalis pupation but not larval survival. The qRT-PCR showed that Csdib and Cmdib expression levels were significantly suppressed when the two pests fed on the transgenic line. Additionally, the transgenic line significantly decreased Cry1C-resistant and Cry1C-susceptible Chilo suppressalis larval survival in detached rice tissue feeding assays, indicating that Cry1C-resistant Chilo suppressalis was not cross-resistant to Csu-novel-260 expressed in miRNA-mediated IRGE rice.

CONCLUSION

Our study demonstrated that miRNA-mediated IRGE rice significantly inhibited Chilo suppressalis and Cnaphalocrocis medinalis pupation. The results provide a new viewpoint for the application of RNAi-based plants and the inspiration for environmental risk assessment.

Identification and Characterization of MicroRNAs in Gonads of Helicoverpa armigera (Lepidoptera: Noctuidae)

Simple Summary

For most insects, the development of the testis and ovary directly determines their reproductive ability. The cotton bollworm, Helicoverpa armigera (Hübner), is a polyphagous crop pest of the Lepidoptera Noctuidae. Owing to its broad range of host plants and strong fertility, H. armigera causes huge economic losses to agricultural production. Acting as a type of post-transcriptional regulatory factor, miRNAs participate in the gonadal development and reproductive regulation of H. arimgera. Our study uses H. armigera as a research object to identify and characterize the miRNAs and study their potential functions in the testis and ovary of this destructive crop pest. A total of 7,592,150 and 8,815,237 clean reads were obtained by constructing small RNA libraries of the testis and ovary, respectively. Length distribution analysis showed that the main types of small RNAs in the testis and ovary were different. Among the 74 known miRNAs, 60 miRNAs existed in the ovary, and 72 existed in the testis. Gene Ontology (GO) and KEGG pathway analyses indicated that the 8 gonad-biased differentially expressed miRNAs (miR-989a, miR-263-5p, miR-34, miR-2763, miR-998, miR-2c, miR-2765, and miR-252a-5p) had many target transcripts involved in the reproduction process.

Abstract

The high fecundity of the most destructive pest Helicoverpa armigera and its great resistance risk to insecticides and Bt crops make the reproductive-destruction-based control of this pest extremely appealing. To find suitable targets for disruption of its reproduction, we observed the testis and ovary development of H. armigera and conducted deep sequencing of the ovary and testis small RNAs of H. armigera and quantitative RT-PCR (RT-qPCR) validation to identify reproduction-related micro RNAs (miRNAs). A total of 7,592,150 and 8,815,237 clean reads were obtained from the testis and ovary tissue, respectively. After further analysis, we obtained 173 novel and 74 known miRNAs from the two libraries. Among the 74 known miRNAs, 60 miRNAs existed in the ovary and 72 existed in the testis. Further RT-qPCR validation of 5 miRNAs from the ovary and 6 miRNAs from the testis confirmed 8 of them were indeed ovary- (miR-989a, miR-263-5p, miR-34) or testis-biased (miR-2763, miR-998, miR-2c, miR-2765, miR-252a-5p). The 8 ovary- or testis-biased miRNAs had a total of 30,172 putative non-redundant target transcripts, as predicted by miRanda and RNAhybrid. Many of these target transcripts are assigned to reproduction-related GO terms (e.g., oocyte maturation, vitellogenesis, spermatogenesis) and are members of multiple reproduction-related KEGG pathways, such as the JAK-STAT signaling pathway, oocyte meiosis, the insulin signaling pathway, and insect hormone biosynthesis. These results suggest that the 8 gonad-biased miRNAs play important roles in reproduction and may be used as the targets for the development of reproductive-destruction-based control of H. armigera and, possibly, other lepidopteran pests.

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.

