Silencing cathepsin L expression reduces Myzus persicae protein content and the nutritional value as prey for Coccinella septempunctata

Silence is not always golden: A closer look at potential environmental and ecotoxicological impacts of large-scale dsRNA application

RNA interference (RNAi) technology has emerged as a pivotal strategy in sustainable pest management, offering a targeted approach that significantly mitigates the environmental and health risks associated with traditional insecticides. Originally implemented through genetically modified organisms (GMOs) to produce specific RNAi constructs, the technology has evolved in response to public and regulatory concerns over GMOs. This evolution has spurred the development of non-transgenic RNAi applications such as spray-induced gene silencing (SIGS), which employs double-stranded RNA (dsRNA) to silence pest genes directly without altering the plant's genetic makeup. Despite its advantages in specificity and reduced ecological footprint, SIGS faces significant obstacles, particularly the instability of dsRNA in field conditions, which limits its practical efficacy. To overcome these limitations, innovative delivery mechanisms have been developed. These include nanotechnology-based systems, minicells, and nanovesicles, which are designed to protect dsRNA from degradation and enhance its delivery to target organisms. While these advancements have improved the stability and application efficiency of dsRNA, comprehensive assessments of their environmental safety and the potential for increased exposure risks to non-target organisms remain incomplete. This comprehensive review aims to elucidate the environmental fate of dsRNA and evaluate the potential risks associated with its widespread application on non-target organisms, encompassing soil microorganisms, beneficial insects, host plants, and mammals. The objective is to establish a more refined framework for RNAi risk assessment within environmental and ecotoxicological contexts, thereby fostering the development of safer, non-transgenic RNAi-based pest control strategies.

Serine proteinase inhibitors from Nicotiana benthamiana, a nonpreferred host plant, inhibit the growth of Myzus persicae (green peach aphid)

BACKGROUND

The green peach aphid (Myzus persicae) is a severe agricultural crop pest that has developed resistance to most current control methods, requiring the urgent development of novel strategies. Plant proteinase inhibitors (PINs) are small proteins that protect plants against pathogens and/or herbivores, likely by preventing efficient protein digestion.

RESULTS

We identified 67 protease genes in the transcriptomes of three M. persicae lineages (USDA-Red, G002 and G006). Comparison of gene expression levels in aphid guts and whole aphids showed that several proteases, including a highly expressed serine protease, are significantly overexpressed in the guts. Furthermore, we identified three genes encoding serine protease inhibitors (SerPIN-II1, 2 and 3) in Nicotiana benthamiana, which is a nonpreferred host for M. persicae. Using virus-induced gene silencing (VIGS) with a tobacco rattle virus (TRV) vector and overexpression with a turnip mosaic virus (TuMV) vector, we demonstrated that N. benthamiana SerPIN-II1 and SerPIN-II2 cause reduced survival and growth, but do not affect aphid protein content. Likewise, SerPIN-II3 overexpression reduced survival and growth, and serpin-II3 knockout mutations, which we generated using CRISPR/Cas9, increased survival and growth. Protein content was significantly increased in aphids fed on SerPIN-II3 overexpressing plants, yet it was decreased in aphids fed on serpin-II3 mutants.

CONCLUSION

Our results show that three PIN-IIs from N. benthamiana, a nonpreferred host plant, effectively inhibit M. persicae survival and growth, thereby representing a new resource for the development of aphid-resistant crop plants. © 2024 Society of Chemical Industry.

Horizontally transferred genes as RNA interference targets for aphid and whitefly control

RNA interference (RNAi)-based technologies are starting to be commercialized as a new approach for agricultural pest control. Horizontally transferred genes (HTGs), which have been transferred into insect genomes from viruses, bacteria, fungi or plants, are attractive targets for RNAi-mediated pest control. HTGs are often unique to a specific insect family or even genus, making it unlikely that RNAi constructs targeting such genes will have negative effects on ladybugs, lacewings and other beneficial predatory insect species. In this study, we sequenced the genome of a red, tobacco-adapted isolate of Myzus persicae (green peach aphid) and bioinformatically identified 30 HTGs. We then used plant-mediated virus-induced gene silencing (VIGS) to show that several HTGs of bacterial and plant origin are important for aphid growth and/or survival. Silencing the expression of fungal-origin HTGs did not affect aphid survivorship but decreased aphid reproduction. Importantly, although there was uptake of plant-expressed RNA by Coccinella septempunctata (seven-spotted ladybugs) via the aphids that they consumed, we did not observe negative effects on ladybugs from aphid-targeted VIGS constructs. To demonstrate that this approach is more broadly applicable, we also targeted five Bemisia tabaci (whitefly) HTGs using VIGS and demonstrated that knockdown of some of these genes affected whitefly survival. As functional HTGs have been identified in the genomes of numerous pest species, we propose that these HTGs should be explored further as efficient and safe targets for control of insect pests using plant-mediated RNA interference.

