Insights into the insect salivary gland proteome: diet-associated changes in caterpillar labial salivary proteins

The effect of chlorogenic acid, a potential botanical insecticide, on gene transcription and protein expression of carboxylesterases in the armyworm (Mythimna separata)

Chlorogenic acid (CGA) is a potential botanical insecticide metabolite that naturally occurs in various plants. Our previous studies revealed CGA is sufficient to control the armyworm Mythimna separata. In this study, we conducted a proteomic analysis of saliva collected from M. separata following exposure to CGA and found that differentially expressed proteins (DEPs) treated with CGA for 6 h and 24 h were primarily enriched in glutathione metabolism and the pentose phosphate pathway. Notably, we observed six carboxylesterase (CarE) proteins that were enriched at both time points. Additionally, these corresponding genes were expressed at levels 5.05 to 130.25 times higher in our laboratory-selected resistance strains. We also noted a significant increase in the enzyme activity of carboxylesterase following treatments with varying CGA concentrations. Finally, we confirmed that knockdown of MsCarE14, MsCarE28, and MsCCE001h decreased the susceptibility to CGA in resistance strain, indicating three CarE genes play crucial roles in CGA detoxification. This study presents the first report on the salivary proteomics of M. separata, offering valuable insights into the role of salivary proteins. Moreover, the determination of CarE mediated susceptibility change to CGA provides new targets for agricultural pest control and highlights the potential insecticide resistance mechanism for pest resistance management.

Protein and volatile contents in the mandibular gland of the sugarcane borer Diatraea saccharalis (Lepidoptera: Crambidae)

The sugarcane borer Diatraea saccharalis (Lepidoptera: Crambidae) is an important sugarcane pest and mechanical injuries caused through the mandibles can allow pathogen infections. The mandibles of D. saccharalis, as well as other insects, are associated with mandibular glands with a possible function in food intake and mouthparts lubrication; however, the chemical composition of the secretion is poorly known and its elucidation is important for the comprehensive understanding of plant-insect interactions. This study characterized some proteins and volatiles in the mandibular glands of D. saccharalis larvae. MALDI-TOF/TOF mass spectrometry allowed the identification of 24 predicted proteins within 10 functional classes, including the transport and metabolism of carbohydrates, lipids, amino acids, and nucleotides; Posttranslational protein modifications; energy conversion; intracellular trafficking; transcription; translation; and cytoskeleton function. Metabolites identified from GC/MS analysis revealed the presence of hydrocarbons classified as alcohols, ether, alkanes, and esters with differences in their relative abundance. Linolenic acid, the most abundant metabolite found in this gland, when conjugated with amino acids, can be an elicitor in the plant-herbivore interaction. The results suggest the occurrence of digestive and defensive biochemical components, which may contribute to understanding of the multifunctional roles of the mandibular gland secretion of D. saccharalis larvae during feeding activity.

Impact of Future Elevated Carbon Dioxide on C3 Plant Resistance to Biotic Stresses

Before the end of the century, atmospheric carbon dioxide levels are predicted to increase to approximately 900 ppm. This will dramatically affect plant physiology and influence environmental interactions and, in particular, plant resistance to biotic stresses. This review is a broad survey of the current research on the effects of elevated CO₂ (eCO₂) on phytohormone-mediated resistance of C₃ agricultural crops and related model species to pathogens and insect herbivores. In general, while plants grown in eCO₂ often have increased constitutive and induced salicylic acid levels and suppressed induced jasmonate levels, there are exceptions that implicate other environmental factors, such as light and nitrogen fertilization in modulating these responses. Therefore, this review sets the stage for future studies to delve into understanding the mechanistic basis behind how eCO₂ will affect plant defensive phytohormone signaling pathways under future predicted environmental conditions that could threaten global food security to inform the best agricultural management practices.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Stomata-mediated interactions between plants, herbivores, and the environment