Regulatory roles of microRNAs in insect pests: prospective targets for insect pest control

At the post-transcriptional level, microRNAs (miRNAs) play an important role in the regulation of gene expression, thereby influencing the outcome of many biological processes in insects, such as development, reproduction, metamorphosis, immunity, and insecticide resistance. The alteration of miRNA expression by mimic/agomir or inhibitor/antagomir via injection/feeding can lead to pest developmental abnormalities, death, or reduced pesticide resistance, indicating that miRNAs are potential targets for pest control. This review provides an overview of recent advances in understanding the regulatory roles of miRNA in agricultural and public health insect pest, and further highlights the potential of miRNAs as prospective targets in pest control.

Engineering pest tolerance through plant-mediated RNA interference

Expression of insect-targeted RNA interference (RNAi) constructs in transgenic plants is a promising approach for agricultural pest control. Compared to conventional chemical insecticides, RNAi target specificity is high and the potential for negative environmental effects is low. However, although numerous laboratory studies show insect growth inhibition by double stranded RNA or artificial microRNA, few of these constructs have been moved into commercial application as genetically engineered plants. Variation in RNA degradation, uptake, processing, and systemic transport in insects can influence interspecific and intraspecific differences in RNAi efficacy and the development of resistance to RNAi in agricultural settings. Further research is needed, both to identify optimal gene targets for efficient RNAi in pest species and to reduce the potential for off-target effects in beneficial species.

MicroRNAs: Potential Targets for Developing Stress-Tolerant Crops

Crop yield is challenged every year worldwide by changing climatic conditions. The forecasted climatic scenario urgently demands stress-tolerant crop varieties to feed the ever-increasing global population. Molecular breeding and genetic engineering approaches have been frequently exploited for developing crops with desired agronomic traits. Recently, microRNAs (miRNAs) have emerged as powerful molecules, which potentially serve as expression markers during stress conditions. The miRNAs are small non-coding endogenous RNAs, usually 20-24 nucleotides long, which mediate post-transcriptional gene silencing and fine-tune the regulation of many abiotic- and biotic-stress responsive genes in plants. The miRNAs usually function by specifically pairing with the target mRNAs, inducing their cleavage or repressing their translation. This review focuses on the exploration of the functional role of miRNAs in regulating plant responses to abiotic and biotic stresses. Moreover, a methodology is also discussed to mine stress-responsive miRNAs from the enormous amount of transcriptome data available in the public domain generated using next-generation sequencing (NGS). Considering the functional role of miRNAs in mediating stress responses, these molecules may be explored as novel targets for engineering stress-tolerant crop varieties.

RNAs on the Go: Extracellular Transfer in Insects with Promising Prospects for Pest Management

RNA-mediated pathways form an important regulatory layer of myriad biological processes. In the last decade, the potential of RNA molecules to contribute to the control of agricultural pests has not been disregarded, specifically via the RNA interference (RNAi) mechanism. In fact, several proofs-of-concept have been made in this scope. Furthermore, a novel research field regarding extracellular RNAs and RNA-based intercellular/interorganismal communication is booming. In this article, we review key discoveries concerning extracellular RNAs in insects, insect RNA-based cell-to-cell communication, and plant-insect transfer of RNA. In addition, we overview the molecular mechanisms implicated in this form of communication and discuss future biotechnological prospects, namely from the insect pest-control perspective.

Dissecting protein domain variability in the core RNA interference machinery of five insect orders

