Three-dimensional reconstruction of a whole insect reveals its phloem sap-sucking mechanism at nano-resolution

Three-dimensional interaction between Cinnamomum camphora and a sap-sucking psyllid insect (Trioza camphorae) revealed by nano-resolution volume electron microscopy

Phloem-feeding insects present significant economic threats worldwide and remain challenging to understand due to their specialized feeding strategies. Significant advances in genetics, genomics, and biochemistry have greatly enriched our comprehension of phloem-insect interactions. However, existing studies relying on two-dimensional discrete images have limited our understanding of visible morphological details. In this study, we leverage volume electron microscopy (vEM) technology to unveil a nanometer-resolution interaction mode between plant and the phloem-feeding insect, Camphor psyllid (Trioza camphorae, Hemiptera: Psyllidae). The stylets penetrate each cell on the way to the feeding site (sieve tube), and new cell walls will form around the salivary sheath, ultimately fusing with the original cell walls to form remarkably thickening cell walls. Our reconstruction findings on pit gall tissues suggest that a significant decrease in cell volume and a drastic increase in cell layers are the primary processes during pit gall formation. These unique findings will set the stage for a robust discussion on the plant cellular response induced by phloem-feeding insects.

Three-dimensional reconstruction of insect chemosensory sensillum

The olfactory system is involved in food and mate recognition in insects. However, 3D structures of chemosensory sensilla in insects are unexplored yet. Here, the internal structures of an olfactory sensillum on the antenna of the brown planthopper, Nilaparvata lugens (Hemiptera: Delphacidae), one of the most important rice pests, are examined and imaged using focused ion beam scanning electron microscopy. Based on these images, a 3D structure is reconstructed in this study. We find that the trichoid olfactory sensillum possesses a multiporous wall encircling a lumen with one sensory cell. Besides, there are three accessory cells (ACs) and a glia cell with different cell contents surrounding the sensory cell. The abundant tubular membrane structures in the tormogen cell suggest its role in secreting proteins like odorant binding proteins into the receptor lymph, while three auxiliary cells with simpler cellular content closely enfold the sensory cell, probably to prevent leaking of the receptor lymph into the surrounding epidermis. In the sensory cell, the microtubules and two tandem basal bodies at the base of the microtubules are also reconstructed. They are considered as a propulsive engine to ensure dendrite vibration or spinning in the receptor lymph, so that the proteins and odorant molecules move faster in the receptor lymph, which improves recognition of environmental odors and enables the insect to immediately respond to this information.

Comparative proteomic analysis of resistant and susceptible cotton genotypes in response to leaf hopper infestation

The cotton leaf hopper is a major pest in cotton, causing a hopper burn in leaves. In this study, a comparative proteomic analysis of NDLH2010 (Resistant) and LRA5166 (Susceptible), infected with leaf hopper, was employed using a nano LC-MS/MS approach. A total of 1402 proteins varied significantly between leaf hopper-infected and control plants. The resistant and susceptible genotypes had differentially expressed proteins (DEPs) of 743 and 659, respectively. Functional annotation of DEPs revealed that the DEPs were primarily associated with stress response, hormone synthesis, photosynthesis, cell wall, and secondary metabolites. Notably, DEPs such as polyphenol oxidase, carboxypeptidase, heat shock proteins, protein BTR1-like isoform X2, chaperone protein ClpB1, and β glucosidase factors associated with environmental stress response were also detected. Quantitative real-time PCR (qRT-PCR) analysis confirmed a positive correlation between protein abundances and transcripts for all genes. Collectively, this study provides the molecular mechanisms associated with cotton defense responses against leaf hopper. SIGNIFICANCE STATEMENT: Cotton, a natural fiber, assumes a pivotal role as a raw material for textile industries, thereby bearing significant importance in the global economy. The cotton production sector is considerably affected by both biotic and abiotic stresses. The cotton leaf hopper (Amrasca biguttula biguttula (Ishida)) stands as a polyphagous insect, emerging as a dominant sap-feeding pest of the cotton crop. The continuous onslaught of sap-feeding insects on cotton plants has a detrimental impact, with leaf hoppers potentially causing yield reductions of up to 50%. Therefore, comprehending the molecular interplay between cotton and leaf hopper, elucidated at the proteome level, holds promise for more effective pest management strategies. This approach holds the potential to offer insights that contribute to the development of leaf hopper-resistant cotton varieties.

