RNAi knockdown of acetyl-CoA carboxylase gene eliminates jinggangmycin-enhanced reproduction and population growth in the brown planthopper, Nilaparvata lugens

The integral role of de novo lipogenesis in the preparation for seasonal dormancy

Animals can alter their body compositions in anticipation of dormancy to endure seasons with limited food availability. Accumulation of lipid reserves, mostly in the form of triglycerides (TAGs), is observed during the preparation for dormancy in diverse animals, including insects (diapause) and mammals (hibernation). However, the mechanisms involved in the regulation of lipid accumulation and the ecological consequences of failure to accumulate adequate lipid stores in preparation for animal dormancy remain understudied. In the broadest sense, lipid reserves can be accumulated in two ways: the animal either receives lipids directly from the environment or converts the sugars and amino acids present in food to fatty acids through de novo lipogenesis and then to TAGs. Here, we show that preparation for diapause in the Colorado potato beetle (Leptinotarsa decemlineata) involves orchestrated upregulation of genes involved in lipid metabolism with a transcript peak in 8- and 10-d-old diapause-destined insects. Regulation at the transcript abundance level was associated with the accumulation of substantial fat stores. Furthermore, the knockdown of de novo lipogenesis enzymes (ACCase and FAS-1) prolonged the preparatory phase, while the knockdown of fatty acid transportation genes shortened the preparatory phase. Our findings suggest a model in which the insects dynamically decide when to transition from the preparation phase into diapause, depending on the progress in lipid accumulation through de novo lipogenesis.

Jinggangmycin stimulates reproduction and increases CHCs-dependent desiccation tolerance in Drosophila melanogaster

Jinggangmycin (JGM), an agricultural antibiotic compound, is mainly used against the rice sheath blight (RSB) Rhizoctonia solani. However, its application may lead to unexpected consequences in insects. In this study, the effects of JGM on the physiological parameters of Drosophila melanogaster were investigated. The results showed that 0.005 g/ml JGM exposure increased female daily egg production and extended the oviposition period, while there was no significant effect on reproduction at 0.016 g/ml. At the same time, desiccation tolerance increased in flies fed 0.005 g/ml JGM. The RT-qPCR results revealed that FAS1 and FAS3 expression were upregulated in 0.005 g/ml JGM treated flies. Consistently, the amount of CHCs accumulated on the cuticle surface increased upon JGM treatment at 0.005 g/ml. Moreover, RNAi for FAS3 decreased desiccation tolerance of JGM-treated flies. These results suggest that JGM affects fatty acid biosynthesis, which in turn enhances reproduction and desiccation tolerance in Drosophila.

Synergistic and Additive Interactions of Zhongshengmycin to the Chemical Insecticide Pymetrozine for Controlling Nilaparvata lugens (Hemiptera: Delphacidae)

Management of the rice brown planthopper Nilaparvata lugens Stål is challenging because it can rapidly adapt to new pesticides within several generations. Combined use of chemical insecticides and antimicrobials was proposed as an alternative strategy to control N. lugens. Our previous experiments identified two effective agents (chemical insecticide: pymetrozine and antimicrobial: zhongshengmycin) that act on different targets in N. lugens. However, conditions and effectiveness of combinations of antimicrobials and insecticides against N. lugens are still unknown. Here, we evaluated separate and combined effects of pymetrozine and zhongshengmycin on third instar nymphs of N. lugens under laboratory and greenhouse conditions. Results showed that zhongshengmycin exerts significant inhibitory effects on the three endosymbionts Pichia guilliermondii, Cryptococcus peneaus, and Pichia anomala cultured in vitro of N. lugens. Combinations of pymetrozine and zhongshengmycin under laboratory conditions produced additive or synergistic effects on N. lugens and caused higher mortality in third instar nymphs than either of them used alone. Experiments under greenhouse conditions further demonstrated that effective component quality ratio of pymetrozine to zhongshengmycin of 1:10 and 1:40 with co-toxicity coefficients of 221.63 and 672.87, respectively, also produced significant synergistic effects against N. lugens. Our results indicated that chemical insecticides combined with antimicrobials may provide a potential novel strategy for controlling N. lugens by inhibiting its endosymbionts.

