Molecular identification and lipid mobilization role of adipokinetic hormone receptor in Spodoptera litura (F.)

Expression and functional analysis of adipokinetic hormone reveal its different roles in larval development and female fecundity in Panonychus citri (McGregor) (Acari: Tetranychidae)

Adipokinetic hormone (AKH), a crucial neuropeptide, participates in the important physiological processes by specially binding to its receptor to activate the AKH signalling pathway. AKH regulates energy metabolism. However, it remains unknown whether AKH affects larval development and adult reproduction by influencing energy metabolism. In the present study, the AKH was identified from Panonychus citri and contained the conserved functional domain 'Q-[LIV]-[NT]-F-[ST]-X (2)-W' that characterises the AKH family. The relative expression levels of PcAKH revealed different patterns of AKH expression at different developmental stages of P. citri. Feeding of double-standard RNA against PcAKH induced decreased fecundity and reduced survival, which was accompanied by the down-regulation of vitellogenin gene expression. In addition, after silencing the PcAKH, lipid metabolism and carbohydrate homeostasis were disrupted, manifested by increased body width and weight, and fasting phenomenon. Further investigation found that compared with the control, physiological changes in trehalose and triglyceride contents were accompanied by variations in the mRNA expression levels of genes related to lipid metabolism and carbohydrate metabolism. The disorder of lipid and carbohydrate metabolism may affect adult female reproduction, which may lead to insufficient vitellogenin deposition. Moreover, the silencing of PcAKH seriously affected the growth and development of larvae, which was manifested as delayed development period and difficulty in moulting. Conclusively, all these results in current study demonstrated that double-stranded RNA silencing system targeting PcAKH effectively inhibited larval development and female fecundity by disturbing lipid and carbohydrate metabolism, and PcAKH is a specific RNAi target for control of P. citri in the design and development of biopesticide in sustainable agriculture.

A larval expressed chemosensory protein involved in recognition of anthocyanins in Spodoptera litura (Lepidoptera: Noctuidae)

Anthocyanins are secondary metabolites which act as diverse functions during plant growth. Insects can discriminate host plants by their sensitive gustatory systems. It is hypothetical that chemosensory proteins (CSPs) play a crucial role in regulating this behavioral process. However, the underlying molecular mechanisms remain obscure. In the present study, we characterized a CSP SlitCSP8 from the Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae). Quantitative real-time-polymerase chain reaction analysis demonstrated that SlitCSP8 was mainly expressed in the head of the 7th S. litura larvae, especially labrum. Further, recombinant SlitCSP8 was obtained using bacterial expression system. Fluorescence competitive binding assays demonstrated that the purified SlitCSP8 exhibited a strong binding affinity to anthocyanins, a natural compound derived from the host plant. Silencing SlitCSP8 through RNAi significantly reduced the sensitivity of S. litura larvae to anthocyanins-treated leaf disks, the development from larva to pupae was not affected. These data provide insight into the molecular basis that CSP8 can detect anthocyanins in host plants by chemosensory system of insects. It can be further used in designing novel optimal food attractant targeting to the CSPs for pest control.

Lipid Metabolism as a Target Site in Pest Control

Lipid metabolism is essential to insect life as insects use lipids for their development, reproduction, flight, diapause, and a wide range of other functions. The central organ for insect lipid metabolism is the fat body, which is analogous to mammalian adipose tissue and liver, albeit less structured. Various other systems including the midgut, brain, and neural organs also contribute functionally to insect lipid metabolism. Lipid metabolism is under the control of core lipogenic [e.g. acetyl-CoA-carboxylase (ACC), fatty acid synthase (FAS), perilipin 2 (LSD2)], and lipolytic (lipases, perilipin 1) enzymes that are primarily expressed in the fat body, as well as hormones [insulin-like peptides (ILP), adipokinetic hormone (AKH)], transcription factors (SREBPs, foxO, and CREB), secondary messengers (calcium) and post-translational modifications (phosphorylation). Essential roles of the fat body, together with the fact that proper coordination of lipid metabolism is critical for insects, render lipid metabolism an attractive target site in pest control. In the current chapter, we focus on pest control tactics that target insect lipid metabolism. Various classes of traditional chemical insecticides [e.g. organophosphates, pyrethroids, neonicotinoids, and chitin synthesis inhibitors (Sects. 2.1 and 2.2)] have been shown to interfere with lipid metabolism, albeit it is not their primary site of action. However, the discovery of "lipid biosynthesis inhibitors", tetronic and tetramic acid derivatives commonly known as ketoenols (Sect. 2.3), was a milestone in applied entomology as they directly target lipid biosynthesis, particularly in sucking pests. Spirodiclofen, spiromesifen, and spirotetramat targeting ACC act against various insect and mite pests, while spiropidion and spidoxamat have been introduced to the market only recently. Efforts have concentrated on the development of chemical alternatives, such as hormone agonists and antagonists (Sect. 2.4), dsRNA-based pesticides that depend on RNA interference, which have great potential in pest control (Sect. 2.5) and other eco-friendly alternatives (Sect. 2.6).

Effect of Insulin Receptor on Juvenile Hormone Signal and Fecundity in Spodoptera litura (F.)

Simple Summary

The tobacco cutworm, Spodoptera litura (F.), exemplifies strong reproductive capacities and damages many agricultural crops. The insulin signaling pathway is known as a key determinant of female reproduction in insects. However, the detailed molecular mechanisms in these processes are poorly studied. Here, we injected bovine insulin into the newly emerged moth, resulting in gene expression changes in the insulin pathway, while knockdown of SlInR caused an inverse gene expression change involved in the insulin pathway. Further studies indicated that the content of JH-III, Vg, total proteins and triacylgycerol could be suppressed by SlInR dsRNA injection. Furthermore, stunted ovaries and lower fecundity were observed by RNAi. Our studies indicated that SlInR plays a key role in JH-III synthesis and the ovarian development in S. litura.

Abstract

Insulin signaling can regulate various physiological functions, such as energy metabolism and reproduction and so on, in many insects, including mosquito and locust. However, the molecular mechanism of this physiological process remains elusive. The tobacco cutworm, Spodoptera litura, is one of the most important pests of agricultural crops around the world. In this study, phosphoinositide 3-kinase (SlPI3K), protein kinase B (SlAKT), target of rapamycin (SlTOR), ribosomal protein S6 kinase (SlS6K) and transcription factor cAMP-response element binding protein (SlCREB) genes, except transcription factor forkhead box class O (SlFoxO), can be activated by bovine insulin injection. Then, we studied the influence of the insulin receptor gene (SlInR) on the reproduction of S. litura using RNA interference technology. qRT-PCR analysis revealed that SlInR was most abundant in the head. The SlPI3K, SlAKT, SlTOR, SlS6K and SlCREB genes were decreased, except SlFoxO, after the SlInR gene knockdown. Further studies revealed that the expression of vitellogenin mRNA and protein, Methoprene-tolerant gene (SlMet), could be down-regulated by the injection of dsRNA of SlInR significantly. Furthermore, a depletion in the insulin receptor by RNAi significantly decreased the content of juvenile hormone III (JH-III), total proteins and triacylgycerol. These changes indicated that a lack of SlInR could impair ovarian development and decrease fecundity in S. litura. Our studies contribute to a comprehensive insight into reproduction, regulated by insulin and the juvenile hormone signaling pathway through nutrition, and a provide theoretical basis for the reproduction process in pest insects.