Micro-CT data of complete metamorphosis process in Harmonia axyridis

Insect metamorphosis involves significant changes in insect internal structure and is thus a critical focus of entomological research. Investigating the morphological transformation of internal structures is vital to understanding the origins of adult insect organs. Beetles are among the most species-rich groups in insects, but the development and transformation of their internal organs have yet to be systematically documented. In this study, we have acquired a comprehensive dataset that includes 27 detailed whole-body tomographic image sets of Harmonia axyridis, spanning from the prepupal to the pupal stages. Utilizing this data, we have created intricate 3D models of key internal organs, encompassing the brain, ventral nerve cord, digestive and excretion systems, as well as the body wall muscles. These data documented the transformation process of these critical organs and correlations between the origin of adult and larval organs and can be used to enhance the understanding of holometabolous adult organ genesis and offers a valuable reference model for investigating complete metamorphosis in insects.

Endosymbiotic Fungal Diversity and Dynamics of the Brown Planthopper across Developmental Stages, Tissues, and Sexes Revealed Using Circular Consensus Sequencing

Endosymbiotic fungi play an important role in the growth and development of insects. Understanding the endosymbiont communities hosted by the brown planthopper (BPH; Nilaparvata lugens Stål), the most destructive pest in rice, is a prerequisite for controlling BPH rice infestations. However, the endosymbiont diversity and dynamics of the BPH remain poorly studied. Here, we used circular consensus sequencing (CCS) to obtain 87,131 OTUs (operational taxonomic units), which annotated 730 species of endosymbiotic fungi in the various developmental stages and tissues. We found that three yeast-like symbionts (YLSs), Polycephalomyces prolificus, Ophiocordyceps heteropoda, and Hirsutella proturicola, were dominant in almost all samples, which was especially pronounced in instar nymphs 4-5, female adults, and the fat bodies of female and male adult BPH. Interestingly, honeydew as the only in vitro sample had a unique community structure. Various diversity indices might indicate the different activity of endosymbionts in these stages and tissues. The biomarkers analyzed using LEfSe suggested some special functions of samples at different developmental stages of growth and the active functions of specific tissues in different sexes. Finally, we found that the incidence of occurrence of three species of Malassezia and Fusarium sp. was higher in males than in females in all comparison groups. In summary, our study provides a comprehensive survey of symbiotic fungi in the BPH, which complements the previous research on YLSs. These results offer new theoretical insights and practical implications for novel pest management strategies to understand the BPH-microbe symbiosis and devise effective pest control strategies.

Modeling the 3-dimensional structure of the silkworm's spinning apparatus in silk production

The silk-spinning process of the silkworms transforms the liquid silk solution to a solid state under mild conditions, making it an attractive model for bioinspiration However, the precise mechanism behind silk expulsion remains largely unknown. Here we selected the silkworms as representative models to investigate the silk-spinning mechanism. We used serial block-face scanning electron microscopy (SBF-SEM) to reconstruct the three-dimensional structures of the spinnerets in silkworms at various stages and with different gene backgrounds. By comparing the musculature and duct deformation of these spinneret models during the spinning process, we were able to simulate the morphological changes of the spinneret. Based on the results, we proposed three essential factors for silkworm spinning: the pressure generated by the silk gland, the opening duct, and the pulling force generated by head movement. Understanding the silkworm spinning process provides insights into clarify the fluid-ejecting mechanism of a group of animals. Moreover, these findings are helpful to the development of biomimetic spinning device that mimics the push-and-pull dual-force system in silkworms. STATEMENT OF SIGNIFICANCE: The silkworms' spinning system produces fibers under mild conditions, making it an ideal candidate for bioinspiration. However, the mechanism of silk expulsion is unknown, and the three-dimensional structure of the spinneret is still uncertain. In this study, we reconstructed a detailed 3-dimensional model of the spinneret at near-nanometer resolution, and for the first time, we observed the changes that occur before and during the silk-spinning process. Our reconstructed models suggested that silkworms have the ability to control the spinning process by opening or closing the spinning duct. During the continuously spinning period, both the pressure generated by the silk gland and the pulling force resulting from head movement work in tandem to expel the silk solution. We believe that gaining a full understanding of the spinning process steps can advance our ability to spin synthetic fibers with properties comparable to those of native fibers by mimicking the natural spinning process.