Ten fatty acyl-CoA reductase family genes were essential for the survival of the destructive rice pest, Nilaparvata lugens

BACKGROUND

Fatty alcohols are the precursors of sex pheromone components, wax esters and hydrocarbons in insects. Fatty acyl-CoA reductases (FARs) are important enzymes required for the reduction of fatty alcohol and thereby contribute to the production of cuticular hydrocarbon (CHC).

RESULTS

Based on bioinformatics analyses we identified 17 FAR genes in the brown planthopper, Nilaparvata lugens. RNA interference against these genes demonstrated that ten NlFAR genes were essential for the survival of N. lugens. For instance, knockdown of NlFAR5, 6, 11 or 15 was lethal and caused a slender body shape, while the old cuticles of the respective animals remained attached to the abdomen or failed to split open from the nota. Knockdown of NlFAR9 resulted in a phenotype, with a smooth body surface and a decrease in CHC amounts. Similarly, CHC deficiency in N. lugens resulted in increased adhesion of water droplets and secreted honeydew to the insect surface and the inability of N. lugens to survive in paddy fields with varying humidity. Knockdown of NlFAR1, 4, 5, 6, 8, 9, 11 and 13 additionally resulted in female adult infertility.

CONCLUSION

The present study illustrates the structural and functional differences of FAR family genes and provides potential targets for RNA interference-based rice planthopper management. © 2020 Society of Chemical Industry.

The Role of Peptide Hormones in Insect Lipid Metabolism

Lipids are the primary storage molecules and an essential source of energy in insects during reproduction, prolonged periods of flight, starvation, and diapause. The coordination center for insect lipid metabolism is the fat body, which is analogous to the vertebrate adipose tissue and liver. The fat body is primarily composed of adipocytes, which accumulate triacylglycerols in intracellular lipid droplets. Genomics and proteomics, together with functional analyses, such as RNA interference and CRISPR/Cas9-targeted genome editing, identified various genes involved in lipid metabolism and elucidated their functions. However, the endocrine control of insect lipid metabolism, in particular the roles of peptide hormones in lipogenesis and lipolysis are relatively less-known topics. In the current review, the neuropeptides that directly or indirectly affect insect lipid metabolism are introduced. The primary lipolytic and lipogenic peptide hormones are adipokinetic hormone and the brain insulin-like peptides (ILP2, ILP3, ILP5). Other neuropeptides, such as insulin-growth factor ILP6, neuropeptide F, allatostatin-A, corazonin, leucokinin, tachykinins and limostatin, might stimulate lipolysis, while diapause hormone-pheromone biosynthesis activating neuropeptide, short neuropeptide F, CCHamide-2, and the cytokines Unpaired 1 and Unpaired 2 might induce lipogenesis. Most of these peptides interact with one another, but mostly with insulin signaling, and therefore affect lipid metabolism indirectly. Peptide hormones are also involved in lipid metabolism during reproduction, flight, diapause, starvation, infections and immunity; these are also highlighted. The review concludes with a discussion of the potential of lipid metabolism-related peptide hormones in pest management.

Pesticide-Induced Planthopper Population Resurgence in Rice Cropping Systems

Planthoppers are serious rice pests in Asia. Their population resurgence was first reported in the early 1960s, caused mainly by insecticides that indiscriminately killed beneficial arthropods and target pests. The subsequent resurgence involved two mechanisms, the loss of beneficial insects and insecticide-enhanced planthopper reproduction. In this review, we identify two forms of resurgence, acute and chronic. Acute resurgence is caused by traditional insecticides with rapid resurgence in the F₁ generation. Chronic resurgence follows application of modern pesticides, including fungicides and herbicides, with low natural enemy toxicity, coupled with stimulated planthopper reproduction. The chemical-driven syndrome of changes leads to later resurgence in the F₂ or later generations. Chronic resurgence poses new threats to global rice production. We review findings on the physiological and molecular mechanisms of chronic planthopper resurgence and suggest research directions that may help manage these new threats.

