Genomic resources for Myzus persicae: EST sequencing, SNP identification, and microarray design

Cardiac glycosides protect wormseed wallflower (Erysimum cheiranthoides) against some, but not all, glucosinolate-adapted herbivores

The chemical arms race between plants and insects is foundational to the generation and maintenance of biological diversity. We asked how the evolution of a novel defensive compound in an already well-defended plant lineage impacts interactions with diverse herbivores. Erysimum cheiranthoides (Brassicaceae), which produces both ancestral glucosinolates and novel cardiac glycosides, served as a model. We analyzed gene expression to identify cardiac glycoside biosynthetic enzymes in E. cheiranthoides and characterized these enzymes via heterologous expression and CRISPR/Cas9 knockout. Using E. cheiranthoides cardiac glycoside-deficient lines, we conducted insect experiments in both the laboratory and field. EcCYP87A126 initiates cardiac glycoside biosynthesis via sterol side-chain cleavage, and EcCYP716A418 has a role in cardiac glycoside hydroxylation. In EcCYP87A126 knockout lines, cardiac glycoside production was eliminated. Laboratory experiments with these lines revealed that cardiac glycosides were highly effective defenses against two species of glucosinolate-tolerant specialist herbivores, but did not protect against all crucifer-feeding specialist herbivores in the field. Cardiac glycosides had lesser to no effect on two broad generalist herbivores. These results begin elucidation of the E. cheiranthoides cardiac glycoside biosynthetic pathway and demonstrate in vivo that cardiac glycoside production allows Erysimum to escape from some, but not all, specialist herbivores.

Serine proteinase inhibitors from Nicotiana benthamiana, a nonpreferred host plant, inhibit the growth of Myzus persicae (green peach aphid)

BACKGROUND

The green peach aphid (Myzus persicae) is a severe agricultural crop pest that has developed resistance to most current control methods, requiring the urgent development of novel strategies. Plant proteinase inhibitors (PINs) are small proteins that protect plants against pathogens and/or herbivores, likely by preventing efficient protein digestion.

RESULTS

We identified 67 protease genes in the transcriptomes of three M. persicae lineages (USDA-Red, G002 and G006). Comparison of gene expression levels in aphid guts and whole aphids showed that several proteases, including a highly expressed serine protease, are significantly overexpressed in the guts. Furthermore, we identified three genes encoding serine protease inhibitors (SerPIN-II1, 2 and 3) in Nicotiana benthamiana, which is a nonpreferred host for M. persicae. Using virus-induced gene silencing (VIGS) with a tobacco rattle virus (TRV) vector and overexpression with a turnip mosaic virus (TuMV) vector, we demonstrated that N. benthamiana SerPIN-II1 and SerPIN-II2 cause reduced survival and growth, but do not affect aphid protein content. Likewise, SerPIN-II3 overexpression reduced survival and growth, and serpin-II3 knockout mutations, which we generated using CRISPR/Cas9, increased survival and growth. Protein content was significantly increased in aphids fed on SerPIN-II3 overexpressing plants, yet it was decreased in aphids fed on serpin-II3 mutants.

CONCLUSION

Our results show that three PIN-IIs from N. benthamiana, a nonpreferred host plant, effectively inhibit M. persicae survival and growth, thereby representing a new resource for the development of aphid-resistant crop plants. © 2024 Society of Chemical Industry.

Discovery of Three Bemisia tabaci Effectors and Their Effect on Gene Expression in Planta

Bemisia tabaci (whitefly) is a polyphagous agroeconomic pest species complex. Two members of this species complex, Mediterranean (MED) and Middle-East-Asia Minor 1 (MEAM1), have a worldwide distribution and have been shown to manipulate plant defenses through effectors. In this study, we used three different strategies to identify three MEAM1 proteins that can act as effectors. Effector B1 was identified using a bioinformatics-driven effector-mining strategy, whereas effectors S1 and P1 were identified in the saliva of whiteflies collected from artificial diet and in phloem exudate of tomato on which nymphs were feeding, respectively. These three effectors were B. tabaci specific and able to increase whitefly fecundity when transiently expressed in tobacco plants (Nicotiana tabacum). Moreover, they reduced growth of Pseudomonas syringae pv. tabaci in Nicotiana benthamiana. All three effectors changed gene expression in planta, and B1 and S1 also changed phytohormone levels. Gene ontology and KEGG pathway enrichment analysis pinpointed plant-pathogen interaction and photosynthesis as the main enriched pathways for all three effectors. Our data thus show the discovery and validation of three new B. tabaci MEAM1 effectors that increase whitefly fecundity and modulate plant immunity. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Fatty alcohols, a minor component of the tree tobacco surface wax, are associated with defence against caterpillar herbivory

Despite decades of research resulting in a comprehensive understanding of epicuticular wax metabolism, the function of these almost ubiquitous metabolites in plant-herbivore interactions remains unresolved. In this study, we examined the effects of CRISPR-induced knockout mutations in four Nicotiana glauca (tree tobacco) wax metabolism genes. These mutations cause a wide range of changes in epicuticular wax composition, leading to altered interactions with insects and snails. Three interaction classes were examined: chewing herbivory by seven caterpillars and one snail species, phloem feeding by Myzus persicae (green peach aphid) and oviposition by Bemisia tabaci (whitefly). Although total wax load and alkane abundance did not affect caterpillar growth, a correlation across species, showed that fatty alcohols, a minor component of N. glauca surface waxes, negatively affected the growth of both a generalist caterpillar (Spodoptera littoralis) and a tobacco-feeding specialist (Manduca sexta). This negative correlation was overshadowed by the stronger effect of anabasine, a nicotine isomer, and was apparent when fatty alcohols were added to an artificial lepidopteran diet. By contrast, snails fed more on waxy leaves. Aphid reproduction and feeding activity were unaffected by wax composition but were potentially affected by altered cutin composition. Wax crystal morphology could explain the preference of B. tabaci to lay eggs on waxy wild-type plants relative to both alkane and fatty alcohol-deficient mutants. Together, our results suggest that the varied responses among herbivore classes and species are likely to be a consequence of the co-evolution that shaped the specific effects of different surface wax components in plant-herbivore interactions.

Aphid resistance segregates independently of cardiac glycoside and glucosinolate content in an Erysimum cheiranthoides (wormseed wallflower) F2 population

Plants in the genus Erysimum produce both glucosinolates and cardiac glycosides as defense against herbivory. Two natural isolates of Erysimum cheiranthoides (wormseed wallflower) differed in their glucosinolate content, cardiac glycoside content, and resistance to Myzus persicae (green peach aphid), a broad generalist herbivore. Both classes of defensive metabolites were produced constitutively and were not induced further by aphid feeding. To investigate the relative importance of glucosinolates and cardiac glycosides in E. cheiranthoides defense, we generated an improved genome assembly, genetic map, and segregating F2 population. Genotypic and phenotypic analysis of the F2 plants identified quantitative trait loci affecting glucosinolates and cardiac glycosides, but not aphid resistance. The abundance of most glucosinolates and cardiac glycosides was positively correlated in the F2 population, indicating that similar processes regulate their biosynthesis and accumulation. Aphid reproduction was positively correlated with glucosinolate content. Although overall cardiac glycoside content had little effect on aphid growth and survival, there was a negative correlation between aphid reproduction and helveticoside abundance. However, this variation in defensive metabolites could not explain the differences in aphid growth on the two parental lines, suggesting that processes other than the abundance of glucosinolates and cardiac glycosides have a predominant effect on aphid resistance in E. cheiranthoides.

