Molecular characterization and expression of laccase genes in the salivary glands of the green rice leafhopper, Nephotettix cincticeps (Hemiptera: Cicadellidae)

Proteomic Analysis of Salivary Secretions from the Tea Green Leafhopper, Empoasca flavescens Fabrecius

Saliva plays a crucial role in shaping the compatibility of piercing-sucking insects with their host plants. Understanding the complex composition of leafhopper saliva is important for developing effective and eco-friendly control strategies for the tea green leafhopper, Empoasca flavescens Fabrecius, a major piercing-sucking pest in Chinese tea plantations. This study explored the saliva proteins of tea green leafhopper adults using a custom collection device, consisting of two layers of Parafilm stretched over a sucrose diet. A total of 152 proteins were identified using liquid chromatography-tandem mass spectrometry (LC-MS/MS) following the filter-aided sample preparation (FASP). These proteins were categorized into six groups based on their functions, including enzymes, transport proteins, regulatory proteins, cell structure proteins, other proteins, and unknown proteins. Bioinformatics analyses predicted 16 secreted proteins, which were successfully cloned and transcriptionally analyzed across various tissues and developmental stages. Genes encoding putative salivary secretory proteins, including Efmucin1, EfOBP1, EfOBP2, EfOBP3, Efmucin2, low-density lipoprotein receptor-related protein (EfLRP), EFVg1, and EFVg2, exhibited high expressions in salivary gland (SG) tissues and feeding-associated expressions at different developmental stages. These findings shed light on the potential elicitors or effectors mediating the leafhopper feeding and defense responses in tea plants, providing insights into the coevolution of tea plants and leafhoppers. The study's conclusions open avenues for the development of innovative leafhopper control technologies that reduce the reliance on pesticides in the tea industry.

Functional and evolutionary implications of protein and metal content of leafhopper brochosomes

Brochosomes derived from the specialized glandular segments of the Malpighian tubules (MTs) form superhydrophobic coatings for insects of Membracoidea, and have multiple hypothetical functions. However, the constituents, biosynthesis and evolutionary origin of brochosomes remain poorly understood. We investigated general chemical and physical characteristics of the integumental brochosomes (IBs) of the leafhopper Psammotettix striatus, determined the constituents of IBs, identified the unigenes involved in brochosomal protein synthesis, and investigated the potential associations among brochosomal protein synthesis, amino acid composition of food source, and the possible roles of endosymbionts in brochosome production. The results show that IBs are mainly composed of glycine- and tyrosine-rich proteins and some metal elements, which contain both essential and non-essential amino acids (EAAs and NEAAs) for insects, including EAAs deficient in the sole food source. All 12 unigenes involved in synthesizing the 12 brochosomal proteins (BPs) with high confidence are exclusively highly expressed in the glandular segment of MTs, confirming that brochosomes are synthesized by this segment. The synthesis of BPs is one of the key synapomorphies of Membracoidea but may be lost secondarily in a few lineages. The synthesis of BPs might be related to the symbiosis of leafhoppers/treehoppers with endosymbionts that provide these insects with EAAs, including those are deficient in the sole diet (i.e., plant sap) and could only be made available by the symbionts. We hypothesize that the functional modification of MTs have combined with the application of BPs enabling Membracoidea to colonize and adapt to novel ecological niches, and evolve to the dramatic diversification of this hemipteran group (in particular the family Cicadellidae). This study highlights the importance of evolutionary plasticity and multiple functions of MTs in driving the adaptations and evolution of sap-sucking insects of Hemiptera.

Groundnut Bud Necrosis Virus Modulates the Expression of Innate Immune, Endocytosis, and Cuticle Development-Associated Genes to Circulate and Propagate in Its Vector, Thrips palmi

Thrips palmi (Thysanoptera: Thripidae) is the predominant tospovirus vector in Asia-Pacific region. It transmits economically damaging groundnut bud necrosis virus (GBNV, family Tospoviridae) in a persistent propagative manner. Thrips serve as the alternate host, and virus reservoirs making tospovirus management very challenging. Insecticides and host plant resistance remain ineffective in managing thrips–tospoviruses. Recent genomic approaches have led to understanding the molecular interactions of thrips–tospoviruses and identifying novel genetic targets. However, most of the studies are limited to Frankliniella species and tomato spotted wilt virus (TSWV). Amidst the limited information available on T. palmi–tospovirus relationships, the present study is the first report of the transcriptome-wide response of T. palmi associated with GBNV infection. The differential expression analyses of the triplicate transcriptome of viruliferous vs. nonviruliferous adult T. palmi identified a total of 2,363 (1,383 upregulated and 980 downregulated) significant transcripts. The Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses showed the abundance of differentially expressed genes (DEGs) involved in innate immune response, endocytosis, cuticle development, and receptor binding and signaling that mediate the virus invasion and multiplication in the vector system. Also, the gene regulatory network (GRN) of most significant DEGs showed the genes like ABC transporter, cytochrome P450, endocuticle structural glycoprotein, gamma-aminobutyric acid (GABA) receptor, heat shock protein 70, larval and pupal cuticle proteins, nephrin, proline-rich protein, sperm-associated antigen, UHRF1-binding protein, serpin, tyrosine–protein kinase receptor, etc., were enriched with higher degrees of interactions. Further, the expression of the candidate genes in response to GBNV infection was validated in reverse transcriptase-quantitative real-time PCR (RT-qPCR). This study leads to an understanding of molecular interactions between T. palmi and GBNV and suggests potential genetic targets for generic pest control.