Stomata play a central role in plant responses to abiotic and biotic stresses. Existing knowledge regarding the roles of stomata in plant stress is centered on abiotic stresses and plant-pathogen interactions, but how stomata influence plant-herbivore interactions remains largely unclear. Here, we summarize the functions of stomata in plant-insect interactions and highlight recent discoveries of how herbivores manipulate plant stomata. Because stomata are linked to interrelated physiological processes in plants, herbivory-induced changes in stomatal dynamics might have cellular, organismic, and/or even community-level impacts. We summarize our current understanding of how stomata mediate plant responses to herbivory and environmental stimuli, propose how herbivores may influence these responses, and identify key knowledge gaps in plant-herbivore interactions.

Transcriptomic analysis of salivary gland and proteomic analysis of oral secretion in Helicoverpa armigera under cotton plant leaves, gossypol, and tannin stresses

Gossypol and tannin are involved in important chemical defense processes in cotton plants. In this study, we used transcriptomics and proteomics to explore the changes in salivary gland functional genes and oral secretion (OS) proteins after feeding with artificial diet (containing gossypols and tannins) and cotton plant leaves. We found that dietary cotton plant leaves, gossypols and tannins exerted adverse impacts on the genes that regulated the functions of peptidase, GTPase, glycosyl hydrolases in the salivary glands of the Helicoverpa armigera (H. armigera). However, GST, UGT, hydrolases, and lipase genes were up-regulated to participate in the detoxification and digestive of H. armigera. The oral secretory proteins of H. armigera were significantly inhibited under the stress of gossypol and tannin, such as enzyme activity, but some proteins (such as PZC71358.1) were up-regulated and involved in immune and digestive functions. The combined analysis of transcriptomics and metabolomics showed a weak correlation, and the genes and proteins involved were mainly in digestive enzyme activities. Our work clarifies the deleterious physiological impacts of gossypols and tannins on H. armigera and reveals the mechanism by which H. armigera effectively mitigate the phytotoxic effects through detoxification and immune systems.

Horizontal Gene Transfer and Gene Duplication of β-Fructofuranosidase Confer Lepidopteran Insects Metabolic Benefits

Horizontal gene transfer (HGT) is a potentially critical source of material for ecological adaptation and the evolution of novel genetic traits. However, reports on posttransfer duplication in organism genomes are lacking, and the evolutionary advantages conferred on the recipient are generally poorly understood. Sucrase plays an important role in insect physiological growth and development. Here, we performed a comprehensive analysis of the evolution of insect β-fructofuranosidase transferred from bacteria via HGT. We found that posttransfer duplications of β-fructofuranosidase were widespread in Lepidoptera and sporadic occurrences of β-fructofuranosidase were found in Coleoptera and Hymenoptera. β-fructofuranosidase genes often undergo modifications, such as gene duplication, differential gene loss, and changes in mutation rates. Lepidopteran β-fructofuranosidase gene (SUC) clusters showed marked divergence in gene expression patterns and enzymatic properties in Bombyx mori (moth) and Papilio xuthus (butterfly). We generated SUC1 mutations in B. mori using CRISPR/Cas9 to thoroughly examine the physiological function of SUC. BmSUC1 mutant larvae were viable but displayed delayed growth and reduced sucrase activities that included susceptibility to the sugar mimic alkaloid found in high concentrations in mulberry. BmSUC1 served as a critical sucrase and supported metabolic homeostasis in the larval midgut and silk gland, suggesting that gene transfer of β-fructofuranosidase enhanced the digestive and metabolic adaptation of lepidopteran insects. These findings highlight not only the universal function of β-fructofuranosidase with a link to the maintenance of carbohydrate metabolism but also an underexplored function in the silk gland. This study expands our knowledge of posttransfer duplication and subsequent functional diversification in the adaptive evolution and lineage-specific adaptation of organisms.