Cardiac glycosides protect wormseed wallflower (Erysimum cheiranthoides) against some, but not all, glucosinolate-adapted herbivores

The chemical arms race between plants and insects is foundational to the generation and maintenance of biological diversity. We asked how the evolution of a novel defensive compound in an already well-defended plant lineage impacts interactions with diverse herbivores. Erysimum cheiranthoides (Brassicaceae), which produces both ancestral glucosinolates and novel cardiac glycosides, served as a model.We analyzed gene expression to identify cardiac glycoside biosynthetic enzymes in E. cheiranthoides and characterized these enzymes via heterologous expression and CRISPR/Cas9 knockout. Using E. cheiranthoides cardiac glycoside-deficient lines, we conducted insect experiments in both the laboratory and field.EcCYP87A126 initiates cardiac glycoside biosynthesis via sterol side chain cleavage, and EcCYP716A418 has a role in cardiac glycoside hydroxylation. In EcCYP87A126 knockout lines, cardiac glycoside production was eliminated. Laboratory experiments with these lines revealed that cardiac glycosides were highly effective defenses against two species of glucosinolate-tolerant specialist herbivores but did not protect against all crucifer-feeding specialist herbivores in the field. Cardiac glycosides had lesser to no effect on two broad generalist herbivores.These results begin elucidation of the E. cheiranthoides cardiac glycoside biosynthetic pathway and demonstrate in vivo that cardiac glycoside production allows Erysimum to escape from some, but not all, specialist herbivores.

Horizontally transferred genes as RNA interference targets for aphid and whitefly control

RNA interference (RNAi)-based technologies are starting to be commercialized as a new approach for agricultural pest control. Horizontally transferred genes (HTGs), which have been transferred into insect genomes from viruses, bacteria, fungi or plants, are attractive targets for RNAi-mediated pest control. HTGs are often unique to a specific insect family or even genus, making it unlikely that RNAi constructs targeting such genes will have negative effects on ladybugs, lacewings and other beneficial predatory insect species. In this study, we sequenced the genome of a red, tobacco-adapted isolate of Myzus persicae (green peach aphid) and bioinformatically identified 30 HTGs. We then used plant-mediated virus-induced gene silencing (VIGS) to show that several HTGs of bacterial and plant origin are important for aphid growth and/or survival. Silencing the expression of fungal-origin HTGs did not affect aphid survivorship but decreased aphid reproduction. Importantly, although there was uptake of plant-expressed RNA by Coccinella septempunctata (seven-spotted ladybugs) via the aphids that they consumed, we did not observe negative effects on ladybugs from aphid-targeted VIGS constructs. To demonstrate that this approach is more broadly applicable, we also targeted five Bemisia tabaci (whitefly) HTGs using VIGS and demonstrated that knockdown of some of these genes affected whitefly survival. As functional HTGs have been identified in the genomes of numerous pest species, we propose that these HTGs should be explored further as efficient and safe targets for control of insect pests using plant-mediated RNA interference.

Acylsugars protect Nicotiana benthamiana against insect herbivory and desiccation

KEY MESSAGE

Nicotiana benthamiana acylsugar acyltransferase (ASAT) is required for protection against desiccation and insect herbivory. Knockout mutations provide a new resource for investigation of plant-aphid and plant-whitefly interactions. Nicotiana benthamiana is used extensively as a transient expression platform for functional analysis of genes from other species. Acylsugars, which are produced in the trichomes, are a hypothesized cause of the relatively high insect resistance that is observed in N. benthamiana. We characterized the N. benthamiana acylsugar profile, bioinformatically identified two acylsugar acyltransferase genes, ASAT1 and ASAT2, and used CRISPR/Cas9 mutagenesis to produce acylsugar-deficient plants for investigation of insect resistance and foliar water loss. Whereas asat1 mutations reduced accumulation, asat2 mutations caused almost complete depletion of foliar acylsucroses. Three hemipteran and three lepidopteran herbivores survived, gained weight, and/or reproduced significantly better on asat2 mutants than on wildtype N. benthamiana. Both asat1 and asat2 mutations reduced the water content and increased leaf temperature. Our results demonstrate the specific function of two ASAT proteins in N. benthamiana acylsugar biosynthesis, insect resistance, and desiccation tolerance. The improved growth of aphids and whiteflies on asat2 mutants will facilitate the use of N. benthamiana as a transient expression platform for the functional analysis of insect effectors and resistance genes from other plant species. Similarly, the absence of acylsugars in asat2 mutants will enable analysis of acylsugar biosynthesis genes from other Solanaceae by transient expression.

Hormesis and insects: Effects and interactions in agroecosystems

Insects in agroecosystems contend with many stressors - e.g., chemicals, heat, nutrient deprivation - that are often encountered at low levels. Exposure to mild stress is now well known to induce hormetic (stimulatory) effects in insects, with implications for insect management, and ecological structure and function in agroecosystems. In this review, we examine the major ecological niches insects occupy or guilds to which they belong in agroecosystems and how hormesis can manifest within and across these groups. The mechanistic underpinnings of hormesis in insects are starting to become established, explaining the many phenotypic hormetic responses observed in insect reproduction, development, and behavior. Whereas potential effects on insect populations are well supported in laboratory experiments, field-based hypothesis-driven research on hormesis is greatly lacking. Furthermore, because most ecological paradigms are founded within the context of communities, entomological agroecologists interested in hormesis need to 'level up' and test hypotheses that explore effects on species interactions, and community structure and functioning. Embedded in this charge is to continue experimentation on herbivorous pest species while shifting more focus towards insect natural enemies, pollinators, and detritivores - guilds that play crucial roles in highly functioning agroecosystems that have been understudied in hormesis research. Important areas for future insect agroecology research on hormesis are discussed.