Toxicity of Ribavirin to Spodoptera litura by Inhibiting the Juvenile Hormone

Ribavirin is an antiviral drug showing high and delayed toxicity to the destructive agricultural pest Spodoptera litura. Larvae fed with artificial diets containing ribavirin could not molt successfully and showed abnormal phenotypes, including cuticle melanization and heavy wrinkle of the newly formed procuticle. RNA-Seq analysis suggested that ribavirin has great negative influence on cuticle. Quantitative real-time-polymerase chain reaction results indicated that ribavirin treatment decreased the expression of key genes in juvenile hormone (JH) biosynthesis (CYP15C1 and JH acid methyltransferase) and most cuticle protein genes, whereas the genes in melanin biosynthesis and bursicon genes were upregulated by ribavirin treatment. These results coincided with the decreased titer of JH I, JH II, and JH III determined by enzyme-linked immunosorbent assay, the much thinner procuticle layer exhibited by histopathological examination, and the cuticle melanization after ribavirin treatment. These results provided a valuable theoretical basis for the creation of green insecticides targeting JH and the development of new insecticide derivatives from 1,2,4-triazole.

Arms and ammunitions: effectors at the interface of rice and it's pathogens and pests

The plant immune system has evolved to resist attack by pathogens and pests. However, successful phytopathogens deliver effector proteins into plant cells where they hijack the host cellular machinery to suppress the plant immune responses and promote infection. This manipulation of the host cellular pathways is done by the pathogen using various enzymatic activities, protein- DNA or protein- protein interactions. Rice is one the major economically important crops and its yield is affected by several pathogens and pests. In this review, we summarize the various effectors at the plant- pathogen/ pest interface for the major pathogens and pests of rice, specifically, on the mode of action and target genes of the effector proteins. We then compare this across the major rice pathogens and pests in a bid to understand probable conserved pathways which are under attack from pathogens and pests in rice. This analysis highlights conserved patterns of effector action, as well as unique host pathways targeted by the pathogens and pests.

Knockdown of calreticulin, laccase, and Snf7 Genes Through RNAi Is Not Effective to Control the Asian Citrus Psyllid (Hemiptera: Livideae)

The Asian citrus psyllid, Diaphorina citri Kuwayama, transmits the bacteria Candidatus Liberibacter associated with huanglongbing (HLB), a devastating disease of the citrus industry. The use of genetically modified plants is an alternative to control this vector. Conversely, technology based on RNA interference (RNAi) for silencing specific genes of a target insect could be attempted. This work evaluated the knockdown effect of the target genes calreticulin (DcCRT), laccase (DcLAC), and Snf7 (DcSnf7) by RNAi through feeding D. citri in Murraya paniculata leaves after the uptake of an aqueous solution with dsRNA homologous to each vector target gene. Confocal microscopy revealed the uptake of the fluorescent-labeled dsRNA by detached leaves and the symplastic movement, allowing the ingestion by the feeding insect. A reduction in the survival rate was observed only 144 h after the beginning of feeding with dsRNA targeting DcSnf7; however, no reduction in transcript accumulation. The knockdown of the DcCRT and DcLAC genes was detected only 12 and 96 h after insect feeding, respectively. Additionally, a reduction in amino acid excretion from insects fed with dsRNA targets to DcCRT and DcLAC was observed 120 h after the beginning of feeding. However, the effects of the dsRNAs tested here appear to be minimal, both at the transcriptional and phenotype levels. For most concentrations and time points, no effects were observed. Therefore, the knockdown of genes DcCRT, DcLAC, and DcSnf7 do not appear to have the potential to control of D. citri through RNAi-mediated gene silencing.

Proteomics of the Honeydew from the Brown Planthopper and Green Rice Leafhopper Reveal They Are Rich in Proteins from Insects, Rice Plant and Bacteria

Honeydew is a watery fluid excreted by plant sap-feeding insects. It is a waste product for the insect hosts. However, it plays important roles for other organisms, such as serving as a nutritional source for beneficial insects and bacteria, as well as elicitors and effectors modulating plant responses. In this study, shotgun LC-MS/MS analyses were used to identify the proteins in the honeydew from two important rice hemipteran pests, the brown planthopper (Nilaparvata lugens, BPH) and green rice leafhopper (Nephotettix cincticeps, GRH). A total of 277 and 210 proteins annotated to insect proteins were identified in the BPH and GRH honeydews, respectively. These included saliva proteins that may have similar functions as the saliva proteins, such as calcium-binding proteins and apolipophorin, involved in rice plant defenses. Additionally, a total of 52 and 32 Oryza proteins were identified in the BPH and GRH honeydews, respectively, some of which are involved in the plant immune system, such as Pathogen-Related Protein 10, ascorbate peroxidase, thioredoxin and glutaredoxin. Coincidently, 570 and 494 bacteria proteins were identified from the BPH and GRH honeydews, respectively, which included several well-known proteins involved in the plant immune system: elongation factor Tu, flagellin, GroEL and cold-shock proteins. The results of our study indicate that the insect honeydew is a complex fluid cocktail that contains abundant proteins from insects, plants and microbes, which may be involved in the multitrophic interactions of plants-insects-microbes.

A novel Laccase gene from Litopenaeus vannamei is involved in the immune responses to pathogen infection and oxidative stress

Laccases (Lacs) are copper-containing oxidase enzymes that are found in various plants, fungi, and microorganisms. For invertebrates, particularly insects and crustaceans, Lacs have been shown to be involved in immune responses. In shrimp, a Lac gene has been cloned and functionally characterized, which revealed that it is involved in shrimp anti-pathogen infection. In the present study, a novel Lac gene (LvLac2) was cloned from Litopenaeus vannamei. Real-time RT-PCR analysis showed that LvLac2 is induced by white spot syndrome virus (WSSV)- or Vibrio alginolyticus infection. In addition, the downregulated expression of LvLac2 decreased the cumulative mortality of WSSV- or V. alginolyticus infected shrimps. Moreover, LvLac2 is also induced by oxidative stress. Knocking down the expression of LvLac2 decreased the severity of hepatopancreatic injury caused by oxidative stress, as well as reduced the cumulative shrimp mortality during oxidative stress. Furthermore, gene reporter assays showed that the expression of LvLac2 is regulated by NF-E2-related factor 2, which is the key transcription factor of the oxidative stress response signaling pathway. Our study revealed that LvLac2 not only participates in immune responses against infections in L. vannamei but is also involved in oxidative stress responses.