Protein expression in submandibular glands of young rats is modified by a high-fat/high-sugar maternal diet

OBJECTIVE

Maternal diet has consequences on many organs of the offspring, but salivary glands have received little attention despite the importance of the saliva secretory function in oral health and control of food intake. The objective of this work was therefore to document in rats the impact of maternal high-fat/high-sugar diet (Western Diet) on submandibular glands of the progeny.

DESIGN

Sprague-Dawley rat dams were fed either a Western diet or control diet during gestation and lactation and their pups were sacrificed 25 days after birth. The pups' submandibular gland protein content was characterized by means of 2D-electrophoresis followed by LC-MS/MS. Data were further analyzed by Gene Ontology enrichment analysis and protein-protein interactions mapping. The expression of two specific proteins was also evaluated using immunohistochemistry.

RESULTS

Combining both male and female pups (n = 18), proteome analysis revealed that proteins involved in protein quality control (e.g. heat shock proteins, proteasome sub-units) and microtubule proteins were over-expressed in Western diet conditions, which may translate intense metabolic activity. A cluster of proteins controlling oxidative stress (e.g. Glutathione peroxidases, peroxiredoxin) and enhancement of the antioxidant activity molecular function were also characteristic of maternal Western diet as well as under-expression of annexin A5. The down-regulating effect of maternal Western diet on Annexin A5 expression was significant only for males (p < 0.05).

CONCLUSIONS

A maternal Western diet modifies the protein composition of the offspring's salivary glands, which may have consequences on the salivary function.

Proteomic analysis of labial saliva of the generalist cabbage looper (Trichoplusia ni) and its role in interactions with host plants

Insect saliva is one of the first secretions to come in contact with plants during feeding. The composition and role of caterpillar saliva has not been as thoroughly studied as that of sucking insects. This study focuses on characterizing the proteome of the cabbage looper (Trichoplusia ni) saliva using iTRAQ labeling and LC-MS/MS. We also measured how the saliva proteome changed when larvae were reared on different diets - cabbage, tomato, and an artificial pinto bean diet. We identified 254 proteins in the saliva out of which 63 were differentially expressed. A large percentage (56%) of the proteins identified function in protein metabolism, followed by proteins involved in vesicle transport (6%) and oxidoreductase activity (5%), among other categories. Several proteins identified are antioxidants or reactive oxygen species (ROS) scavengers. Among these ROS scavengers, we identified a catalase and further analyzed its gene expression and enzymatic activity. We also applied commercial, purified catalase on tomato and measured the activity of defensive proteins - trypsin proteinase inhibitor, polyphenol oxidase and peroxidase. Catalase gene expression was significantly higher in the salivary glands of larvae fed on tomato. Also, catalase suppressed the induction of tomato trypsin proteinase inhibitor levels, but not the induction of polyphenol oxidase or peroxidase. These results add to our understanding of proteomic plasticity in saliva and its role in herbivore offense against plant defenses.

Altering Plant Defenses: Herbivore-Associated Molecular Patterns and Effector Arsenal of Chewing Herbivores

Chewing herbivores, such as caterpillars and beetles, while feeding on the host plant, cause extensive tissue damage and release a wide array of cues to alter plant defenses. Consequently, the cues can have both beneficial and detrimental impacts on the chewing herbivores. Herbivore-associated molecular patterns (HAMPs) are molecules produced by herbivorous insects that aid them to elicit plant defenses leading to impairment of insect growth, while effectors suppress plant defenses and contribute to increased susceptibility to subsequent feeding by chewing herbivores. Besides secretions that originate from glands (e.g., saliva) and fore- and midgut regions (e.g., oral secretions) of chewing herbivores, recent studies have shown that insect frass and herbivore-associated endosymbionts also play a critical role in modulating plant defenses. In this review, we provide an update on a growing body of literature that discusses the chewing insect HAMPs and effectors and the mechanisms by which they modulate host defenses. Novel "omic" approaches and availability of new tools will help researchers to move forward this discipline by identifying and characterizing novel insect HAMPs and effectors and how these herbivore-associated cues are perceived by host plant receptors.