A sugarcane mosaic virus vector for rapid in planta screening of proteins that inhibit the growth of insect herbivores

Spodoptera frugiperda (fall armyworm) is a notorious pest that threatens maize production worldwide. Current control measures involve the use of chemical insecticides and transgenic maize expressing Bacillus thuringiensis (Bt) toxins. Although additional transgenes have confirmed insecticidal activity, limited research has been conducted in maize, at least partially due to the technical difficulty of maize transformation. Here, we describe implementation of a sugarcane mosaic virus (SCMV) vector for rapidly testing the efficacy of both endogenous maize genes and heterologous genes from other organisms for the control of S. frugiperda in maize. Four categories of proteins were tested using the SCMV vector: (i) maize defence signalling proteins: peptide elicitors (Pep1 and Pep3) and jasmonate acid conjugating enzymes (JAR1a and JAR1b); (ii) maize defensive proteins: the previously identified ribosome-inactivating protein (RIP2) and maize proteinase inhibitor (MPI), and two proteins with predicted but unconfirmed anti-insect activities, an antimicrobial peptide (AMP) and a lectin (JAC1); (iii) lectins from other plant species: Allium cepa agglutinin (ACA) and Galanthus nivalis agglutinin (GNA); and (iv) scorpion and spider toxins: peptides from Urodacus yaschenkoi (UyCT3 and UyCT5) and Hadronyche versuta (Hvt). In most cases, S. frugiperda larval growth was reduced by transient SCMV-mediated overexpression of genes encoding these proteins. Additionally, experiments with a subset of the SCMV-expressed genes showed effectiveness against two aphid species, Rhopalosiphum maidis (corn leaf aphid) and Myzus persicae (green peach aphid). Together, these results demonstrate that SCMV vectors are a rapid screening method for testing the efficacy and insecticidal activity of candidate genes in maize.

Global patterns in genomic diversity underpinning the evolution of insecticide resistance in the aphid crop pest Myzus persicae

The aphid Myzus persicae is a destructive agricultural pest that displays an exceptional ability to develop resistance to both natural and synthetic insecticides. To investigate the evolution of resistance in this species we generated a chromosome-scale genome assembly and living panel of >110 fully sequenced globally sampled clonal lines. Our analyses reveal a remarkable diversity of resistance mutations segregating in global populations of M. persicae. We show that the emergence and spread of these mechanisms is influenced by host-plant associations, uncovering the widespread co-option of a host-plant adaptation that also offers resistance against synthetic insecticides. We identify both the repeated evolution of independent resistance mutations at the same locus, and multiple instances of the evolution of novel resistance mechanisms against key insecticides. Our findings provide fundamental insights into the genomic responses of global insect populations to strong selective forces, and hold practical relevance for the control of pests and parasites.

An Aphid-Secreted Salivary Protease Activates Plant Defense in Phloem

Recent studies have reported that aphids facilitate their colonization of host plants by secreting salivary proteins into host tissues during their initial probing and feeding. Some of these salivary proteins elicit plant defenses, but the molecular and biochemical mechanisms that underlie the activation of phloem-localized resistance remain poorly understood. The aphid Myzus persicae, which is a generalized phloem-sucking pest, encompasses a number of lineages that are associated with and adapted to specific host plant species. The current study found that a cysteine protease Cathepsin B3 (CathB3), and the associated gene CathB3, was upregulated in the salivary glands and saliva of aphids from a non-tobacco-adapted (NTA) aphid lineage, when compared to those of a tobacco-adapted lineage. Furthermore, the knockdown of CathB3 improved the performance of NTA lineages on tobacco, and the propeptide domain of CathB3 was found to bind to tobacco cytoplasmic kinase ENHANCED DISEASE RESISTANCE 1-like (EDR1-like), which triggers the accumulation of reactive oxygen species in tobacco phloem, thereby suppressing both phloem feeding and colonization by NTA lineages. These findings reveal a novel function for a cathepsin-type protease in aphid saliva that elicits effective host plant defenses and warranted the theory of host specialization for generalist aphids.

Less Is More: a Mutation in the Chemical Defense Pathway of Erysimum cheiranthoides (Brassicaceae) Reduces Total Cardenolide Abundance but Increases Resistance to Insect Herbivores

Erysimum cheiranthoides L (Brassicaceae; wormseed wallflower) accumulates not only glucosinolates, which are characteristic of the Brassicaceae, but also abundant and diverse cardenolides. These steroid toxins, primarily glycosylated forms of digitoxigenin, cannogenol, and strophanthidin, inhibit the function of essential Na⁺/K⁺-ATPases in animal cells. We screened a population of 659 ethylmethanesulfonate-mutagenized E. cheiranthoides plants to identify isolates with altered cardenolide profiles. One mutant line exhibited 66% lower cardenolide content, resulting from greatly decreased cannogenol and strophanthidin glycosides, partially compensated for by increases in digitoxigenin glycosides. This phenotype was likely caused by a single-locus recessive mutation, as evidenced by a wildtype phenotype of F1 plants from a backcross, a 3:1 wildtype:mutant segregation in the F2 generation, and genetic mapping of the altered cardenolide phenotype to one position in the genome. The mutation created a more even cardenolide distribution, decreased the average cardenolide polarity, but did not impact most glucosinolates. Growth of generalist herbivores from two feeding guilds, Myzus persicae Sulzer (Hemiptera: Aphididae; green peach aphid) and Trichoplusia ni Hübner (Lepidoptera: Noctuidae; cabbage looper), was decreased on the mutant line compared to wildtype. Both herbivores accumulated cardenolides in proportion to the plant content, with T. ni accumulating higher total concentrations than M. persicae. Helveticoside, a relatively abundant cardenolide in E. cheiranthoides, was not detected in M. persicae feeding on these plants. Our results support the hypothesis that increased digitoxigenin glycosides provide improved protection against M. persicae and T. ni, despite an overall decrease in cardenolide content of the mutant line.

Resistance of Amaranthus Spp. to the Green Peach Aphid (Hemiptera: Aphididae)

The green peach aphid [Myzus persicae (Sulzer)] is an important pest of amaranth grown for leaf consumption (i.e., leafy amaranth) in the tropics. Aphids reduce the amount of fresh leaf yield of amaranth and the value of leafy amaranth as aphid-infested leaves are not marketable. Our objective was to evaluate Amaranthus species selected by a breeding program in East Africa to develop cultivars for leaf consumption with resistance to M. persicae. We focused on antibiosis to determine whether varieties of Amaranthus spp. could be grown without producing an aphid population. Artificial infestations of aphids were placed on multiple selections of three species of Amaranthus: two selections of A. blitum, four selections of A. hybridus and one selection of A. hypochondriacus. Aphid populations were assessed over a 5-wk period. Evaluations of vegetative yield, leaf damage symptoms, and specific leaf area (SLA) were made of the seven selections at the end of this experiment. Aphid populations assessed 49 d after planting differed significantly (P ≤ 0.001) among the amaranth species and within selections of the same species. The selections of A. blitum had the lowest aphid populations, and A. hybridus had the highest populations. Selections of A. hybridus produced the most marketable leaves (i.e., aphid free). The fresh weight of A. blitum were the lowest of the seven selections, whereas A. hybridus had the greatest fresh leaf weight. Implications of these finding for further promotion of amaranth breeding are discussed related to pest management for leaf production.