Laccase Properties, Physiological Functions, and Evolution

Discovered in 1883, laccase is one of the first enzymes ever described. Now, after almost 140 years of research, it seems that this copper-containing protein with a number of unique catalytic properties is widely distributed across all kingdoms of life. Laccase belongs to the superfamily of multicopper oxidases (MCOs)—a group of enzymes comprising many proteins with different substrate specificities and diverse biological functions. The presence of cupredoxin-like domains allows all MCOs to reduce oxygen to water without producing harmful byproducts. This review describes structural characteristics and plausible evolution of laccase in different taxonomic groups. The remarkable catalytic abilities and broad substrate specificity of laccases are described in relation to other copper-containing MCOs. Through an exhaustive analysis of laccase roles in different taxa, we find that this enzyme evolved to serve an important, common, and protective function in living systems.

Characterization of multicopper oxidase genes in the green rice leafhopper, Nephotettix cincticeps (Hemiptera: Cicadellidae), with focus on salivary gland-specific genes

Multicopper oxidase (MCO) enzymes are present ubiquitously and act on diverse substrates. Recently, the presence of multiple MCO genes has been described in many insects. Based on sialotranscriptome data, we identified and comprehensively characterized six MCO genes: NcLac1S, 1G, and 2-5 in the green rice leafhopper, Nephotettix cincticeps (Hemiptera: Cicadellidae). NcLac1S and NcLac1G belong to the MCO1 ortholog of other insects. NcLac2 forms a clade with MCO2s involved in the sclerotization and pigmentation of the cuticle. NcLac3 and NcLac4 form a clade with NlMCO3 -5 of the hemipteran Nilaparvata luges. NcLac5 forms a clade with MCORPs (MCO-related proteins) that lack amino acid residues normally highly conserved in copper-coordinated MCOs. NcLac1S and NcLac3 were specifically expressed in the salivary glands; whereas NcLac5 was primarily expressed in the salivary glands. Only NcLac3 protein is considered to have laccase activity in the salivary glands and salivary sheaths ejected by the insect. NcLac1G expression was relatively high in the testis. NcLac2 and NcLac4 were specifically expressed in the integument and in Malpighian tubules, respectively. Knockdown by RNA interference (RNAi) of either NcLac2 and NcLac5 in nymphs caused high mortality. All NcLac2-knockdown nymphs showed depigmentation and soft cuticle, and eventually died, as did other MCO2-knockdown insects. DsNcLac5-injected nymphs (third, fourth, and fifth-instar) showed high mortality, but injection into adults had no effect on survival or number of eggs deposited, suggesting that NcLac5 is not essential for survival after molting (eclosion). NcLac5 could be a promising target gene for control of N. cincticeps.

Transcriptome-wide responses of adult melon thrips (Thrips palmi) associated with capsicum chlorosis virus infection

Thrips palmi is a widely distributed major agricultural pest in the tropics and subtropics, causing significant losses in cucurbit and solanaceous crops through feeding damage and transmission of tospoviruses. Thrips palmi is a vector of capsicum chlorosis virus (CaCV) in Australia. The present understanding of transmission biology and potential effects of CaCV on T. palmi is limited. To gain insights into molecular responses to CaCV infection, we performed RNA-Seq to identify thrips transcripts that are differentially-abundant during virus infection of adults. De-novo assembly of the transcriptome generated from whole bodies of T. palmi adults generated 166,445 contigs, of which ~24% contained a predicted open reading frame. We identified 1,389 differentially-expressed (DE) transcripts, with comparable numbers up- (708) and down-regulated (681) in virus-exposed thrips compared to non-exposed thrips. Approximately 59% of these DE transcripts had significant matches to NCBI non-redundant proteins (Blastx) and Blast2GO identified provisional functional categories among the up-regulated transcripts in virus-exposed thrips including innate immune response-related genes, salivary gland and/or gut-associated genes and vitellogenin genes. The majority of the immune-related proteins are known to serve functions in lysosome activity and melanisation in insects. Most of the up-regulated oral and extra-oral digestion-associated genes appear to be involved in digestion of proteins, lipids and plant cell wall components which may indirectly enhance the likelihood or frequency of virus transmission or may be involved in the regulation of host defence responses. Most of the down-regulated transcripts fell into the gene ontology functional category of 'structural constituent of cuticle'. Comparison to DE genes responsive to tomato spotted wilt virus in Frankliniella occidentalis indicates conservation of some thrips molecular responses to infection by different tospoviruses. This study assembled the first transcriptome in the genus Thrips and provides important data to broaden our understanding of networks of molecular interactions between thrips and tospoviruses.