Application progress of plant-mediated RNAi in pest control

RNA interference (RNAi)-based biopesticides are novel biologic products, developed using RNAi principles. They are engineered to target genes of agricultural diseases, insects, and weeds, interfering with their target gene expression so as to hinder their growth and alleviate their damaging effects on crops. RNAi-based biopesticides are broadly classified into resistant plant-based plant-incorporated protectants (PIPs) and non-plant-incorporated protectants. PIP RNAi-based biopesticides are novel biopesticides that combine the advantages of RNAi and resistant transgenic crops. Such RNAi-based biopesticides are developed through nuclear or plastid transformation to breed resistant plants, i.e., dsRNA-expressing transgenic plants. The dsRNA of target genes is expressed in the plant cell, with pest and disease control being achieved through plant-target organism interactions. Here, we review the action mechanism and strategies of RNAi for pest management, the development of RNAi-based transgenic plant, and the current status and advantages of deploying these products for pest control, as well as the future research directions and problems in production and commercialization. Overall, this study aims to elucidate the current development status of RNAi-based biopesticides and provide guidelines for future research.

Evolved transcriptional responses and their trade-offs after long-term adaptation of Bemisia tabaci to a marginally-suitable host

Although generalist insect herbivores can migrate and rapidly adapt to a broad range of host plants, they can face significant difficulties when accidentally migrating to novel and marginally-suitable hosts. What happens, both in performance and gene expression regulation, if these marginally-suitable hosts must be used for multiple generations before migration to a suitable host can take place, largely remains unknown. In this study, we established multigenerational colonies of the whitefly Bemisia tabaci, a generalist phloem-feeding species, adapted to a marginally-suitable host (habanero pepper) or an optimal host (cotton). We used reciprocal host tests to estimate the differences in performance of the populations on both hosts under optimal (30 oC) and mild-stressful (24 oC) temperature conditions, and documented the associated transcriptomic changes. The habanero pepper-adapted population greatly improved its performance on habanero pepper but did not reach its performance level on cotton, the original host. It also showed reduced performance on cotton, relative to the non-adapted population, and an antagonistic effect of the lower-temperature stressor. The transcriptomic data revealed that most of the expression changes, associated with long-term adaptation to habanero pepper, can be categorized as "evolved" with no initial plastic response. Three molecular functions dominated: enhanced formation of cuticle structural constituents, enhanced activity of oxidation-reduction processes involved in neutralization of phytotoxins and reduced production of proteins from the cathepsin B family. Taken together, these findings indicate that generalist insects can adapt to novel host plants by modifying the expression of a relatively small set of specific molecular functions.

Phloem: At the center of action in plant defense against aphids

The location of the phloem deep inside the plant, the high hydrostatic pressure in the phloem, and the composition of phloem sap, which is rich in sugar with a high C:N ratio, allows phloem sap feeding insects to occupy a unique ecological niche. The anatomy and physiology of aphids, a large group of phytophagous insects that use their mouthparts, which are modified into stylets, to consume large amounts of phloem sap, has allowed aphids to successfully exploit this niche, however, to the detriment of agriculture and horticulture. The ability to reproduce asexually, a short generation time, the development of resistance to commonly used insecticides, and their ability to vector viral diseases makes aphids among the most damaging pests of plants. Here we review how plants utilize their ability to occlude sieve elements and accumulate antibiotic and antinutritive factors in the phloem sap to limit aphid infestation. In addition, we summarize progress on understanding how plants perceive aphids to activate defenses in the phloem.