Ultrastructure of male accessory glands in the scorpionfly Sinopanorpa tincta (Navás, 1931) (Mecoptera: Panorpidae)

The ultrastructure of male reproductive accessory glands was investigated in the scorpionfly Sinopanorpa tincta (Navás, 1931) (Mecoptera: Panorpidae) using light and transmission electron microscopy. The male accessory glands comprise one pair of mesodermal glands (mesadenia) and six pairs of ectodermal glands (ectadenia). The former opens into the vasa deferentia and the latter into the ejaculatory sac. The mesadenia consist of a mono-layered elongated columnar epithelium, the cells of which are highly microvillated and extrude secretory granules by means of merocrine mechanisms. The epithelium of ectadenia consists of two types of cells: the large secretory cells and the thin duct-forming cells. These two types of cells that join with a cuticular duct constitute a functional glandular unit, corresponding to the class III glandular cell type of Noirot and Quennedey. The cuticular duct consists of a receiving canal and a conducting canal. The secretory granules were taken up by the receiving canal and then plunged into the lumen through the conducting canal.

Quantitative proteomic analysis of the fall armyworm saliva

Lepidopteran larvae secrete saliva on plant tissues during feeding. Components in the saliva may aid in food digestion, whereas other components are recognized by plants as cues to elicit defense responses. Despite the ecological and economical importance of these plant-feeding insects, knowledge of their saliva composition is limited to a few species. In this study, we identified the salivary proteins of larvae of the fall armyworm (FAW), Spodoptera frugiperda; determined qualitative and quantitative differences in the salivary proteome of the two host races-corn and rice strains-of this insect; and identified changes in total protein concentration and relative protein abundance in the saliva of FAW larvae associated with different host plants. Quantitative proteomic analyses were performed using labeling with isobaric tags for relative and absolute quantification followed by liquid chromatography-tandem mass spectrometry. In total, 98 proteins were identified (>99% confidence) in the FAW saliva. These proteins were further categorized into five functional groups: proteins potentially involved in (1) plant defense regulation, (2) herbivore offense, (3) insect immunity, (4) detoxification, (5) digestion, and (6) other functions. Moreover, there were differences in the salivary proteome between the FAW strains that were identified by label-free proteomic analyses. Thirteen differentially identified proteins were present in each strain. There were also differences in the relative abundance of eleven salivary proteins between the two FAW host strains as well as differences within each strain associated with different diets. The total salivary protein concentration was also different for the two strains reared on different host plants. Based on these results, we conclude that the FAW saliva contains a complex mixture of proteins involved in different functions that are specific for each strain and its composition can change plastically in response to diet type.

Arabidopsis glucosinolates trigger a contrasting transcriptomic response in a generalist and a specialist herbivore

Phytophagous insects have to deal with toxic defense compounds from their host plants. Although it is known that insects have evolved genes and mechanisms to detoxify plant allochemicals, how specialist and generalist precisely respond to specific secondary metabolites at the molecular level is less understood. Here we studied the larval performance and transcriptome of the generalist moth Heliothis virescens and the specialist butterfly Pieris brassicae feeding on Arabidopsis thaliana genotypes with different glucosinolate (GS) levels. H. virescens larvae gained significantly more weight on the GS-deficient mutant quadGS compared to wild-type (Col-0) plants. On the contrary, P. brassicae was unaffected by the presence of GS and performed equally well on both genotypes. Strikingly, there was a considerable differential gene expression in H. virescens larvae feeding on Col-0 compared to quadGS. In contrast, compared to H. virescens, P. brassicae displayed a much-reduced transcriptional activation when fed on both plant genotypes. Transcripts coding for putative detoxification enzymes were significantly upregulated in H. virescens, along with digestive enzymes and transposable elements. These data provide an unprecedented view on transcriptional changes that are specifically activated by GS and illustrate differential molecular responses that are linked to adaptation to diet in lepidopteran herbivores.