The green rice leafhopper, Nephotettix cincticeps (Hemiptera: Cicadellidae), salivary protein NcSP75 is a key effector for successful phloem ingestion

Nephotettix cincticeps, a prevalent rice pest, injects gelling and watery saliva into plant tissues during the sucking process. Certain components within the saliva are believed to interact with plant cellular constituents and play important roles in overcoming host plant defense responses. Based on our previous analysis of the salivary gland transcriptome and secreted saliva proteome of N. cincticeps, in this study, we analyzed the biological functions of salivary protein, NcSP75 (N. cincticepssalivary protein 75 kD). NcSP75, a salivary glands-specific gene, showed low similarities to any previously reported sequences. Knockdown of NcSP75 by RNA interference (RNAi) reduced the longevity of treated nymphs to approximately half of the longevity of controls and caused severe developmental retardation. Furthermore, the knockdown of NcSP75 decreased the survival rate of adults, and reduced the number of deposited eggs and hatched nymphs. Thus, the adverse effects caused by the knockdown of NcSP75 were observed throughout the lifetime of N. cincticeps, when feeding on rice plants. In contrast, no reduction was observed in the survival rate of the knockdown of NcSP75 adults fed on an artificial diet. Electrical penetration graph measurements taken from adult females feeding on rice plants showed a significantly shorter duration of phloem ingestion associated with the knockdown of NcSP75 than the knockdown of the enhanced green fluorescent protein (EGFP). Furthermore, the total sugar content of the honeydew was lower when NcSP75 was knocked down. These results suggest that the NcSP75 protein contribute to successful and sustainable ingestion from the sieve elements of rice plants. The NcSP75 protein of N. cincticeps can, accordingly, be considered as a key effector for establishing compatible interaction with rice plants and could be a potential target for controlling this species.

Laccase 1 gene from Plutella xylostella (PxLac1) and its functions in humoral immune response

Laccase (EC 1.10.3.2) is a phenoloxidase found in many insect species. The Laccase 1 gene from Plutella xylostella (PxLac1) was cloned, and its expression patterns and functions were determined using qPCR and RNAi methods. The results showed that the expression levels of PxLac1 were consistently high in all larval stages, and the most abundant was in the midgut during the 4th instar stage. Moreover, the expression of PxLac1 was up-regulated in response to bacterial infection, and decreased 24 h after being parasitized by Cotesia vestalis. Further analyses indicated that the effect of parasitization on PxLac1 was induced by active C. vestalis Bracovirus (CvBV). Haemocyte-free hemolymph phenoloxidase (PO) activity was suppressed when PxLac1 was treated with RNAi. Our results provide evidence for a connection between the Laccase 1 gene and insect immunity, and revealed that parasitoid polydnavirus suppresses host PO activity via PxLac1 regulation.

Molecular characterization and gene silencing of Laccase 1 in the grain aphid, Sitobion avenae

Laccase 1 (Lac1), a polyphenol oxidase, has been proposed to be involved in insect iron metabolism and immunity responses. However, little information is available on the roles of Lac 1 in insect-plant interactions. The grain aphid Sitobion avenae is one of the most destructive pests of cereal, directly drawing phloem sap and transmitting viruses. In the present study, we first cloned the open reading frame (ORF) of Lac 1 from S. avenae, and the putative protein sequence was predicted to have a carboxyl-terminal transmembrane domain. We found that SaLac1 had higher expression levels in the fourth and adult stages using reverse transcription real-time quantitative PCR (RT-qPCR). SaLac 1 was highly expressed in the salivary gland and midgut and also in wingless compared with winged morphs. After feeding on aphid-resistant wheat with a high total phenol content, the expression level of SaLac 1 increased significantly. RNA interference (RNAi) by oral feeding successfully inhibited the transcript levels of SaLac 1, and the knockdown of Lac 1 significantly decreased the survival rate of S. avenae on aphid-resistant wheat. Our study demonstrated that S. avenae Lac1 was involved in the detoxification of phenolic compounds in wheat and was essential for the aphid to adapt to resistant plants.

Laccase SilA from Streptomyces ipomoeae CECT 3341, a key enzyme for the degradation of lignin from agricultural residues?

The role of laccase SilA produced by Streptomyces ipomoeae CECT 3341 in lignocellulose degradation was investigated. A comparison of the properties and activities of a laccase-negative mutant strain (SilA-) with that of the wild-type was studied in terms of their ability to degrade lignin from grass lignocellulose. The yields of solubilized lignin (acid precipitable polymeric lignin, APPL) obtained from wheat straw by both strains in Solid State Fermentation (SSF) conditions demonstrated the importance of SilA laccase in lignin degradation with the wild-type showing 5-fold more APPL produced compared with the mutant strain (SilA-). Analytical pyrolysis and FT-IR (Fourier Transform Infrared Spectroscopy) confirmed that the APPL obtained from the substrate fermented by wild-type strain was dominated by lignin derived methoxyphenols whereas those from SilA- and control APPLs were composed mainly of polysaccharides. This is the first report highlighting the role of this laccase in lignin degradation.

Comparative study of three phenoloxidases in the sea cucumber Apostichopus japonicus

In order to preliminarily illustrate the functional differences of phenoloxidases (POs) in Apostichopus japonicus, the full-length cDNAs of two POs (named as AjPOⅡ and AjPOⅢ, respectively) were cloned from the coelomocytes of A. japonicus using 3'- and 5'-rapid amplification of cDNA ends method, and combined with the previously acquired full-length cDNA of a laccase-type PO from A. japonicus (Accession No. KF040052, named as AjPOⅠ), the sequence structure and phylogenic status of POs from A. japonicus (AjPOs) were comparatively analyzed, and the transcriptional expression of AjPOs in different tissues, at different developmental stages and after different bacterial challenges was determined with quantitative real-time PCR method. Sequence analysis indicated AjPOⅡ and AjPOⅢ were both laccase-type POs, coincident to the results of phylogenic analysis. Sequence analysis also showed that AjPOⅠ had a transmembrane domain (J. Jiang et al., 2014), AjPOⅡ contained a signal peptide, and AjPOⅢ possessed a signal peptide and a transmembrane domain, implying that three AjPOs might play different roles in immune and physiological processes. Transcriptional expression analysis showed that AjPOⅡ and AjPOⅢ were most abundant in tube feet, while AjPOⅠhad the highest expression level in coelomocytes (J. Jiang et al., 2014), suggesting that AjPOⅠ may be mainly involved in immune response, while AjPOⅡ and AjPOⅢ are probably responsible for other physiological processes in addition to immune response. Besides, three AjPOs were determined to have different expression patterns during organism development and different spectrums of response against bacteria, which further indicated that there might be immune and physiological functional differentiation among three AjPOs.