RNA interference (RNAi)-mediated gene silencing can be used to control specific insect pest populations. Unfortunately, the variable efficiency in the knockdown levels of target genes has narrowed the applicability of this technology to a few species. Here, we examine the current state of knowledge regarding the miRNA (micro RNA) and siRNA (small interfering RNA) pathways in insects and investigate the structural variability at key protein domains of the RNAi machinery. Our goal was to correlate domain variability with mechanisms affecting the gene silencing efficiency. To this end, the protein domains of 168 insect species, encompassing the orders Coleoptera, Diptera, Hemiptera, Hymenoptera, and Lepidoptera, were analysed using our pipeline, which takes advantage of meticulous structure-based sequence alignments. We used phylogenetic inference and the evolutionary rate coefficient (K) to outline the variability across domain regions and surfaces. Our results show that four domains, namely dsrm, Helicase, PAZ and Ribonuclease III, are the main contributors of protein variability in the RNAi machinery across different insect orders. We discuss the potential roles of these domains in regulating RNAi-mediated gene silencing and the role of loop regions in fine-tuning RNAi efficiency. Additionally, we identified several order-specific singularities which indicate that lepidopterans have evolved differently from other insect orders, possibly due to constant coevolution with plants and viruses. In conclusion, our results highlight several variability hotspots that deserve further investigation in order to improve the application of RNAi technology in the control of insect pests.

Multiple microRNAs control ecdysone signaling in the midgut of Spodoptera litura

Metamorphosis is one of the most important physiological processes in insects. It is regulated by a serial of ecdysone cascade genes. Recently, lots of microRNAs (miRNAs) were investigated in insects; however, their function in metamorphosis is largely unknown. In the present study, the dynamics of a small RNA population was investigated by RNA sequencing from the midgut of a lepidopteran pest Spodoptera litura during larval-pupal metamorphosis. A total of 101 miRNAs were identified, and 75 miRNAs were differentially expressed during the metamorphic process. The relationship between these differentially expressed miRNAs and 12 ecdysone cascade genes was analyzed by four classical software programs, and a multiple-to-multiple regulatory network was found to exist between these miRNAs and their targets. Among them, miR-14-3p and its two targets (EcR and E75) were chosen for further validation. MiR-14-3p had higher expression level in the 6th instar larvae as compared with either the prepupae or pupae, which was opposite to that of both EcR and E75, two ecdysone cascade genes. Luciferase reporter assay confirmed that both EcR and E75 were regulated by miR-14-3p. Interestingly, the 3' untranslated regions are nearly identical to each other among different transcript variants of the ecdysone cascade genes, including EcR, USP, E75, E74, E78, E93, Hr3, Hr4, Hr39, Krh1 and Ftzf1. Thus, different transcript variants of one ecdysone cascade gene could be regulated by the same miRNA. The above data suggest that the ecdysone signaling pathway is under the tight control of miRNA. These findings expand our understanding of the mechanism of insect metamorphosis and may also provide a novel possibility for the control of pest insects in the future.

Host-derived artificial miRNA-mediated silencing of ecdysone receptor gene provides enhanced resistance to Helicoverpa armigera in tomato

Helicoverpa armigera causes huge crop losses due to its polyphagous nature. The present study demonstrates the use of artificial microRNA (amiRNA) mediated gene silencing approach to generate insect resistant tomato plants. Ecdysone receptor (HaEcR) gene of the target pest, H. armigera, which is involved in the regulation of all developmental stages of the insect life cycle, was silenced by sequence-specific amiRNA (amiRNA-HaEcR). Continuous feeding on detached tomato leaves expressing the amiRNA-319a-HaEcR resulted in reduced target gene transcripts and affected the overall growth and survival of H. armigera. Not only the target gene was down-regulated but, the feeding also affected the expression of down-stream genes involved in the ecdysone signaling pathway. The resistant trait was also observed in T₁ generation of tomato transgenic lines. These results further established the role of EcR as a master regulator in insect development and effectiveness of amiRNA technology for efficient control of H. armigera.

Insights Into Genetic and Molecular Elements for Transgenic Crop Development

Climate change and the exploration of new areas of cultivation have impacted the yields of several economically important crops worldwide. Both conventional plant breeding based on planned crosses between parents with specific traits and genetic engineering to develop new biotechnological tools (NBTs) have allowed the development of elite cultivars with new features of agronomic interest. The use of these NBTs in the search for agricultural solutions has gained prominence in recent years due to their rapid generation of elite cultivars that meet the needs of crop producers, and the efficiency of these NBTs is closely related to the optimization or best use of their elements. Currently, several genetic engineering techniques are used in synthetic biotechnology to successfully improve desirable traits or remove undesirable traits in crops. However, the features, drawbacks, and advantages of each technique are still not well understood, and thus, these methods have not been fully exploited. Here, we provide a brief overview of the plant genetic engineering platforms that have been used for proof of concept and agronomic trait improvement, review the major elements and processes of synthetic biotechnology, and, finally, present the major NBTs used to improve agronomic traits in socioeconomically important crops.