An aphid RNA transcript migrates systemically within plants and is a virulence factor

Aphids are sap-feeding insects that colonize a broad range of plant species and often cause feeding damage and transmit plant pathogens, including bacteria, viruses, and viroids. These insects feed from the plant vascular tissue, predominantly the phloem. However, it remains largely unknown how aphids, and other sap-feeding insects, establish intimate long-term interactions with plants. To identify aphid virulence factors, we took advantage of the ability of the green peach aphid Myzus persicae to colonize divergent plant species. We found that a M. persicae clone of near-identical females established stable colonies on nine plant species of five representative plant eudicot and monocot families that span the angiosperm phylogeny. Members of the novel aphid gene family Ya are differentially expressed in aphids on the nine plant species and are coregulated and organized as tandem repeats in aphid genomes. Aphids translocate Ya transcripts into plants, and some transcripts migrate to distal leaves within several plant species. RNAi-mediated knockdown of Ya genes reduces M. persicae fecundity, and M. persicae produces more progeny on transgenic plants that heterologously produce one of the systemically migrating Ya transcripts as a long noncoding (lnc) RNA. Taken together, our findings show that beyond a range of pathogens, M. persicae aphids translocate their own transcripts into plants, including a Ya lncRNA that migrates to distal locations within plants, promotes aphid fecundity, and is a member of a previously undescribed host-responsive aphid gene family that operate as virulence factors.

Salivary proteins of Phloeomyzus passerinii, a plant-manipulating aphid, and their impact on early gene responses of susceptible and resistant poplar genotypes

Successful plant colonization by parasites requires the circumvention of host defenses, and sometimes a reprogramming of host metabolism, mediated by effector molecules delivered into the host. Using transcriptomic and enzymatic approaches, we characterized salivary glands and saliva of Phloeomyzus passerinii, an aphid exhibiting an atypical feeding strategy. Plant responses to salivary extracts of P. passerinii and Myzus persicae were assessed with poplar protoplasts of a susceptible and a resistant genotype, and in a heterologous Arabidopsis system. We predict that P. passerinii secretes a highly peculiar saliva containing effectors potentially interfering with host defenses, biotic stress signaling and plant metabolism, notably phosphatidylinositol phosphate kinases which seemed specific to P. passerinii. Gene expression profiles indicated that salivary extracts of M. persicae markedly affected host defenses and biotic stress signaling, while salivary extracts of P. passerinii induced only weak responses. The effector-triggered susceptibility was characterized by downregulations of genes involved in cytokinin signaling and auxin homeostasis. This suggests that P. passerinii induces an intracellular accumulation of auxin in susceptible host genotypes, which is supported by histochemical assays in Arabidopsis. This might in turn affect biotic stress signaling and contribute to host tissue manipulation by the aphid.

Silencing cathepsin L expression reduces Myzus persicae protein content and the nutritional value as prey for Coccinella septempunctata

Gut-expressed aphid genes, which may be more easily inhibited by RNA interference (RNAi) constructs, are attractive targets for pest control efforts involving transgenic plants. Here we show that expression of cathepsin L, which encodes a cysteine protease that functions in aphid guts, can be reduced by expression of an RNAi construct in transgenic tobacco. The effectiveness of this approach is demonstrated by up to 80% adult mortality, reduced fecundity, and delayed nymph production of Myzus persicae (green peach aphids) when cathepsin L expression was reduced by plant-mediated RNAi. Consistent with the function of cathepsin L as a gut protease, M. persicae fed on the RNAi plants had a lower protein content in their bodies and excreted more protein and/or free amino acids in their honeydew. Larvae of Coccinella septempunctata (seven-spotted ladybugs) grew more slowly on aphids having reduced cathepsin L expression, suggesting that prey insect nutritive value, and not just direct negative effects of the RNAi construct, needs to be considered when producing transgenic plants for RNAi-mediated pest control.

Gene Expression and Diet Breadth in Plant-Feeding Insects: Summarizing Trends

Transcriptomic studies lend insights into the role of transcriptional plasticity in adaptation and specialization. Recently, there has been growing interest in understanding the relationship between variation in herbivorous insect gene expression and the evolution of diet breadth. We review the studies that have emerged on insect gene expression and host plant use, and outline the questions and approaches in the field. Many candidate genes underlying herbivory and specialization have been identified, and a few key studies demonstrate increased transcriptional plasticity associated with generalist compared with specialist species. Addressing the roles that transcriptional variation plays in insect diet breadth will have important implications for our understanding of the evolution of specialization and the genetic and environmental factors that govern insect-plant interactions.

Potential role of the heat shock protein 90 (hsp90) in buffering mutations to favour cyclical parthenogenesis in the peach potato aphid Myzus persicae (Aphididae, Hemiptera)

Heat-shock proteins 90 (hsp90s) are a class of molecules able to stabilize a network of 'client' proteins that are involved in several processes. Furthermore, recent studies indicated that mutations in the hsp90-encoding gene induce a wide range of phenotypic abnormalities, which have been interpreted as an increased sensitivity of different developmental pathways to hidden/cryptic mutations. In order to verify the role of hsp90 in aphids, we amplified and sequenced the hsp90 gene in 17 lineages of the peach potato aphid Myzus persicae (Sulzer, 1776) looking for the presence of mutations. In particular, we compared lineages with different reproductive modes (obligate vs. cyclical parthenogenesis), propensity to develop winged females and karyotype stability. Differently from the cyclical parthenogenetic lineages that possessed functional hsp90 genes, the seven analysed asexual lineages showed severe mutations (including frameshift and non-sense mutations). In vivo functional assays with the hsp90-inhibitor geldanamycin showed that some lineages with cyclical parthenogenesis may lose their ability to induce sexuales in the absence of active hsp90 revealing the presence of cryptic mutations in their genomes. As a whole, our data suggest that hsp90 could play in aphids a role in buffering hidden/cryptic mutations that disrupt cyclical parthenogenesis.

The Evolutionary History and Functional Divergence of Trehalase (treh) Genes in Insects

Trehalases (treh) have been found in different organisms, such as bacteria, fungi, yeast, nematodes, insects, vertebrates, and plants. Their biochemical properties are extremely variable and not yet fully understood. Gene expression patterns have shown differences among insect species suggesting a potential functional diversification of trehalase enzymes during their evolution. A second gene family encoding for enzymes with hypothetical trehalase activity has been repeatedly annotated in insect genome as acid trehalases/acid trehalase-like (ath), but its functional role is still not clear. The currently available large amount of genomic data from many insect species may enable a better understanding of the evolutionary history, phylogenetic relationships and possible roles of trehalase encoding genes in this taxon. The aim of the present study is to infer the evolutionary history of trehalases and acid trehalase genes in insects and analyze the trehalase functional divergence during their evolution, combining phylogenetic and genomic synteny/colinearity analyses.