Carboxylesterase Precursor (EST-1) Mediated the Fungicide Jinggangmycin-Suppressed Reproduction of Sogatella furcifera (Hemiptera: Delphacidae)

The jinggangmycin (JGM) is a widely used fungicide for controlling the rice sheath blight, Rhizoctonia solani, in China. Previous experiments under lab conditions showed that JGM foliar spray suppressed Sogatella furcifera (Horvath) reproduction. However, the molecular mechanisms of JGM-driven changes in S. furcifera reproduction are unclear. Therefore, we selected carboxylesterase precursor (EST-1) as a target gene for silencing by RNAi based on gene expression profiles. The present results demonstrated that JGM and control + dsSfEST-1 treatments significantly reduced the number of eggs laid (down by 58% and 54%, respectively), oviposition period (down by 57% and 38%, respectively), and longevity (down by 32% and 38%, respectively) in adult females compared with untreated controls, while no pronounced differences in the preoviposition period were observed. Meanwhile, the dietary control + dsSfEST-1 treatment also severely impeded protein synthesis, specifically soluble ovarian protein content (down by 20% and 24%, respectively) and soluble sugar content (down by 42% and 35%, respectively), which led to stunted growth and reduced body weight in adult females. We thereby speculate that downregulated SfEST-1 expression may be one molecular mechanism underlying JGM-driven reproduction in S. furcifera.

RNA Interference in Insect Vectors for Plant Viruses

Insects and other arthropods are the most important vectors of plant pathogens. The majority of plant pathogens are disseminated by arthropod vectors such as aphids, beetles, leafhoppers, planthoppers, thrips and whiteflies. Transmission of plant pathogens and the challenges in managing insect vectors due to insecticide resistance are factors that contribute to major food losses in agriculture. RNA interference (RNAi) was recently suggested as a promising strategy for controlling insect pests, including those that serve as important vectors for plant pathogens. The last decade has witnessed a dramatic increase in the functional analysis of insect genes, especially those whose silencing results in mortality or interference with pathogen transmission. The identification of such candidates poses a major challenge for increasing the role of RNAi in pest control. Another challenge is to understand the RNAi machinery in insect cells and whether components that were identified in other organisms are also present in insect. This review will focus on summarizing success cases in which RNAi was used for silencing genes in insect vector for plant pathogens, and will be particularly helpful for vector biologists.

Driving Pest Insect Populations: Agricultural Chemicals Lead to an Adaptive Syndrome in Nilaparvata Lugens Stål (Hemiptera: Delphacidae)

The brown planthopper (BPH) is a devastating pest of rice throughout Asia. In this paper we document the BPH biogeographic range expansion in China over the 20-year period, 1992 to 2012. We posed the hypothesis that the range expansion is due to a syndrome of adaptations to the continuous presence of agricultural chemicals (insecticides and a fungicide) over the last 40 years. With respect to biogeography, BPH ranges have expanded by 13% from 1992 to 1997 and by another 3% from 1997 to 2012. In our view, such expansions may follow primarily from the enhancing effects of JGM, among other agricultural chemicals, and from global warming. JGM treatments led to increased thermotolerance, recorded as decreased mortality under heat stress at 40 ± 1 °C (down from 80% to 55%) and increased fecundity (by 49%) at 34 °C. At the molecular level, JGM treatments led to increased abundances of mRNA encoding Acetyl Co-A carboxylase (Acc) (up 25%) and Hsp70 (up 32%) in experimental BPH. RNAi silencing of Hsp70 and Acc eliminated the JGM effects on fecundity and silencing Hsp70 reduced JGM-induced thermotolerance. Integrated with global climate change scenarios, such syndromes in pest insect species have potential for regional- and global-scale agricultural disasters.