Genomics of Lepidoptera saliva reveals function in herbivory

Lepidoptera herbivores deposit copious amounts of saliva when feeding. Their saliva is produced by the paired mandibular and labial glands and evidence indicates that it may play an important role in allowing an herbivore to establish on its host plant. Genomic studies of Lepidoptera saliva are beginning to reveal the role of saliva in herbivory. Molecules involved in digestion, detoxification, immunity, defense against plant secondary chemicals, chemoreception and so on have been identified using high throughput genomic tools. These genomic tools have also revealed changes that occur in Lepidoptera saliva when caterpillars feed on different host plants. However, there are other factors either biotic or abiotic (e.g., larval stage, larval health, temperature, water stress, etc.) that might also affect its composition. Though further functional and ecological studies are still necessary to fully understand the role of Lepidoptera saliva on herbivory, here we review current trends.

Resource allocation and compensation during development in holometabolous insects

We provide an extensive review on current knowledge and future research paths on the topic of resource allocation and compensation during development in holometabolous insects, emphasizing the role of resource management during development, and how compensatory mechanisms may be acting to remediate nutritional deficiencies carried over from earlier stages of development. We first review resource allocation in "open" and "closed" developmental stages and then move on to the topic of modelling resource allocation and its trade-offs. In doing so, we review novel methodological developments such as response-surface methods and mixture experiments as well as nutritional geometry. We also dwell on the fascinating topic of compensatory physiology and behavior. We finish by discussing future research paths, among them the emerging field of nutrigenomics and gut microbiome, which will shed light into the yet poorly understood role of the symbiotic microbiota in nutrient compensation or assimilation.

Insect-induced effects on plants and possible effectors used by galling and leaf-mining insects to manipulate their host-plant

Gall-inducing insects are iconic examples in the manipulation and reprogramming of plant development, inducing spectacular morphological and physiological changes of host-plant tissues within which the insect feeds and grows. Despite decades of research, effectors involved in gall induction and basic mechanisms of gall formation remain unknown. Recent research suggests that some aspects of the plant manipulation shown by gall-inducers may be shared with other insect herbivorous life histories. Here, we illustrate similarities and contrasts by reviewing current knowledge of metabolic and morphological effects induced on plants by gall-inducing and leaf-mining insects, and ask whether leaf-miners can also be considered to be plant reprogrammers. We review key plant functions targeted by various plant reprogrammers, including plant-manipulating insects and nematodes, and functionally characterize insect herbivore-derived effectors to provide a broader understanding of possible mechanisms used in host-plant manipulation. Consequences of plant reprogramming in terms of ecology, coevolution and diversification of plant-manipulating insects are also discussed.

Shared weapons of blood- and plant-feeding insects: Surprising commonalities for manipulating hosts

Insects that reprogram host plants during colonization remind us that the insect side of plant-insect story is just as interesting as the plant side. Insect effectors secreted by the salivary glands play an important role in plant reprogramming. Recent discoveries point to large numbers of salivary effectors being produced by a single herbivore species. Since genetic and functional characterization of effectors is an arduous task, narrowing the field of candidates is useful. We present ideas about types and functions of effectors from research on blood-feeding parasites and their mammalian hosts. Because of their importance for human health, blood-feeding parasites have more tools from genomics and other - omics than plant-feeding parasites. Four themes have emerged: (1) mechanical damage resulting from attack by blood-feeding parasites triggers "early danger signals" in mammalian hosts, which are mediated by eATP, calcium, and hydrogen peroxide, (2) mammalian hosts need to modulate their immune responses to the three "early danger signals" and use apyrases, calreticulins, and peroxiredoxins, respectively, to achieve this, (3) blood-feeding parasites, like their mammalian hosts, rely on some of the same "early danger signals" and modulate their immune responses using the same proteins, and (4) blood-feeding parasites deploy apyrases, calreticulins, and peroxiredoxins in their saliva to manipulate the "danger signals" of their mammalian hosts. We review emerging evidence that plant-feeding insects also interfere with "early danger signals" of their hosts by deploying apyrases, calreticulins and peroxiredoxins in saliva. Given emerging links between these molecules, and plant growth and defense, we propose that these effectors interfere with phytohormone signaling, and therefore have a special importance for gall-inducing and leaf-mining insects, which manipulate host-plants to create better food and shelter.