Plant miRNA Cross-Kingdom Transfer Targeting Parasitic and Mutualistic Organisms as a Tool to Advance Modern Agriculture

MicroRNAs (miRNAs), defined as small non-coding RNA molecules, are fine regulators of gene expression. In plants, miRNAs are well-known for regulating processes spanning from cell development to biotic and abiotic stress responses. Recently, miRNAs have been investigated for their potential transfer to distantly related organisms where they may exert regulatory functions in a cross-kingdom fashion. Cross-kingdom miRNA transfer has been observed in host-pathogen relations as well as symbiotic or mutualistic relations. All these can have important implications as plant miRNAs can be exploited to inhibit pathogen development or aid mutualistic relations. Similarly, miRNAs from eukaryotic organisms can be transferred to plants, thus suppressing host immunity. This two-way lane could have a significant impact on understanding inter-species relations and, more importantly, could leverage miRNA-based technologies for agricultural practices. Additionally, artificial miRNAs (amiRNAs) produced by engineered plants can be transferred to plant-feeding organisms in order to specifically regulate their cross-kingdom target genes. This minireview provides a brief overview of cross-kingdom plant miRNA transfer, focusing on parasitic and mutualistic relations that can have an impact on agricultural practices and discusses some opportunities related to miRNA-based technologies. Although promising, miRNA cross-kingdom transfer remains a debated argument. Several mechanistic aspects, such as the availability, transfer, and uptake of miRNAs, as well as their potential to alter gene expression in a cross-kingdom manner, remain to be addressed.

The Role of MicroRNAs in Genome Response to Plant-Lepidoptera Interaction

RNA interference is a known phenomenon of plant immune responses, involving the regulation of gene expression. The key components triggering the silencing of targeted sequences are double-stranded RNA molecules. The regulation of host-pathogen interactions is controlled by miRNA molecules, which regulate the expression of host resistance genes or the genes of the pathogen. The review focused on basic principles of RNA interference as a gene-silencing-based defense mechanism and the role of miRNA molecules in insect genomes. RNA interference as a tool for plant protection management is discussed. The review summarizes current miRNA-based biotechnology approaches for plant protection management.

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.

microRNA-92a regulates the expression of aphid bacteriocyte-specific secreted protein 1

OBJECTIVE

Aphids harbor a nutritional obligate endosymbiont in specialized cells called bacteriocytes, which aggregate to form an organ known as the bacteriome. Aphid bacteriomes display distinct gene expression profiles that facilitate the symbiotic relationship. Currently, the mechanisms that regulate these patterns of gene expression are unknown. Recently using computational pipelines, we identified miRNAs that are conserved in expression in the bacteriomes of two aphid species and proposed that they function as important regulators of bacteriocyte gene expression. Here using a dual luciferase assay in mouse NIH/3T3 cell culture, we aimed to experimentally validate the computationally predicted interaction between Myzus persicae miR-92a and the predicted target region of M. persicae bacteriocyte-specific secreted protein 1 (SP1) mRNA.

RESULTS

In the dual luciferase assay, miR-92a interacted with the SP1 target region resulting in a significant downregulation of the luciferase signal. Our results demonstrate that miR-92a interacts with SP1 to alter expression in a heterologous expression system, thereby supporting our earlier assertion that miRNAs are regulators of the aphid/Buchnera symbiotic interaction.