Secretory laccase 1 in Bemisia tabaci MED is involved in whitefly-plant interaction

The whitefly Bemisia tabaci is a phloem-feeding pest that lives predominantly on herbaceous species and causes serious damage to hosts. Whitefly saliva is thought to contain proteins that modulate plant defences and facilitate feeding. A predicted secreted protein, laccase 1 (LAC1), was found in the salivary gland transcriptome of B. tabaci and might be existed in the watery saliva of B. tabaci. As LAC1 has a potential role in detoxification of secondary plant compounds in insects, we speculated that it may participate in the insect’s response to plant defences. Here, we cloned the complete cDNA of LAC1 and found that (1) LAC1 was highly expressed in the salivary gland (SG) and midgut; (2) LAC1 transcript level in head (containing SG) was 2.1 times higher in plant-fed than in diet-fed whiteflies and 1.6 times higher in the head and 23.8 times higher in the midgut of whiteflies that fed on jasmonic acid (JA)-sprayed plants than on control plants; and (3) silencing LAC1 decreased the survival rate of plant-fed whiteflies but had a marginal effect on whiteflies raised on an artificial diet. These results indicate that LAC1 enables whiteflies to overcome the chemical defences of host plants and might act as an effector in saliva.

Suppression of Laccase 2 severely impairs cuticle tanning and pathogen resistance during the pupal metamorphosis of Anopheles sinensis (Diptera: Culicidae)

Background

Phenol oxidases (POs) catalyze the oxidation of dopa and dopamine to melanin, which is crucial for cuticle formation and innate immune maintenance in insects. Although, Laccase 2, a member of the PO family, has been reported to be a requirement for melanin-mediated cuticle tanning in the development stages of some insects, whether it participates in cuticle construction and other physiological processes during the metamorphosis of mosquito pupae is unclear.

Methods

The association between the phenotype and the expression profile of Anopheles sinensis Laccase 2 (AsLac2) was assessed from pupation to adult eclosion. Individuals showing an expression deficiency of AsLac2 that was produced by RNAi and their phenotypic defects and physiological characterizations were compared in detail with the controls.

Results

During the dominant expression period, knockdown of AsLac2 in pupae caused the cuticle to be unpigmented, and produced thin and very soft cuticles, which further impeded the eclosion rate of adults as well as their fitness. Moreover, melanization immune responses in the pupae were sharply decreased, leading to poor resistance to microorganism infection. Both the high conservation among Laccase 2 homologs and a very similar genomic synteny of the neighborhood in Anopheles genus implies a conservative function in the pupal stage.

Conclusions

To our knowledge, this is the first study to report the serious phenotypic defects in mosquito pupae caused by the dysfunction of Laccase 2. Our findings strongly suggest that Laccase 2 is crucial for Anopheles cuticle construction and melanization immune responses to pathogen infections during pupal metamorphosis. This irreplaceability provides valuable information on the application of Lacccase 2 and/or other key genes in the melanin metabolism pathway for developing mosquito control strategies.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-2118-4) contains supplementary material, which is available to authorized users.

GENE SILENCING BY PARENTAL RNA INTERFERENCE IN THE GREEN RICE LEAFHOPPER, Nephotettix cincticeps (HEMIPTERA: CICADELLIDAE)

RNA interference (RNAi) has been widely used for investigating gene function in many nonmodel insect species. Parental RNAi causes gene knockdown in the next generation through the administration of double-strand RNA (dsRNA) to the mother generation. In this study, we demonstrate that parental RNAi mediated gene silencing is effective in determining the gene function of the cuticle and the salivary glands in green rice leafhopper (GRH), Nephotettix cincticeps (Uhler). Injection of dsRNA of NcLac2 (9 ng/female) to female parents caused a strong knockdown of laccase-2 gene of first instar nymphs, which eventually led to high mortality rates and depigmentation of side lines on the body. The effects of parental RNAi on the mortality of the nymphs were maintained through 12-14 days after the injections. We also confirmed the effectiveness of parental RNAi induced silencing on the gene expressed in the salivary gland, the gene product of which is passed from instar to instar. The parental RNAi method can be used to examine gene function by phenotyping many offspring nymphs with injection of dsRNA into a small number of parent females, and may be applicable to high-efficiency determination of gene functions in this species.

Forager bees (Apis mellifera) highly express immune and detoxification genes in tissues associated with nectar processing

Pollinators, including honey bees, routinely encounter potentially harmful microorganisms and phytochemicals during foraging. However, the mechanisms by which honey bees manage these potential threats are poorly understood. In this study, we examine the expression of antimicrobial, immune and detoxification genes in Apis mellifera and compare between forager and nurse bees using tissue-specific RNA-seq and qPCR. Our analysis revealed extensive tissue-specific expression of antimicrobial, immune signaling, and detoxification genes. Variation in gene expression between worker stages was pronounced in the mandibular and hypopharyngeal gland (HPG), where foragers were enriched in transcripts that encode antimicrobial peptides (AMPs) and immune response. Additionally, forager HPGs and mandibular glands were enriched in transcripts encoding detoxification enzymes, including some associated with xenobiotic metabolism. Using qPCR on an independent dataset, we verified differential expression of three AMP and three P450 genes between foragers and nurses. High expression of AMP genes in nectar-processing tissues suggests that these peptides may contribute to antimicrobial properties of honey or to honey bee defense against environmentally-acquired microorganisms. Together, these results suggest that worker role and tissue-specific expression of AMPs, and immune and detoxification enzymes may contribute to defense against microorganisms and xenobiotic compounds acquired while foraging.