Cues from chewing insects - the intersection of DAMPs, HAMPs, MAMPs and effectors

Chewing herbivores cause massive damage when crushing plant tissues with their mandibles, thus releasing a vast array of cues that may be perceived by the plant to mobilize defenses. Besides releasing damage cues in wounded tissues, herbivores deposit abundant cues from their saliva, regurgitant and feces that trigger herbivore specific responses in plants. Herbivores can manipulate the perception mechanisms and defense signals to suppress plant defenses by secreting effectors and/or by exploiting their associated oral microbes. Recent studies indicate that both the composition of herbivore cues and the plant's ability to recognize them are highly dependent upon the specific plant-herbivore system. There is a growing amount of work on identifying herbivore elicitors and effectors, but the most significant bottleneck in the discipline is the identification and characterization of plant receptors that perceive these herbivore-specific cues.

Mechanisms and ecological consequences of plant defence induction and suppression in herbivore communities

BACKGROUND

Plants are hotbeds for parasites such as arthropod herbivores, which acquire nutrients and energy from their hosts in order to grow and reproduce. Hence plants are selected to evolve resistance, which in turn selects for herbivores that can cope with this resistance. To preserve their fitness when attacked by herbivores, plants can employ complex strategies that include reallocation of resources and the production of defensive metabolites and structures. Plant defences can be either prefabricated or be produced only upon attack. Those that are ready-made are referred to as constitutive defences. Some constitutive defences are operational at any time while others require activation. Defences produced only when herbivores are present are referred to as induced defences. These can be established via de novo biosynthesis of defensive substances or via modifications of prefabricated substances and consequently these are active only when needed. Inducibility of defence may serve to save energy and to prevent self-intoxication but also implies that there is a delay in these defences becoming operational. Induced defences can be characterized by alterations in plant morphology and molecular chemistry and are associated with a decrease in herbivore performance. These alterations are set in motion by signals generated by herbivores. Finally, a subset of induced metabolites are released into the air as volatiles and function as a beacon for foraging natural enemies searching for prey, and this is referred to as induced indirect defence.

SCOPE

The objective of this review is to evaluate (1) which strategies plants have evolved to cope with herbivores and (2) which traits herbivores have evolved that enable them to counter these defences. The primary focus is on the induction and suppression of plant defences and the review outlines how the palette of traits that determine induction/suppression of, and resistance/susceptibility of herbivores to, plant defences can give rise to exploitative competition and facilitation within ecological communities "inhabiting" a plant.

CONCLUSIONS

Herbivores have evolved diverse strategies, which are not mutually exclusive, to decrease the negative effects of plant defences in order to maximize the conversion of plant material into offspring. Numerous adaptations have been found in herbivores, enabling them to dismantle or bypass defensive barriers, to avoid tissues with relatively high levels of defensive chemicals or to metabolize these chemicals once ingested. In addition, some herbivores interfere with the onset or completion of induced plant defences, resulting in the plant's resistance being partly or fully suppressed. The ability to suppress induced plant defences appears to occur across plant parasites from different kingdoms, including herbivorous arthropods, and there is remarkable diversity in suppression mechanisms. Suppression may strongly affect the structure of the food web, because the ability to suppress the activation of defences of a communal host may facilitate competitors, whereas the ability of a herbivore to cope with activated plant defences will not. Further characterization of the mechanisms and traits that give rise to suppression of plant defences will enable us to determine their role in shaping direct and indirect interactions in food webs and the extent to which these determine the coexistence and persistence of species.