MicroRNAs and new biotechnological tools for its modulation and improving stress tolerance in plants

MicroRNAs (miRNAs) modulate the abundance and spatial-temporal accumulation of target mRNAs and indirectly regulate several plant processes. Transcriptional regulation of the genes encoding miRNAs (MIR genes) can be activated by numerous transcription factors, which themselves are regulated by other miRNAs. Fine-tuning of MIR genes or miRNAs is a powerful biotechnological strategy to improve tolerance to abiotic or biotic stresses in crops of economic importance. Current approaches for miRNA fine-tuning are based on the down- or up-regulation of MIR gene transcription and the use of genetic engineering tools to manipulate the final concentration of these miRNAs in the cytoplasm. Transgenesis, cisgenesis, intragenesis, artificial MIR genes, endogenous and artificial target mimicry, MIR genes editing using Meganucleases, ZNF proteins, TALENs and CRISPR/Cas9 or CRISPR/Cpf1, CRISPR/dCas9 or dCpf1, CRISPR13a, topical delivery of miRNAs and epigenetic memory have been successfully explored to MIR gene or miRNA modulation and improve agronomic traits in several model or crop plants. However, advantages and drawbacks of each of these new biotechnological tools (NBTs) are still not well understood. In this review, we provide a brief overview of the biogenesis and role of miRNAs in response to abiotic or biotic stresses, we present critically the main NBTs used for the manipulation of MIR genes and miRNAs, we show current efforts and findings with the MIR genes and miRNAs modulation in plants, and we summarize the advantages and drawbacks of these NBTs and provide some alternatives to overcome. Finally, challenges and future perspectives to miRNA modulating in important crops are also discussed.

RNA Interference in the Tobacco Hornworm, Manduca sexta, Using Plastid-Encoded Long Double-Stranded RNA

RNA interference (RNAi) is a promising method for controlling pest insects by silencing the expression of vital insect genes to interfere with development and physiology; however, certain insect Orders are resistant to this process. In this study, we set out to test the ability of in planta-expressed dsRNA synthesized within the plastids to silence gene expression in an insect recalcitrant to RNAi, the lepidopteran species, Manduca sexta (tobacco hornworm). Using the Manduca vacuolar-type H+ ATPase subunit A (v-ATPaseA) gene as the target, we first evaluated RNAi efficiency of two dsRNA products of different lengths by directly feeding the in vitro-synthesized dsRNAs to M. sexta larvae. We found that a long dsRNA of 2222 bp was the most effective in inducing lethality and silencing the v-ATPaseA gene, when delivered orally in a water droplet. We further transformed the plastid genome of the M. sexta host plant, Nicotiana tabacum, to produce this long dsRNA in its plastids and performed bioassays with M. sexta larvae on the transplastomic plants. In the tested insects, the plastid-derived dsRNA had no effect on larval survival and no statistically significant effect on expression of the v-ATPaseA gene was observed. Comparison of the absolute quantities of the dsRNA present in transplastomic leaf tissue for v-ATPaseA and a control gene, GFP, of a shorter size, revealed a lower concentration for the long dsRNA product compared to the short control product. We suggest that stability and length of the dsRNA may have influenced the quantities produced in the plastids, resulting in inefficient RNAi in the tested insects. Our results imply that many factors dictate the effectiveness of in planta RNAi, including a likely trade-off effect as increasing the dsRNA product length may be countered by a reduction in the amount of dsRNA produced and accumulated in the plastids.