Multicopper oxidase-1 is required for iron homeostasis in Malpighian tubules of Helicoverpa armigera

Multicopper oxidases (MCOs) are enzymes that contain 10 conserved histidine residues and 1 cysteine residue. MCO1 has been extensively investigated in the midgut because this MCO is implicated in ascorbate oxidation, iron homeostasis and immune responses. However, information regarding the action of MCO1 in Malpighian tubules is limited. In this study, Helicoverpa armigera was used as a model to investigate the function of MCO1 in Malpighian tubules. Sequence analysis results revealed that HaMCO1 exhibits typical MCO characteristics, with 10 histidine and 1 cysteine residues for copper ion binding. HaMCO1 was also found to be highly abundant in Malpighian tubules. Temporal expression patterns indicated that HaMCO1 is mainly expressed during larval molting stages. Hormone treatments [the molting hormone 20-hydroxyecdysone (20E) and juvenile hormone (JH)] revealed that 20E inhibits HaMCO1 transcript expression via its heterodimer receptor, which consists of ecdysone receptor (EcR) and ultraspiracle (USP), and that JH counteracts the action of 20E to activate HaMCO1 transcript expression via its intracellular receptor methoprene-tolerant (Met). HaMCO1 knockdown caused a significant decrease in iron accumulation and also significantly reduced transferrin and ferritin transcript expression. Therefore, HaMCO1 is coordinately regulated by 20E and JH and is required for iron homeostasis in Malpighian tubules.

Lignin engineering through laccase modification: a promising field for energy plant improvement

Laccase (p-diphenol:dioxygen oxidoreductase, EC 1.10.3.2) is a member of the multicopper oxidases and catalyzes the one-electron oxidation of a wide range of substrates, coupled with the reduction of oxygen to water. It is widely distributed in bacteria, fungi, plants and insects. Laccases are encoded by multigene family, and have been characterized mostly from fungi till now, with abundant industrial applications in pulp and paper, textile, food industries, organic synthesis, bioremediation and nanobiotechnology, while limited researches have been performed in plants, and no application has been reported. Plant laccases share the common molecular architecture and reaction mechanism with fungal ones, despite of difference in redox potential and pH optima. Plant laccases are implicated in lignin biosynthesis since genetic evidence was derived from the Arabidopsis LAC4 and LAC17. Manipulation of plant laccases has been considered as a promising and innovative strategy in plant biomass engineering for desirable lignin content and/or composition, since lignin is the major recalcitrant component to saccharification in biofuel production from lignocellulose, and therefore directly limits the fermentation yields. Moreover, plant laccases have been reported to be involved in wound healing, maintenance of cell wall structure and integrity, and plant responses to environmental stresses. Here, we summarize the properties and functions of plant laccase, and discuss the potential of biotechnological application, thus providing a new insight into plant laccase, an old enzyme with a promising beginning in lignocellulose biofuel production.

The multicopper oxidase gene family in the brown planthopper, Nilaparvata lugens

The multicopper oxidase (MCO) family of enzymes includes laccases, ascorbate oxidases, bilirubin oxidases and a subgroup of metal oxidases. On the basis of a bioinformatics investigation, we identified 7 genes encoding putative multicopper oxidase proteins in the genome of the brown planthopper (BPH), Nilaparvata lugens (Hemiptera: Delphacidae). MCO1 and MCO2 are conserved, while others diverse in insects. Analysis of developmental and tissue-specific expression patterns revealed the following: NlMCO2 was mainly expressed in the integument, and its expression peaked periodically during molting; NlMCO3 was an ovary-specific MCO gene with a high expression level only at the adult stage; NlMCO4 was a salivary gland-specific MCO gene that was expressed at all developmental stages; NlMCO5 only had short-term expression in the middle of the fourth instar stage and was expressed mainly in the gut; NlMCO6 had a developmental expression pattern similar to that of NlMCO2 and was expressed in most N. lugens tissues; and NlMCO1 was expressed in most N. lugens tissues except for the testis, whereas NlMCO7 was mainly expressed in the gut and the Malpighian tube. BPHs injected with double-stranded RNA (dsRNA) targeting NlMCO2 failed to pigment and sclerotize, were colorless and soft-bodied and subsequently died in a short time. Lethal phenotypes were also observed in insects challenged by dsRNA targeting NlMCO6. However, no observable morphological or internal structural abnormality was obtained in the insects treated with dsRNA for NlMCO1, NlMCO3, NlMCO4, NlMCO5 or NlMCO7.

Proteome Analysis of Watery Saliva Secreted by Green Rice Leafhopper, Nephotettix cincticeps

The green rice leafhopper, Nephotettix cincticeps, is a vascular bundle feeder that discharges watery and gelling saliva during the feeding process. To understand the potential functions of saliva for successful and safe feeding on host plants, we analyzed the complexity of proteinaceous components in the watery saliva of N. cincticeps. Salivary proteins were collected from a sucrose diet that adult leafhoppers had fed on through a membrane of stretched parafilm. Protein concentrates were separated using SDS-PAGE under reducing and non-reducing conditions. Six proteins were identified by a gas-phase protein sequencer and two proteins were identified using LC-MS/MS analysis with reference to expressed sequence tag (EST) databases of this species. Full -length cDNAs encoding these major proteins were obtained by rapid amplification of cDNA ends-PCR (RACE-PCR) and degenerate PCR. Furthermore, gel-free proteome analysis that was performed to cover the broad range of salivary proteins with reference to the latest RNA-sequencing data from the salivary gland of N. cincticeps, yielded 63 additional protein species. Out of 71 novel proteins identified from the watery saliva, about 60 % of those were enzymes or other functional proteins, including GH5 cellulase, transferrin, carbonic anhydrases, aminopeptidase, regucalcin, and apolipoprotein. The remaining proteins appeared to be unique and species- specific. This is the first study to identify and characterize the proteins in watery saliva of Auchenorrhyncha species, especially sheath-producing, vascular bundle-feeders.