Aphid effector Me10 interacts with tomato TFT7, a 14-3-3 isoform involved in aphid resistance

We demonstrated previously that expression of Macrosiphum euphorbiae salivary protein Me10 enhanced aphid reproduction on its host tomato (Solanum lycopersicum). However, the mechanism of action of Me10 remained elusive. To confirm the secretion of Me10 by the aphid into plant tissues, we produced Me10 polyclonal antibodies. To identify the plant targets of Me10, we developed a tomato immune induced complementary DNA yeast two-hybrid library and screened it with Me10 as bait. Immunoprecipitation and bimolecular fluorescence complementation (BiFC) assays were performed to validate one of the interactions in planta, and virus-induced gene silencing was used for functional characterization in tomato. We demonstrated that Me10 is secreted into the plant tissues and interacts with tomato 14-3-3 isoform 7 (TFT7) in yeast. Immunoprecipitation assays confirmed that Me10 and its homologue in Aphis gossypii, Ag10k, interact with TFT7 in planta. Further, BiFC revealed that Me10 interaction with TFT7 occurs in the plant cell cytoplasm. While silencing of TFT7 in tomato leaves did not affect tomato susceptibility to M. euphorbiae, it enhanced longevity and fecundity of A. gossypii, the non-host aphid. Our results suggest the model whereby TFT7 plays a role in aphid resistance in tomato and effectors of the Me10/Ag10k family interfere with TFT7 function during aphid infestation.

Host-mediated RNA interference targeting a cuticular protein gene impaired fecundity in the green peach aphid Myzus persicae

BACKGROUND

The green peach aphid (Myzus persicae) is a devastating sap-sucking insect pest that damages many host plants worldwide and causes billions of dollars of crop losses. Induction of RNA interference (RNAi) through oral feeding of small interfering RNA (siRNA) has been demonstrated in aphids. Therefore, host-mediated delivery of double-stranded RNA (dsRNA) specific to vital structural genes of aphids has been envisaged as a tool for the development of resistance against this aphid species.

RESULTS

Cuticular protein (CP) senses seasonal photoperiodism and drives a shift from clonal to sexual generation in aphids. Thus, attenuation of CP gene expression is likely to result in a different reproductive orientation in aphids and thereby affect their fecundity. A gene encoding CP in M. persicae has been targeted for RNAi-mediated knockdown. Transgenic Arabidopsis expressing dsRNA homologous to the MyCP gene was developed. The dsRNA-transgenics produced gene-specific siRNAs fed by aphids infesting the transgenics. A reverse transcription-quantitative polymerase chain reaction (RT-qPCR) study revealed an attenuated level of transcripts of the CP gene in aphid nymphs reared on the transgenic plants. Decreased expression of the CP gene resulted in a noticeable decline in aphid fecundity on the transgenic Arabidopsis plants.

CONCLUSION

Increasing genetic resistance is the only sustainable way of minimizing the use of toxic agrochemicals to protect plants. Host-mediated RNAi of important insect genes has been proposed as a potential avenue for developing crop resistance against insect pests. This study demonstrated the potential of MyCP dsRNA in developing RNAi-based resistance to M. persicae. RNAi-mediated resistance is expected to be more durable compared with other transgenic strategies. © 2018 Society of Chemical Industry.

Evolutionary loss and replacement of Buchnera, the obligate endosymbiont of aphids

Symbiotic interactions between organisms create new ecological niches. For example, many insects survive on plant-sap with the aid of maternally transmitted bacterial symbionts that provision essential nutrients lacking in this diet. Symbiotic partners often enter a long-term relationship in which the co-evolutionary fate of lineages is interdependent. Obligate symbionts that are strictly maternally transmitted experience genetic drift and genome degradation, compromising symbiont function and reducing host fitness unless hosts can compensate for these deficits. One evolutionary solution is the acquisition of a novel symbiont with a functionally intact genome. Whereas almost all aphids host the anciently acquired bacterial endosymbiont Buchnera aphidicola (Gammaproteobacteria), Geopemphigus species have lost Buchnera and instead contain a maternally transmitted symbiont closely related to several known insect symbionts from the bacterial phylum Bacteroidetes. A complete genome sequence shows the symbiont has lost many ancestral genes, resulting in a genome size intermediate between that of free-living and symbiotic Bacteroidetes. The Geopemphigus symbiont retains biosynthetic pathways for amino acids and vitamins, as in Buchnera and other insect symbionts. This case of evolutionary replacement of Buchnera provides an opportunity to further understand the evolution and functional genomics of symbiosis.

microRNA regulation in an ancient obligate endosymbiosis

Although many insects are associated with obligate bacterial endosymbionts, the mechanisms by which these host/endosymbiont associations are regulated remain mysterious. While microRNAs (miRNAs) have been recently identified as regulators of host/microbe interactions, including host/pathogen and host/facultative endosymbiont interactions, the role miRNAs may play in mediating host/obligate endosymbiont interactions is virtually unknown. Here, we identified conserved miRNAs that potentially mediate symbiotic interactions between aphids and their obligate endosymbiont, Buchnera aphidicola. Using small RNA sequence data from Myzus persicae and Acyrthosiphon pisum, we annotated 93 M. persicae and 89 A. pisum miRNAs, among which 69 were shared. We found 14 miRNAs that were either highly expressed in aphid bacteriome, the Buchnera-housing tissue, or differentially expressed in bacteriome vs. gut, a non-Buchnera-housing tissue. Strikingly, 10 of these 14 miRNAs have been implicated previously in other host/microbe interaction studies. Investigating the interaction networks of these miRNAs using a custom computational pipeline, we identified 103 miRNA::mRNA interactions shared between M. persicae and A. pisum. Functional annotation of the shared mRNA targets revealed only two over-represented cluster of orthologous group categories: amino acid transport and metabolism, and signal transduction mechanisms. Our work supports a role for miRNAs in mediating host/symbiont interactions between aphids and their obligate endosymbiont Buchnera. In addition, our results highlight the probable importance of signal transduction mechanisms to host/endosymbiont coevolution.

Evaluation of the reference genes for expression analysis using quantitative real-time polymerase chain reaction in the green peach aphid, Myzus persicae

The green peach aphid, Myzus persicae Sulzer (Hemiptera, Aphididae), is an important cosmopolitan pest. Real time qRT-PCR has been used for target gene expression analysis on M. persicae. Using real time qRT-PCR, the expression levels are normalized on the basis of the reliable reference genes. However, to date, the stability of available reference genes has been insufficient. In this study, we evaluated nine candidate reference genes from M. persicae under diverse experimental conditions. The tested candidate genes were comprehensively ranked based on five alternative methods (RefFinder, geNorm, Normfinder, BestKeeper and the comparative ΔC_t method). 18s, Actin and ribosomal protein L27 (L27) were recommended as the most stable reference genes for M. persicae, whereas ribosomal protein L27 (L27) was found to be the least stable reference genes for abiotic studies (photoperiod, temperature and insecticide susceptibility). Our finding not only sheds light on establishing an accurate and reliable normalization of real time qRT-PCR data in M. persicae but also lays a solid foundation for further studies of M. persicae involving RNA interference and functional gene research.

Rapid transcriptional plasticity of duplicated gene clusters enables a clonally reproducing aphid to colonise diverse plant species

Background

The prevailing paradigm of host-parasite evolution is that arms races lead to increasing specialisation via genetic adaptation. Insect herbivores are no exception and the majority have evolved to colonise a small number of closely related host species. Remarkably, the green peach aphid, Myzus persicae, colonises plant species across 40 families and single M. persicae clonal lineages can colonise distantly related plants. This remarkable ability makes M. persicae a highly destructive pest of many important crop species.