Transgenic microRNA-14 rice shows high resistance to rice stem borer

Rice stem borer (RSB, Chilo suppressalis) is an insect pest that causes huge economic losses every year. Control efforts rely heavily on chemical insecticides, which leads to serious problems such as insecticide resistance, environment pollution, and food safety issues. Therefore, developing alternative pest control methods is an important task. Here, we identified an insect-specific microRNA, miR-14, in RSB, which was predicted to target Spook (Spo) and Ecdysone receptor (EcR) in the ecdysone signalling network. In-vitro dual luciferase assays using HEK293T cells confirmed the interactions of Csu-miR-14 with CsSpo and with CsEcR. Csu-miR-14 exhibited high levels of expression at the end of each larval instar stage, and its expression was negatively correlated with the expression of its two target genes. Overexpression of Csu-miR-14 at the third day of the fifth instar stage led to high mortality and developmental defects in RSB individuals. We produced 35 rice transformants to express miR-14 and found that three lines had a single copy with highly abundant miR-14 mature transcripts. Feeding bioassays using both T0 and T1 generations of transgenic miR-14 rice indicated that at least one line (C#24) showed high resistance to RSB. These results indicated that the approach of miRNAs as targets has potential for improving pest control methods. Moreover, using insect-specific miRNAs rather than protein-encoding genes for pest control may prove benign to non-insect species, and thus is worthy of further exploration.

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.

miRNA-Mediated Interactions in and between Plants and Insects

Our understanding of microRNA (miRNA) regulation of gene expression and protein translation, as a critical area of cellular regulation, has blossomed in the last two decades. Recently, it has become apparent that in plant-insect interactions, both plants and insects use miRNAs to regulate their biological processes, as well as co-opting each others' miRNA systems. In this review article, we discuss the current paradigms of miRNA-mediated cellular regulation and provide examples of plant-insect interactions that utilize this regulation. Lastly, we discuss the potential biotechnological applications of utilizing miRNAs in agriculture.

Knockdown of RNA interference pathway genes impacts the fitness of western corn rootworm

Western corn rootworm (Diabrotica virgifera virgifera) is a serious agricultural pest known for its high adaptability to various management strategies, giving rise to a continual need for new control options. Transgenic maize expressing insecticidal RNAs represents a novel mode of action for rootworm management that is dependent on the RNA interference (RNAi) pathways of the insect for efficacy. Preliminary evidence suggests that western corn rootworm could develop broad resistance to all insecticidal RNAs through changes in RNAi pathway genes; however, the likelihood of field-evolved resistance occurring through this mechanism remains unclear. In the current study, eight key genes involved in facilitating interference in the microRNA and small interfering RNA pathways were targeted for knockdown in order to evaluate impact on fitness of western corn rootworm. These genes include drosha, dicer-1, dicer-2, pasha, loquacious, r2d2, argonaute 1, and argonaute 2. Depletion of targeted transcripts in rootworm larvae led to changes in microRNA expression, decreased ability to pupate, reduced adult beetle emergence, and diminished reproductive capacity. The observed effects do not support evolution of resistance through changes in expression of these eight genes due to reduced insect fitness.

Overexpression of ghr-miR166b generates resistance against Bemisia tabaci infestation in Gossypium hirsutum plants

MAIN CONCLUSION

In silico identified Gossypium hirsutum ghr-miR166b shows multi-compatible targets in mitochondrial ATP synthase of Bemisia tabaci. Its overexpression in planta has the potential to act as a biopesticide in reducing B. tabaci population, and consequently the spread of whitefly-transmitted plant viruses. Whiteflies (B. tabaci) are hemipterous insects that act as a vector to transmit plant viruses causing enormous losses to the plants. In the present study, G. hirsutum-encoded miRNAs targeting expressed sequence tags (ESTs) of B. tabaci, based on sequence complimentarity and miRNA-target mRNA thermodynamics, were in silico identified. Out of 108 G. hirsutum miRNAs, 55 targeted the protein encoding ESTs. Among them, ghr-miR166b was selected owing to its intrinsic affinity for ATP synthase. Its functional role was validated following expression of ghr-MIR166b (precursor) sequence in G. hirsutum cv. HS6 plants through Agrobacterium-mediated transformation. Total of seven independent transformed (T0) G. hirsutum lines were obtained. The transcript level of ghr-MIR166b in the transgenic lines was observed to be 2.0- to 17-fold higher as compared to non-transformed plants. Northern-blot analysis of small RNAs isolated from the transgenic plants confirmed the presence of the ghr-miR166b. After feeding on the leaves of transgenic line (HS6-166-30) having highest level of ghr-miR166b expression, B. tabaci population was reduced up to 91% as compared to non-transformed leaves. Further, in the whole plant assay, a maximum of 78% B. tabaci mortality was observed in the same line, while there was an increase in B. tabaci population on the non-transformed plants. Our results revealed that ghr-miR166b supposedly targeting ATP synthase gene of B. tabaci, and subsequently its overexpression in planta has potential to act as biopesticide for reducing B. tabaci population and consequently spread of whitefly transmitted viruses.