Anatomy of an agricultural antagonist: Feeding complex structure and function of three xylem sap-feeding insects illuminated with synchrotron-based 3D imaging

Many insects feed on xylem or phloem sap of vascular plants. Although physical damage to the plant is minimal, the process of insect feeding can transmit lethal viruses and bacterial pathogens. Disparities between insect-mediated pathogen transmission efficiency have been identified among xylem sap-feeding insects; however, the mechanistic drivers of these trends are unclear. Identifying and understanding the structural factors and associated integrated functional components that may ultimately determine these disparities are critical for managing plant diseases. Here, we applied synchrotron-based X-ray microcomputed tomography to digitally reconstruct the morphology of three xylem sap-feeding insect vectors of plant pathogens: Graphocephala atropunctata (blue-green sharpshooter; Hemiptera, Cicadellidae) and Homalodisca vitripennis (glassy-winged sharpshooter; Hemiptera, Cicadellidae), and the spittlebug Philaenus spumarius (meadow spittlebug; Hemiptera, Aphrophoridae). The application of this technique revealed previously undescribed anatomical features of these organisms, such as key components of the salivary complex. The visualization of the 3D structure of the precibarial valve led to new insights into the mechanism of how this structure functions. Morphological disparities with functional implications between taxa were highlighted as well, including the morphology and volume of the cibarial dilator musculature responsible for extracting xylem sap, which has implications for force application capabilities. These morphological insights will be used to target analyses illuminating functional differences in feeding behavior.

A plant cytorhabdovirus modulates locomotor activity of insect vectors to enhance virus transmission

Transmission of many plant viruses relies on phloem-feeding insect vectors. However, how plant viruses directly modulate insect behavior is largely unknown. Barley yellow striate mosaic virus (BYSMV) is transmitted by the small brown planthopper (SBPH, Laodelphax striatellus). Here, we show that BYSMV infects the central nervous system (CNS) of SBPHs, induces insect hyperactivity, and prolongs phloem feeding duration. The BYSMV accessory protein P6 interacts with the COP9 signalosome subunit 5 (LsCSN5) of SBPHs and suppresses LsCSN5-regulated de-neddylation from the Cullin 1 (CUL1), hereby inhibiting CUL1-based E3 ligases-mediated degradation of the circadian clock protein Timeless (TIM). Thus, virus infection or knockdown of LsCSN5 compromises TIM oscillation and induces high insect locomotor activity for transmission. Additionally, expression of BYSMV P6 in the CNS of transgenic Drosophila melanogaster disturbs circadian rhythm and induces high locomotor activity. Together, our results suggest the molecular mechanisms whereby BYSMV modulates locomotor activity of insect vectors for transmission.

Minimal resin embedding of SBF-SEM samples reduces charging and facilitates finding a surface-linked region of interest

BACKGROUND

For decoding the mechanism of how cells and organs function information on their ultrastructure is essential. High-resolution 3D imaging has revolutionized morphology. Serial block face scanning electron microscopy (SBF-SEM) offers non-laborious, automated imaging in 3D of up to ~ 1 mm3 large biological objects at nanometer-scale resolution. For many samples there are obstacles. Quality imaging is often hampered by charging effects, which originate in the nonconductive resin used for embedding. Especially, if the imaged region of interest (ROI) includes the surface of the sample and neighbours the empty resin, which insulates the object. This extra resin also obscures the sample's morphology, thus making navigation to the ROI difficult.

RESULTS

Using the example of small arthropods and a fish roe we describe a workflow to prepare samples for SBF-SEM using the minimal resin (MR) embedding method. We show that for imaging of surface structures this simple approach conveniently tackles and solves both of the two major problems-charging and ROI localization-that complicate imaging of SBF-SEM samples embedded in an excess of overlying resin. As the surface ROI is not masked by the resin, samples can be precisely trimmed before they are placed into the imaging chamber. The initial approaching step is fast and easy. No extra trimming inside the microscope is necessary. Importantly, charging is absent or greatly reduced meaning that imaging can be accomplished under good vacuum conditions, typically at the optimal high vacuum. This leads to better resolution, better signal to noise ratio, and faster image acquisition.

CONCLUSIONS

In MR embedded samples charging is minimized and ROI easily targeted. MR embedding does not require any special equipment or skills. It saves effort, microscope time and eventually leads to high quality data. Studies on surface-linked ROIs, or any samples normally surrounded by the excess of resin, would benefit from adopting the technique.