Results

To investigate the exceptional phenotypic plasticity of M. persicae, we sequenced the M. persicae genome and assessed how one clonal lineage responds to host plant species of different families. We show that genetically identical individuals are able to colonise distantly related host species through the differential regulation of genes belonging to aphid-expanded gene families. Multigene clusters collectively upregulate in single aphids within two days upon host switch. Furthermore, we demonstrate the functional significance of this rapid transcriptional change using RNA interference (RNAi)-mediated knock-down of genes belonging to the cathepsin B gene family. Knock-down of cathepsin B genes reduced aphid fitness, but only on the host that induced upregulation of these genes.

Conclusions

Previous research has focused on the role of genetic adaptation of parasites to their hosts. Here we show that the generalist aphid pest M. persicae is able to colonise diverse host plant species in the absence of genetic specialisation. This is achieved through rapid transcriptional plasticity of genes that have duplicated during aphid evolution.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-016-1145-3) contains supplementary material, which is available to authorized users.

An Immuno-Suppressive Aphid Saliva Protein Is Delivered into the Cytosol of Plant Mesophyll Cells During Feeding

Herbivore selection of plant hosts and plant responses to insect colonization have been subjects of intense investigations. A growing body of evidence suggests that, for successful colonization to occur, (effector/virulence) proteins in insect saliva must modulate plant defense responses to the benefit of the insect. A range of insect saliva proteins that modulate plant defense responses have been identified, but there is no direct evidence that these proteins are delivered into specific plant tissues and enter plant cells. Aphids and other sap-sucking insects of the order Hemiptera use their specialized mouthparts (stylets) to probe plant mesophyll cells until they reach the phloem cells for long-term feeding. Here, we show, by immunogold-labeling of ultrathin sections of aphid feeding sites, that an immuno-suppressive aphid effector localizes in the cytoplasm of mesophyll cells near aphid stylets but not in cells further away from aphid feeding sites. In contrast, another aphid effector protein localizes in the sheaths composed of gelling saliva that surround the aphid stylets. Thus, insects deliver effectors directly into plant tissue. Moreover, different aphid effectors locate extracellularly in the sheath saliva or are introduced into the cytoplasm of plant cells. [Formula: see text] Copyright © 2016 The Author(s). This is an open-access article distributed under the CC BY-NC-ND 4.0 International license .

Transcriptome Analysis of Green Peach Aphid (Myzus persicae): Insight into Developmental Regulation and Inter-Species Divergence

Green peach aphid (Myzus persicae) and pea aphid (Acyrthosiphon pisum) are two phylogenetically closely related agricultural pests. While pea aphid is restricted to Fabaceae, green peach aphid feeds on hundreds of plant species from more than 40 families. Transcriptome comparison could shed light on the genetic factors underlying the difference in host range between the two species. Furthermore, a large scale study contrasting gene expression between immature nymphs and fully developed adult aphids would fill a previous knowledge gap. Here, we obtained transcriptomic sequences of green peach aphid nymphs and adults, respectively, using Illumina sequencing technology. A total of 2244 genes were found to be differentially expressed between the two developmental stages, many of which were associated with detoxification, hormone production, cuticle formation, metabolism, food digestion, and absorption. When searched against publically available pea aphid mRNA sequences, 13,752 unigenes were found to have no homologous counterparts. Interestingly, many of these unigenes that could be annotated in other databases were involved in the "xenobiotics biodegradation and metabolism" pathway, suggesting the two aphids differ in their adaptation to secondary metabolites of host plants. Conversely, 3989 orthologous gene pairs between the two species were subjected to calculations of synonymous and nonsynonymous substitutions, and 148 of the genes potentially evolved in response to positive selection. Some of these genes were predicted to be associated with insect-plant interactions. Our study has revealed certain molecular events related to aphid development, and provided some insight into biological variations in two aphid species, possibly as a result of host plant adaptation.

Vector-Borne Bacterial Plant Pathogens: Interactions with Hemipteran Insects and Plants

Hemipteran insects are devastating pests of crops due to their wide host range, rapid reproduction, and ability to transmit numerous plant-infecting pathogens as vectors. While the field of plant-virus-vector interactions has flourished in recent years, plant-bacteria-vector interactions remain poorly understood. Leafhoppers and psyllids are by far the most important vectors of bacterial pathogens, yet there are still significant gaps in our understanding of their feeding behavior, salivary secretions, and plant responses as compared to important viral vectors, such as whiteflies and aphids. Even with an incomplete understanding of plant-bacteria-vector interactions, some common themes have emerged: (1) all known vector-borne bacteria share the ability to propagate in the plant and insect host; (2) particular hemipteran families appear to be incapable of transmitting vector-borne bacteria; (3) all known vector-borne bacteria have highly reduced genomes and coding capacity, resulting in host-dependence; and (4) vector-borne bacteria encode proteins that are essential for colonization of specific hosts, though only a few types of proteins have been investigated. Here, we review the current knowledge on important vector-borne bacterial pathogens, including Xylella fastidiosa, Spiroplasma spp., Liberibacter spp., and 'Candidatus Phytoplasma spp.'. We then highlight recent approaches used in the study of vector-borne bacteria. Finally, we discuss the application of this knowledge for control and future directions that will need to be addressed in the field of vector-plant-bacteria interactions.

Hemipteran and dipteran pests: Effectors and plant host immune regulators

Hemipteran and dipteran insects have behavioral, cellular and chemical strategies for evading or coping with the host plant defenses making these insects particularly destructive pests worldwide. A critical component of a host plant's defense to herbivory is innate immunity. Here we review the status of our understanding of the receptors that contribute to perception of hemipteran and dipteran pests and highlight the gaps in our knowledge in these early events in immune signaling. We also highlight recent advances in identification of the effectors that activate pattern-triggered immunity and those involved in effector-triggered immunity.

Organ-specific transcriptome response of the small brown planthopper toward rice stripe virus

Rice stripe virus (RSV) causes rice stripe disease and is transmitted by the small brown planthopper (Laodelphax striatellus, SBPH) in a persistent, circulative, and propagative manner. The alimentary canal and salivary gland of SBPH play important roles in viral replication and transmission. However, little is known about the underlying molecular functions of these two organs in the interaction between RSV and SBPH. In this study, organ-specific transcriptomes of the alimentary canal and salivary gland were analyzed in viruliferous and naïve SBPH. The number of differentially expressed unigenes in the alimentary canal was considerably greater than that in the salivary gland after RSV infection, and only 23 unigenes were co-regulated in the two organs. In the alimentary canal, genes involved in lysosome, digestion and detoxification were activated upon RSV infection, whereas the genes related to DNA replication and repair were suppressed. RSV activated RNA transport and repressed the MAPK, mTOR, Wnt, and TGF-beta signaling pathways in the salivary gland. The overall immune reaction toward RSV was much stronger in the salivary gland than in the alimentary canal. RSV activated the pattern recognition molecules and Toll pathway in the salivary gland but inhibited these two reactions in the alimentary canal. The responses from reactive oxygen and the immune-responsive effectors were stronger in the salivary gland than in the alimentary canal after RSV infection. These findings provide clues on the roles of the two organs in confronting RSV infection and aid in the understanding of the interaction between RSV and SBPHs.