Silencing of HaAce1 gene by host-delivered artificial microRNA disrupts growth and development of Helicoverpa armigera

The polyphagous insect-pest, Helicoverpa armigera, is a serious threat to a number of economically important crops. Chemical application and/or cultivation of Bt transgenic crops are the two strategies available now for insect-pest management. However, environmental pollution and long-term sustainability are major concerns against these two options. RNAi is now considered as a promising technology to complement Bt to tackle insect-pests menace. In this study, we report host-delivered silencing of HaAce1 gene, encoding the predominant isoform of H. armigera acetylcholinesterase, by an artificial microRNA, HaAce1-amiR1. Arabidopsis pre-miRNA164b was modified by replacing miR164b/miR164b* sequences with HaAce1-amiR1/HaAce1-amiR1* sequences. The recombinant HaAce1-preamiRNA1 was put under the control of CaMV 35S promoter and NOS terminator of plant binary vector pBI121, and the resultant vector cassette was used for tobacco transformation. Two transgenic tobacco lines expressing HaAce1-amiR1 was used for detached leaf insect feeding bioassays. Larval mortality of 25% and adult deformity of 20% were observed in transgenic treated insect group over that control tobacco treated insect group. The reduction in the steady-state level of HaAce1 mRNA was 70-80% in the defective adults compared to control. Our results demonstrate promise for host-delivered amiRNA-mediated silencing of HaAce1 gene for H. armigera management.

Distinct fitness costs associated with the knockdown of RNAi pathway genes in western corn rootworm adults

RNA interference (RNAi) based approaches can potentially be used to control insect pests. These approaches may depend on the usage of microRNA (miRNA) or double stranded RNA (dsRNA) mediated gene knockdown, which likely involves proteins that regulate these pathways, such as Argonaute 1 (Ago1), Argonaute 2 (Ago2), Dicer 1 (Dcr1), Dicer 2 (Dcr2), and Drosha in insects. We previously performed functional characterization of Ago2 and Dcr2 of western corn rootworm (WCR), Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae) and observed that knockdown of Ago2 and Dcr2 ameliorated the lethal effect induced by the dsRNA-mediated knockdown of an essential gene in WCR, thereby confirming the involvement of Ago2 and Dcr2 in the dsRNA pathway. In the current study, we identified and characterized additional members of the Argonaute and Dicer gene families, namely Ago1, Ago3, Aubergine, and Dcr1, in a previously developed WCR transcriptome. We also identified a Drosha homolog in the same transcriptome. We evaluated the impacts on WCR adult fitness associated with the dsRNA-mediated knockdown of Ago1, Ago2, Dcr1, Dcr2, and Drosha genes. Among these putative RNAi pathway genes, only the knockdown of Ago1 incurred significant fitness costs such as reduced survival and oviposition rate, as well as decreased egg viability. The present study, to our knowledge, represents the first report showing that Ago1 is critical to the survival of insect adults. Our findings suggest that Ago1 plays an essential role in broader life stages of an insect than previously thought. Importantly, since fitness costs were not observed, downregulation or loss of function of RNAi pathway genes such as Ago2 or Dcr2 may confer resistance to pest control measures that rely on the normal functions of these genes. However, the precise roles of these genes under field conditions (i.e., in the presence of possible viral pathogens) requires further investigation.