Transcriptome analysis of the salivary glands of Nephotettix cincticeps (Uhler)

The green rice leafhopper (GRH), Nephotettix cincticeps, is one of the most important pests of rice in temperate Asian countries. GRH, a vascular feeder, secretes watery and gelling saliva in the process of feeding on phloem and xylem sap. It is known that GRH saliva contains several bioactive proteins, including enzymes such as laccase and beta-glucosidase. In this study, we performed transcriptome analysis of salivary glands of GRH using Illumina paired-end sequencing. Of 51,788 assembled contigs, 16,017 (30.9%) showed significant similarity to known proteins in the NCBI nr database, while 34,978 (67.5%) could not be annotated by similarity search, Pfam, or gene ontology (GO). Contigs (905) with predicted signal peptides and no putative transmembrane domains are suggested to represent secreted protein coding genes. Among the 76 most highly expressed putative secretory protein contigs, 68 transcripts were found to be salivary gland-specific or at least -dominant, but not expressed in stomach or Malpighian tubules. However, 45 of the 68 transcripts were unknown proteins. These findings suggest that most of the GRH transcripts encoding secreted proteins expressed in salivary glands are species and/or tissue specific. Our results provide a fundamental list of genes involved in GRH-Poaceae host plant interactions including successful feeding and plant pathogen transmission.

Salivary proteins of plant-feeding hemipteroids - implication in phytophagy

Many hemipteroids are major pests and vectors of microbial pathogens, infecting crops. Saliva of the hemipteroids is critical in enabling them to be voracious feeders on plants, including the economically important ones. A plethora of hemipteroid salivary enzymes is known to inflict stress in plants, either by degrading the plant tissue or by affecting their normal metabolism. Hemipteroids utilize one of the following three strategies of feeding behaviour: salivary sheath feeding, osmotic-pump feeding and cell-rupture feeding. The last strategy also includes several different tactics such as lacerate-and-flush, lacerate-and-sip and macerate-and-flush. Understanding hemipteroid feeding mechanisms is critical, since feeding behaviour directs salivary composition. Saliva of the Heteroptera that are specialized as fruit and seed feeders, includes cell-degrading enzymes, auchenorrhynchan salivary composition also predominantly consists of cell-degrading enzymes such as amylase and protease, whereas that of the Sternorhyncha includes a variety of allelochemical-detoxifying enzymes. Little is known about the salivary composition of the Thysanoptera. Cell-degrading proteins such as amylase, pectinase, cellulase and pectinesterase enable stylet entry into the plant tissue. In contrast, enzymes such as glutathione peroxidase, laccase and trehalase detoxify plant chemicals, enabling the circumvention of plant-defence mechanisms. Salivary enzymes such as M1-zinc metalloprotease and CLIP-domain serine protease as in Acyrthosiphon pisum (Aphididae), and non-enzymatic proteins such as apolipophorin, ficolin-3-like protein and 'lava-lamp' protein as in Diuraphis noxia (Aphididae) have the capacity to alter host-plant-defence mechanisms. A majority of the hemipteroids feed on phloem, hence Ca++-binding proteins such as C002 protein, calreticulin-like isoform 1 and calmodulin (critical for preventing sieve-plate occlusion) are increasingly being recognized in hemipteroid-plant interactions. Determination of a staggering variety of proteins shows the complexity of hemipteroid saliva: effector proteins localized in hemipteran saliva suggest a similarity to the physiology of pathogen-plant interactions.

Insights into the saliva of the brown marmorated stink bug Halyomorpha halys (Hemiptera: Pentatomidae)

We examined the salivary gland structure of the brown marmorated stink bug (Pentatomidae: Halyomorpha halys) and developed methods for independent collection of watery saliva and sheath saliva. This stink bug has become a serious invasive pest of agriculture in the United States and its saliva is largely responsible for the damage it causes. We determined by protein gel analysis and shotgun proteomics that the suite of proteins comprising the sheath and watery saliva are very distinct. Our results indicate that a substantial amount of sheath proteins are derived from tomato when stink bugs feed on tomato fruit. Consequently, the sheath saliva is comprised of both insect and plant-derived proteins. Both sheath and watery saliva possessed amylase activities, but polyphenol oxidase and glucose oxidase activities were not detected in either saliva. Peroxidase activity was only detected in salivary sheaths, but only when stink bugs fed on tomato. Proteomic analysis indicated that the peroxidase was likely of plant origin. We also determined that sheath saliva, but not watery saliva elicited the jasmonate inducible defense gene proteinase inhibitor 2 (Pin2), but this induction was only observed when sheaths had been collected from tomato. This indicates that the eliciting factor of the saliva is likely of plant origin. Lastly, neither watery or sheath saliva affected the expression of the salicylate inducible gene pathogenesis related gene (Pr1a-P4).