Gene Family Expansions in Aphids Maintained by Endosymbiotic and Nonsymbiotic Traits

Facilitating the evolution of new gene functions, gene duplication is a major mechanism driving evolutionary innovation. Gene family expansions relevant to host/symbiont interactions are increasingly being discovered in eukaryotes that host endosymbiotic microbes. Such discoveries entice speculation that gene duplication facilitates the evolution of novel, endosymbiotic relationships. Here, using a comparative transcriptomic approach combined with differential gene expression analysis, we investigate the importance of endosymbiosis in retention of amino acid transporter paralogs in aphid genomes. To pinpoint the timing of amino acid transporter duplications we inferred gene phylogenies for five aphid species and three outgroups. We found that while some duplications arose in the aphid common ancestor concurrent with endosymbiont acquisition, others predate aphid divergence from related insects without intracellular symbionts, and still others appeared during aphid diversification. Interestingly, several aphid-specific paralogs have conserved enriched expression in bacteriocytes, the insect cells that host primary symbionts. Conserved bacteriocyte enrichment suggests that the transporters were recruited to the aphid/endosymbiont interface in the aphid common ancestor, consistent with a role for gene duplication in facilitating the evolution of endosymbiosis in aphids. In contrast, the temporal variability of amino acid transporter duplication indicates that endosymbiosis is not the only trait driving selection for retention of amino acid transporter paralogs in sap-feeding insects. This study cautions against simplistic interpretations of the role of gene family expansion in the evolution of novel host/symbiont interactions by further highlighting that multiple complex factors maintain gene family paralogs in the genomes of eukaryotes that host endosymbiotic microbes.

Transcription of densovirus endogenous sequences in the Myzus persicae genome

Integration of non-retroviral sequences in the genome of different organisms has been observed and, in some cases, a relationship of these integrations with immunity has been established. The genome of the green peach aphid, Myzus persicae (clone G006), was screened for densovirus-like sequence (DLS) integrations. A total of 21 DLSs localized on 10 scaffolds were retrieved that mostly shared sequence identity with two aphid-infecting viruses, Myzus persicae densovirus (MpDNV) and Dysaphis plantaginea densovirus (DplDNV). In some cases, uninterrupted potential ORFs corresponding to non-structural viral proteins or capsid proteins were found within DLSs identified in the aphid genome. In particular, one scaffold harboured a complete virus-like genome, while another scaffold contained two virus-like genomes in reverse orientation. Remarkably, transcription of some of these ORFs was observed in M. persicae, suggesting a biological effect of these viral integrations. In contrast to most of the other densoviruses identified so far that induce acute host infection, it has been reported previously that MpDNV has only a minor effect on M. persicae fitness, while DplDNV can even have a beneficial effect on its aphid host. This suggests that DLS integration in the M. persicae genome may be responsible for the latency of MpDNV infection in the aphid host.

Circulative, "nonpropagative" virus transmission: an orchestra of virus-, insect-, and plant-derived instruments

Species of plant viruses within the Luteoviridae, Geminiviridae, and Nanoviridae are transmitted by phloem-feeding insects in a circulative, nonpropagative manner. The precise route of virus movement through the vector can differ across and within virus families, but these viruses all share many biological, biochemical, and ecological features. All share temporal and spatial constraints with respect to transmission efficiency. The viruses also induce physiological changes in their plant hosts resulting in behavioral changes in the insects that optimize the transmission of virus to new hosts. Virus proteins interact with insect, endosymbiont, and plant proteins to orchestrate, directly and indirectly, virus movement in insects and plants to facilitate transmission. Knowledge of these complex interactions allows for the development of new tools to reduce or prevent transmission, to quickly identify important vector populations, and to improve the management of these economically important viruses affecting agricultural and natural plant populations.

Engineering plants for aphid resistance: current status and future perspectives

The current status of development of transgenic plants for improved aphid resistance, and the pros and cons of different strategies are reviewed and future perspectives are proposed. Aphids are major agricultural pests that cause significant yield losses of crop plants each year. Excessive dependence on insecticides for aphid control is undesirable because of the development of insecticide resistance, the potential negative effects on non-target organisms and environmental pollution. Transgenic plants engineered for resistance to aphids via a non-toxic mode of action could be an efficient alternative strategy. In this review, the distribution of major aphid species and their damages on crop plants, the so far isolated aphid-resistance genes and their applications in developments of transgenic plants for improved aphid resistance, and the pros and cons of these strategies are reviewed and future perspectives are proposed. Although the transgenic plants developed through expressing aphid-resistant genes, manipulating plant secondary metabolism and plant-mediated RNAi strategy have been demonstrated to confer improved aphid resistance to some degree. So far, no aphid-resistant transgenic crop plants have ever been commercialized. This commentary is intended to be a helpful insight into the generation and future commercialization of aphid-resistant transgenic crops in a global context.

Adaptation to nicotine in the facultative tobacco-feeding hemipteran Bemisia tabaci

BACKGROUND

Plant defensive metabolites such as nicotine can provide barriers to host-range expansion by generalist herbivores. Nicotine is one of the most abundant and toxic plant secondary metabolites in nature and is defined by high toxicity to plant-feeding insects. There is significant variation in nicotine tolerance among Bemisia tabaci (tobacco whitefly) isolates. Some nicotine-tolerant B. tabaci strains can consume 40-fold higher nicotine levels than susceptible strains, and also show cross-resistance to neonicotinoid insecticides. In this study, biological and molecular assays were used to investigate the responses of B. tabaci strains that differ in their ability to tolerate dietary nicotine.

RESULTS

Egg laying and honeydew secretion bioassays as well as gene expression microarrays were used to measure B. tabaci biological parameters and gene transcripts misregulated in response to nicotine in resistant and susceptible strains. The resistant B. tabaci strain laid significantly fewer eggs and excreted more honeydew on a tobacco strain with high levels of nicotine, suggesting a fitness cost effect. The molecular response was drastic in the susceptible strain, while the resistant strain exhibited moderate response. Higher expression of the previously identified CYP6CM1 P450 monooxygenase gene related to the resistance to neonicotinoids, as well as other P450s and metabolic genes, was identified in the resistant and susceptible strains after exposure to nicotine.

CONCLUSIONS

Nicotine is a very toxic plant natural compound, and its mode of action resembles that of synthetic neonicotinoids. The biological and molecular responses observed in this study suggest that nicotine may play an important role in providing barriers for host-plant expansion by generalists, and may act as a natural factor that contributes to the development of insect populations resistant to synthetic pesticides.