Sakuranetin protects rice from brown planthopper attack by depleting its beneficial endosymbionts

Plants produce chemical defenses that poison insect herbivores or deter their feeding, but herbivores are also accompanied by microbial endosymbionts crucial for their nutrition, reproduction, and fitness. Hence, plant defenses could target a herbivore's beneficial endosymbionts, but this has not yet been demonstrated. Here, we studied flavonoids that are induced when rice is attacked by a phloem-feeding pest, the brown planthopper (BPH), which harbors beneficial yeast-like symbionts (YLS) essential for insect nutrition, such as by remedying deficiencies in sterols. BPH attack dramatically increased sakuranetin accumulations in leaf sheaths and phloem exudates. Sakuranetin is an antifungal phytoalexin derived from the antibacterial precursor, naringenin, via catalysis of naringenin-O-methyltransferase (NOMT). When added to artificial diets, sakuranetin decreased BPH survivorship, suggesting that it functions as an induced defense. Mutation of NOMT abolished sakuranetin accumulation and increased BPH oviposition and hatching rates. High-throughput amplicon sequencing revealed that BPH fed on sakuranetin-deficient nomt lines were enriched in YLS with only minor changes in the bacterial endosymbionts, compared to those feeding on sakuranetin-rich wild-type (WT) plants. In-vitro feeding of sakuranetin suggested that this flavonoid directly inhibited the growth of YLS. BPH feeding on nomt lines accumulated higher cholesterol levels, which might be attributed to increases in the supply of sterol precursors from the YLS, while nomt lines suffered more damage than WT plants did from BPH herbivory. BPH-elicited accumulation of sakuranetin requires intact jasmonate (JA) signaling. This study reveals that rice uses a JA-induced antifungal flavonoid phytoalexin in defense against BPH by inhibiting its beneficial endosymbionts.

RNAi-mediated silencing of an egg-specific gene Nllet1 results in hatch failure in the brown planthopper

BACKGROUND

The brown planthopper (BPH) is one of the most destructive agricultural pests in Asia. RNA interference (RNAi)-mediated pest management has been under development for years, and the selection of appropriate target genes is important for pest-targeted RNAi. C-type lectins (CTLs) are a class of genes that perform a variety of functions, such as the regulation of growth and development.

RESULTS

A CTL-S protein named Nllet1, containing a single calcium ion (Ca²⁺ )-dependent carbohydrate-binding domain (CRD) with a conserved triplet motif QPD was identified and functionally characterized in BPH. Expression profiles at both the transcriptional and translational levels show that Nllet1 accumulates during the serosal cuticle (SC) formation period. Immunofluorescence and immunogold labeling further demonstrated that Nllet1 is located in the serosal endocuticle (en-SC). Maternal RNAi-mediated silencing of Nllet1 disrupted the SC structure, accompanied by a loss of the outward barrier and 100% embryo mortality. Injection of 10 ng dsNllet1 or dsNllet1' per female adult BPH resulted in a total failure of egg hatching.

CONCLUSION

Nllet1 is essential for SC formation and embryonic development in BPH, which helps us understand the important roles of CTL-Ss. Additionally, BPH eggs show high sensitivity to the depletion of Nllet1. This study indicates that Nllet1 is a promising candidate gene that can be used to develop RNAi-based control strategies at the BPH egg stage, and it can also be used as a target for developing novel ovicides. © 2022 Society of Chemical Industry.

Evolution and ontogeny of bacteriocytes in insects

The ontogenetic origins of the bacteriocytes, which are cells that harbour bacterial intracellular endosymbionts in multicellular animals, are unknown. During embryonic development, a series of morphological and transcriptional changes determine the fate of distinct cell types. The ontogeny of bacteriocytes is intimately linked with the evolutionary transition of endosymbionts from an extracellular to an intracellular environment, which in turn is linked to the diet of the host insect. Here we review the evolution and development of bacteriocytes in insects. We first classify the endosymbiotic occupants of bacteriocytes, highlighting the complex challenges they pose to the host. Then, we recall the historical account of the discovery of bacteriocytes. We then summarize the molecular interactions between the endosymbiont and the host. In addition, we illustrate the genetic contexts in which the bacteriocytes develop, with examples of the genetic changes in the hosts and endosymbionts, during specific endosymbiotic associations. We finally address the evolutionary origin as well as the putative ontogenetic or developmental source of bacteriocytes in insects.