Rapid Screening of Myzus persicae (Green Peach Aphid) RNAi Targets Using Tobacco Rattle Virus

Plant-mediated RNA interference (RNAi) can be used to reduce the growth of insect pests, including Myzus persicae (green peach aphid), a prolific pest of numerous dicot crop species. In one approach, viruses that have been engineered to carry an aphid gene fragment are used to infect plants and thereby silence target gene expression in the aphids feeding on these plants, a process called virus-induced gene silencing, or VIGS. Tobacco Rattle Virus (TRV) in the model plant, Nicotiana benthamiana, was the first of many VIGS systems that have been developed for different plant species. In this chapter, we describe a method for silencing M. persicae gene expression using an established TRV-VIGS vector that infects and spreads in N. benthamiana. The two parts of the TRV genome, RNA1 and RNA2, have been cloned into Agrobacterium T-DNA vectors for initiation of plant infections. The RNA2 construct is modified with a Gateway-compatible cloning site to allow insertion of aphid genes. When feeding on TRV-infected N. benthamiana plants, aphids ingest dsRNAs that silence specific target genes. TRV-VIGS of aphid genes allows rapid identification of essential gene targets that can be used for the control of M. persicae by this and other RNAi methods.

Knockdown of Helicoverpa armigera protease genes affects its growth and mortality via RNA interference

Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), the cotton bollworm, is a destructive pest which is famous for its resistance to a variety of insecticides. RNA interference is a posttranscriptional gene silencing mechanism that has become a popular tool to control insect pests, triggered by double-stranded RNAs (dsRNAs). The effect of ingestion and injection delivery methods of dsRNA related to some protease genes including Trypsin (Ha-TRY39 and Ha-TRY96), Chymotrypsin (Ha-CHY), and Cathepsin L (Ha-CAT) on growth and development of H. armigera was investigated in this study. All protease genes encoded full ORFs and were expressed in all H. armigera larvae stages and tissues. In both injection and feeding bioassays, Ha-RNAi CHY's performance outperformed that of other protease genes. CHY enzyme activity in the midgut of larvae was significantly reduced after treatment with ds-HaCHY. Oral administration of ds-CHY also resulted in significant mortality of H. armigera larvae. However, because of the high RNase activity in the midgut lumen of lepidoptera, a large amount of dsRNA was needed to effectively kill instars of H. armigera. To reduce dsRNA degradation, bacterial expression and dsRNA formulation were used. After oral administration, it was toxic to H. armigera larvae. Before oral administration, bacterial cells were sonicated to increase dsRNA release. The RNA interference efficiency of sonicated bacteria was significantly increased, resulting in higher larval mortality when administered orally. All of these findings point to Ha-CHY as a new candidate for developing an effective dsRNA-based pesticide for H. armigera control.

Inhibition of Rhopalosiphum maidis (Corn Leaf Aphid) Growth on Maize by Virus-Induced Gene Silencing with Sugarcane Mosaic Virus

The corn leaf aphid (Rhopalosiphum maidis), a damaging pest of maize (Zea mays), is not controlled by the insecticidal proteins in commercially available transgenic crop varieties. One promising approach is to reduce aphid growth and fecundity by targeting the expression of essential genes using plant-mediated RNA interference (RNAi). Here we describe a method whereby Sugarcane Mosaic Virus (SCMV), a positive-strand RNA virus in the Potyviridae family, is used for virus-induced gene silencing (VIGS) of gene expression in R. maidis. A segment of the R. maidis target gene is cloned into SCMV, maize plants are infected with the transgenic virus, aphids are placed on the virus-infected plants and, after a few days of feeding, decreases in target gene expression and aphid reproduction are assessed. This VIGS method can be used for rapid screening of suitable RNAi targets for aphid pest control, as well as to study the in vivo function of specific aphid genes.

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.

Evaluation of a cuticle protein gene as a potential RNAi target in aphids

BACKGROUND

RNA interference (RNAi) has potential as a pest insect control technique. One possible RNAi target is the cuticle protein, which is important in insect molting and development. As an example, here we evaluate the possibility of designing double-stranded RNA (RNA) that is effective for silencing the cuticle protein 19 gene (CP19) in aphids but is harmless to non-target predator insects.

RESULTS

The sequences of CP19s were similar (86.6-94.4%) among the tested aphid species (Aphis citricidus, Acyrthosiphon pisum, and Myzus persicae) but different in the predator Propylaea japonica. Ingestion of species-specific dsRNAs of CP19 by the three aphids produced 39.3-64.2% gene silencing and 45.8-55.8% mortality. Ingestion of non-species-specific dsRNA (dsAcCP19) by Ac. pisum and M. persicae gave gene silencing levels ranging from 40.4% to 50.3% and 43.3-50.8% mortality. The dsApCP19 did not affect PjCP19 expression or developmental duration in P. japonica.

CONCLUSION

The results demonstrate that CP19 is a promising RNAi target for aphid control via one dsRNA design. The targeting of genes that are conserved in insect pests but not present in beneficial insects is a useful RNAi-based pest control strategy. © 2019 Society of Chemical Industry.