Transmission Electron Microscopy (TEM) Observations of Female Oocytes From Nilaparvata lugens (Hemiptera: Delphacidae): Antibiotic Jinggangmycin (JGM)-Induced Stimulation of Reproduction and Associated Changes in Hormone Levels

Previous studies have demonstrated that the agricultural antibiotic jinggangmycin (JGM) stimulates reproduction in the brown planthopper Nilaparvata lugens Stål and that the stimulation of brown planthopper reproduction induced by JGM is regulated by the fatty acid synthase (FAS) and acetyl-CoA carboxylase (ACC) genes. However, a key issue in the stimulation of reproduction induced by pesticides involves the growth and development of oocytes. Therefore, the present study investigated oocyte changes via transmission electron microscopy (TEM) and changes in hormone levels (juvenile hormones (JH) and 20-hydroxyecdysone (20 E)) in JGM-treated females. TEM observations showed that the size of the lipid droplets in the oocytes of JGM-treated females, compared with those in the oocytes of the control females, significantly reduced by 32.6 and 29.8% at 1 and 2 d after emergence (1 and 2 DAE), respectively. In addition, the JH levels of JGM-treated females at 1 and 2 DAE were increased by 49.7 and 45.7%, respectively, whereas 20 E levels decreased by 36.0 and 30.0%, respectively. We conclude that JGM treatments lead to substantial changes in lipid metabolism, which are directly and indirectly related to stimulation of reproduction of brown planthopper together with our previous findings.

ATP phosphoribosyltransferase from symbiont Entomomyces delphacidicola invovled in histidine biosynthesis of Nilaparvata lugens (Stål)

Histidine is an essential amino acid assumed to be synthesized by an obligatory yeast-like symbiont (Entomomyces delphacidicola str. NLU) in Nilaparvata lugens, an important rice pest. The adenosine-triphosphate phosphoribosyltransferase (ATP-PRTase) facilities the committed first step of the histidine biosynthesis pathway. In the current study, a putative ATP-PRTase was cloned and verified to be of E. delphacidicola origin (EdePRTase). The expression of the gene was spatial and temporal universal with a profile that matched the distribution of the fungal symbiont. RNA interference aided the knockdown of the EdePRTase-suppressed EdePRTase expression by 32-48 %. Hemolymph histidine level was also reduced followed by significant reduction of adult body weight. However, other performance characters including nymph development, survival, and adult sex ratio were not adversely affected by the knockdown. Furthermore, forced histidine exposure (through injection or feeding) significantly inhibited the EdePRTase mRNA levels at higher concentrations, but significantly increased EdePRTase expression levels at lower concentrations (feeding only). The significance of these findings support that the EdePRTase is from symbiont E. delphacidicola, and its involvement in histidine biosynthesis of N. lugens was discussed. The results provide a better understanding of EdePRTase and the encoded functional ATP-PRTase enzyme regulation in N. lugens and insects in general.

Functional characterization of three trehalase genes regulating the chitin metabolism pathway in rice brown planthopper using RNA interference

RNA interference (RNAi) is an effective gene-silencing tool, and double stranded RNA (dsRNA) is considered a powerful strategy for gene function studies in insects. In the present study, we aimed to investigate the function of trehalase (TRE) genes (TRE 1-1, TRE 1-2, and TRE-2) isolated from the brown planthopper Nilaparvata lugens, a typical piercing-sucking insect in rice, and investigate their regulating roles in chitin synthesis by injecting larvae with dsRNA. The results showed that TRE1 and TRE2 had compensatory function, and the expression of each increased when the other was silenced. The total rate of insects with phenotypic deformities ranged from 19.83 to 24.36% after dsTRE injection, whereas the mortality rate ranged from 14.16 to 31.78%. The mRNA levels of genes involved in the chitin metabolism pathway in RNA-Seq and DGEP, namely hexokinase (HK), glucose-6-phosphate isomerase (G6PI) and chitinase (Cht), decreased significantly at 72 h after single dsTREs injection, whereas two transcripts of chitin synthase (CHS) genes decreased at 72 h after dsTRE1-1 and dsTREs injection. These results demonstrated that TRE silencing could affect the regulation of chitin biosynthesis and degradation, causing moulting deformities. Therefore, expression inhibitors of TREs might be effective tools for the control of planthoppers in rice.