Volume electron microscopy

Life exists in three dimensions, but until the turn of the century most electron microscopy methods provided only 2D image data. Recently, electron microscopy techniques capable of delving deep into the structure of cells and tissues have emerged, collectively called volume electron microscopy (vEM). Developments in vEM have been dubbed a quiet revolution as the field evolved from established transmission and scanning electron microscopy techniques, so early publications largely focused on the bioscience applications rather than the underlying technological breakthroughs. However, with an explosion in the uptake of vEM across the biosciences and fast-paced advances in volume, resolution, throughput and ease of use, it is timely to introduce the field to new audiences. In this Primer, we introduce the different vEM imaging modalities, the specialized sample processing and image analysis pipelines that accompany each modality and the types of information revealed in the data. We showcase key applications in the biosciences where vEM has helped make breakthrough discoveries and consider limitations and future directions. We aim to show new users how vEM can support discovery science in their own research fields and inspire broader uptake of the technology, finally allowing its full adoption into mainstream biological imaging.

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.

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.

Chitin synthase 1 and five cuticle protein genes are involved in serosal cuticle formation during early embryogenesis to enhance eggshells in Nilaparvata lugens

Many holo- and hemimetabolous insects enhance their eggshells during embryogenesis by forming a serosal cuticle (SC). To date, scholarly understanding of the SC composition and SC-related gene functions has been limited, especially for hemimetabolous insects. In this study, we initially performed transmission electron microscopic (TEM) observation and chitin staining of the SC in Nilaparvata lugens, a hemimetabolous rice pest known as the brown planthopper (BPH). We confirmed that the SC was a chitin-rich lamellar structure deposited gradually during the early embryogenesis. Parental RNA interference (RNAi) against Nilaparvata lugens chitin synthase 1 (NlCHS1) in newly emerged and matured females resulted in decreases of egg hatchability by 100% and 76%, respectively. Ultrastructural analyses revealed loss of the lamellar structure of the SC in dsNlCHS1-treated eggs. According to temporal expression profiles, five cuticle protein coding genes, NlugCpr1/2/3/8/90, were specifically or highly expressed during the SC formation period, and NlugCpr1/2/3/90 were further detected in 72 h eggshells extract by ultra-performance liquid chromatography-tandem mass spectrometry/mass spectrometry. NlugCpr2/3/90 were likely three SC-specific cuticle proteins. TEM observations of the SC following parental RNAi against NlugCpr1/2/3/8/90 demonstrated that NlugCpr3/8/90 were essential for SC formation. The study provided an understanding of the SC formation process and SC-related cuticle proteins in BPHs, which offer potential targets for pest control in the egg stage as well.

Intersex Plays a Role in Microbial Homeostasis in the Brown Planthopper

Simple Summary

RNAi-mediated knockdown of intersex in the newly emerged Nilaparvata lugens leads to abnormal expansion of the copulatory bursa by infection filled with bacteria. RNA-seq analysis shows a significant enrichment of immune defense genes responsive to bacteria in differentially expressed genes (DEGs). Moreover, inhibition of intersex expression by dsRNA treatment results in changes in the richness index of symbiotic microorganisms in copulatory bursa, fat body, and midgut of the planthopper. Specifically, significant changes are observed in the microbial community composition of the copulatory bursa. These findings reveal the function of intersex in maintaining microbial homeostasis in this insect, thereby providing insight to improve the pest control strategies.

Abstract

Insects harbor a wide variety of symbiotic microorganisms that are capable of regulating host health and promoting host adaptation to their environment and food sources. However, there is little knowledge concerning the mechanisms that maintain the microbial community homeostasis within insects. In this study, we found that the intersex (ix) gene played an essential role in maintaining microbial homeostasis in the brown planthopper (BPH), Nilaparvata lugens. Injection of the double-strand RNA targeting N. lugens ix (Nlix) into the newly emerged females resulted in abnormal expansion of the copulatory bursa of BPH after mating. Further observation by transmission electron microscopy (TEM) revealed that the abnormally enlarged copulatory bursa resulting from dsNlix treatment was full of microorganisms, while in contrast, the copulatory bursa of dsGFP-treated individuals stored a large number of sperm accompanied by a few bacteria. Moreover, RNA-seq analysis showed that the gene responses to bacteria were remarkably enriched in differentially expressed genes (DEGs). In addition, 16s rRNA sequencing indicated that, compared with control samples, changes in the composition of microbes presented in dsNlix-treated copulatory bursa. Together, our results revealed the immune functions of the Nlix gene in maintaining microbial homeostasis and combating infection in BPH.