High Laccase2 expression is likely involved in the formation of specific cuticular structures during soldier differentiation of the termite Reticulitermes speratus

Termite soldiers are morphologically specialized for colony defense. Analysis of the mechanisms of soldier differentiation is important for understanding the establishment of termite societies. Soldiers differentiate from workers through a presoldier stage and have well-sclerotized and pigmented cuticles. These characteristics are important for nest defense and are likely to be caused by soldier-specific mechanisms of cuticular tanning. The molecular mechanisms leading to cuticular tanning have not been elucidated. Laccase2 (Lac2) plays important roles in this process in insects, and we hypothesized that Lac2 expression may be involved in soldier-specific cuticular tanning. We observed inner and outer head cuticle changes and compared the Lac2 expression patterns among three molts (worker-worker, worker-presoldier and presoldier-soldier) in the termite Reticulitermes speratus. Quantitative analyses of head cuticle colors showed that the color properties changed more conspicuously in presoldier-soldier molts than in the other two molts. Histological observations showed that the exocuticles of soldier heads were substantially thicker than those of worker and presoldier heads, underwent tanning before or just after ecdysis, and were pigmented at earlier time points than other molts. Finally, markedly higher Lac2 expression levels were observed just before and after ecdysis only in the presoldier-soldier molt. These results suggest that specific cuticular formation occurs in the exocuticles during soldier differentiation, and that the high level of Lac2 expression during the presoldier-soldier molt is related to soldier-specific cuticular tanning. We speculate that evolution of the regulatory mechanisms of Lac2 expression were important for the acquisition of soldier-specific cuticles.

Molecular cloning of a novel calcium-binding protein in the secreted saliva of the green rice leafhopper Nephotettix cincticeps

Green rice leafhoppers (Nephotettix cincticeps) secrete watery and coagulable saliva in the feeding process. In our study, the watery salivary secretion was concentrated by ultrafiltration from "fed diet" and subjected to SDS-PAGE. The N-terminal amino acid sequence of the most predominant band at 84 kDa (designated NcSP84) was analyzed by Edman degradation. This sequence was completely consistent with the most abundant protein in the salivary gland extracts, which was separated by two-dimensional gel electrophoresis. Based on the N-terminal amino acid sequence, the complete cDNA of this protein was cloned by 5'- and 3'-RACE using degenerate primers. The deduced NcSP84 contained an open reading frame of 2061 bp encoding a putative 687 amino acids with a putative signal sequence composed of 19 amino acids. The nucleotide and amino acid sequences of NcSP84 did not share statistically significant homology with any sequences in public databases. Motif search predicted that this protein had EF-hands, the most common motif found in Ca(2+) -binding proteins. As predicted, NcSP84 exhibited Ca(2+)-binding activity. The SDS-PAGE mobility of purified NcSP84 bound to Ca(2+) tended to decline discretely, depending on the concentration of CaCl(2) with which it was mixed for 1h before adding SDS buffer. In situ hybridization and immunohistochemistry showed that the NcSP84 gene and gene product were expressed and stored in type III cells, which are the largest lobes in the primary salivary glands. The NcSP84 protein was detected in the phloem sap of rice exposed to leafhoppers, verifying that the NcSP84 protein was injected into the sieve tubes. These results suggest that NcSP84 could be secreted into the sieve tubes during feeding, which might bind Ca(2+) ions that flow into sieve tubes in response to stylet puncturing. This might suppress sieve-element clogging and facilitate continuous ingestion from sieve tubes.

Laccase: microbial sources, production, purification, and potential biotechnological applications

Laccase belongs to the blue multicopper oxidases and participates in cross-linking of monomers, degradation of polymers, and ring cleavage of aromatic compounds. It is widely distributed in higher plants and fungi. It is present in Ascomycetes, Deuteromycetes and Basidiomycetes and abundant in lignin-degrading white-rot fungi. It is also used in the synthesis of organic substance, where typical substrates are amines and phenols, the reaction products are dimers and oligomers derived from the coupling of reactive radical intermediates. In the recent years, these enzymes have gained application in the field of textile, pulp and paper, and food industry. Recently, it is also used in the design of biosensors, biofuel cells, as a medical diagnostics tool and bioremediation agent to clean up herbicides, pesticides and certain explosives in soil. Laccases have received attention of researchers in the last few decades due to their ability to oxidize both phenolic and nonphenolic lignin-related compounds as well as highly recalcitrant environmental pollutants. It has been identified as the principal enzyme associated with cuticular hardening in insects. Two main forms have been found: laccase-1 and laccase-2. This paper reviews the occurrence, mode of action, general properties, production, applications, and immobilization of laccases within different industrial fields.

Laccase2 is required for cuticular pigmentation in stinkbugs

During the maturation of insect cuticle, protein-protein and protein-chitin crosslinkages are formed by the action of diphenoloxidases. Two types of diphenoloxidases, laccases and tyrosinases, are present in the insect cuticle. In coleopteran and hymenopteran insects, laccase2 gene has been identified as encoding an enzyme principally responsible for cuticular pigmentation and hardening, whereas biological roles of laccase genes in hemimetabolous insects remain to be established. Here we identified laccase2 genes from three hemipteran stinkbugs, Riptortus pedestris (Alydidae), Nysius plebeius (Lygaeidae) and Megacopta punctatissima (Plataspidae). In R. pedestris, laccase2 gene was highly expressed in epidermal tissues prior to molting. When the gene expression was suppressed by an RNA interference technique, cuticular pigmentation after molting were blocked depending on the dose of injected double-stranded RNA targeting the laccase2 gene. Similar results were obtained for N. plebeius and M. punctatissima. In all the stinkbug species, injecting 20 ng of double-stranded RNA was sufficient to prevent the cuticular maturation. These results indicate that laccase2 gene is generally required for cuticular pigmentation in different stinkbug families, highlighting its conserved biological function across diverse insect taxa.