Adaptation to nicotine feeding in Myzus persicae

Lineages of the generalist hemipteran herbivore Myzus persicae (green peach aphid) that have expanded their host range to include tobacco often have elevated nicotine tolerance. The tobacco-adapted M. persicae lineage used in this study was able to reproduce on nicotine-containing artificial diets at concentrations that were 15-fold higher than those that were lethal to a non-adapted M. persicae lineage. Fecundity of the nicotine-tolerant M. persicae lineage was increased by 100 μM nicotine in artificial diet, suggesting that this otherwise toxic alkaloid can serve as a feeding stimulant at low concentrations. This lineage also was pre-adapted to growth on tobacco, exhibiting no drop in fecundity when it was moved onto tobacco from a different host plant. Although growth of the non-tobacco-adapted M. persicae lineage improved after three generations on tobacco, this higher reproductive rate was not associated with increased nicotine tolerance. Myzus persicae gene expression microarrays were used to identify transcripts that are up-regulated in response to nicotine in the tobacco-adapted lineage. Induced expression was found for CYP6CY3, which detoxifies nicotine in M. persicae, other genes encoding known classes of detoxifying enzymes, and genes encoding secreted M. persicae salivary proteins.

Suppression of plant defenses by a Myzus persicae (green peach aphid) salivary effector protein

The complex interactions between aphids and their host plant are species-specific and involve multiple layers of recognition and defense. Aphid salivary proteins, which are released into the plant during phloem feeding, are a likely mediator of these interactions. In an approach to identify aphid effectors that facilitate feeding from host plants, eleven Myzus persicae (green peach aphid) salivary proteins and the GroEL protein of Buchnera aphidicola, a bacterial endosymbiont of this aphid species, were expressed transiently in Nicotiana tabacum (tobacco). Whereas two salivary proteins increased aphid reproduction, expression of three other aphid proteins and GroEL significantly decreased aphid reproduction on N. tabacum. These effects were recapitulated in stable transgenic Arabidopsis thaliana plants. Further experiments with A. thaliana expressing Mp55, a salivary protein that increased aphid reproduction, showed lower accumulation of 4-methoxyindol-3-ylmethylglucosinolate, callose and hydrogen peroxide in response to aphid feeding. Mp55-expressing plants also were more attractive for aphids in choice assays. Silencing Mp55 gene expression in M. persicae using RNA interference approaches reduced aphid reproduction on N. tabacum, A. thaliana, and N. benthamiana. Together, these results demonstrate a role for Mp55, a protein with as-yet-unknown molecular function, in the interaction of M. persicae with its host plants.

Gene expression during imidacloprid-induced hormesis in green peach aphid

Imidacloprid-induced hormesis in the form of stimulated reproduction has previously been reported in green peach aphid, Myzus persicae. Changes in gene expression accompanying this hormetic response have not been previously investigated. In this study, expression of stress response (Hsp60), dispersal (OSD, TOL and ANT), and developmental (FPPS I) genes were examined for two generations during imidacloprid-induced reproductive stimulation in M. persicae. Global DNA methylation was also measured to test the hypothesis that changes in gene expression are heritable. At hormetic concentrations, down-regulation of Hsp60 was followed by up-regulation of this gene in the subsequent generation. Likewise, expression of dispersal-related genes and FPPS I varied with concentration, life stage, and generation. These results indicate that reproductive hormesis in M. persicae is accompanied by a complex transgenerational pattern of up- and down-regulation of genes that likely reflects trade-offs in gene expression and related physiological processes during the phenotypic dose-response. Moreover, DNA methylation in second generation M. persicae occurred at higher doses than in first-generation aphids, suggesting that heritable adaptability to low doses of the stressor might have occurred.

Immunodetection of a brown planthopper (Nilaparvata lugens Stål) salivary catalase-like protein into tissues of rice, Oryza sativa

Saliva plays an important role in host plant-phloem-feeding insect molecular interactions. To better elucidate the role of insect saliva, a series of experiments were conducted to establish if catalase from the salivary glands of the brown planthopper (BPH; Nilaparvata lugens Stål) was secreted into rice host plant tissue during feeding. Catalase is the main enzyme that decomposes hydrogen peroxide (H2O2) at high concentrations. H2O2 is a part of the free radicals system that mediates important physiological roles including signalling and defence. Previous studies have suggested that H2O2 is involved in the rice endogenous response to BPH feeding. If, the BPH secretes catalase into host plant tissue this will counter the effects of H2O2, from detoxification to interfering with plant signalling and defence mechanisms. When BPHs were fed on a hopper-resistant rice variety for 24 h, catalase activity in the salivary glands increased 3.5-fold compared with hoppers fed on a susceptible rice variety. Further supporting evidence of the effects of BPH catalase was demonstrated by immunodetection analyses where results from two independent sources: BPH-infested rice tissue and BPH-probed artificial diets, suggest that the BPH secretes catalase-like protein during feeding. The possible physiological roles of BPH-secreted catalase are discussed.

Comparative analyses of salivary proteins from three aphid species

Saliva is a critical biochemical interface between aphids and their host plants; however, the biochemical nature and physiological functions of aphid saliva proteins are not fully elucidated. In this study we used a multidisciplinary proteomics approach combining liquid chromatography-electrospray ionization tandem mass spectrometry and two-dimensional differential in-gel electrophoresis/matrix-assisted laser desorption/ionization time-of-flight/mass spectrometry to compare the salivary proteins from three aphid species including Acyrthosiphon pisum, Megoura viciae and Myzus persicae. Comparative analyses revealed variability among aphid salivary proteomes. Among the proteins that varied, 22% were related to DNA-binding, 19% were related to GTP-binding, and 19% had oxidoreductase activity. In addition, we identified a peroxiredoxin enzyme and an ATP-binding protein that may be involved in the modulation of plant defences. Knowledge of salivary components and how they vary among aphid species may reveal how aphids target plant processes and how the aphid and host plant interact.

Mp10 and Mp42 from the aphid species Myzus persicae trigger plant defenses in Nicotiana benthamiana through different activities

Aphids are phloem-feeding insects that, like other plant parasites, deliver effectors inside their host to manipulate host responses. The Myzus persicae (green peach aphid) candidate effectors Mp10 and Mp42 were previously found to reduce aphid fecundity upon intracellular transient overexpression in Nicotiana benthamiana. We performed functional analyses of these proteins to investigate whether they activate defenses through similar activities. We employed a range of functional characterization experiments based on intracellular transient overexpression in N. benthamiana to determine the subcellular localization of Mp10 and Mp42 and investigate their role in activating plant defense signaling. Mp10 and Mp42 showed distinct subcellular localization in planta, suggesting that they target different host compartments. Also, Mp10 reduced the levels of Agrobacterium-mediated overexpression of proteins. This reduction was not due to an effect on Agrobacterium viability. Transient overexpression of Mp10 but not Mp42 activated jasmonic acid and salicylic acid signaling pathways and decreased susceptibility to the hemibiotrophic plant pathogen Phytophthora capsici. We found that two candidate effectors from the broad-host-range aphid M. persicae can trigger aphid defenses through different mechanisms. Importantly, we found that some (candidate) effectors such as Mp10 interfere with Agrobacterium-based overexpression assays, an important tool to study effector activity and function.

Comparative analysis of genome sequences from four strains of the Buchnera aphidicola Mp endosymbion of the green peach aphid, Myzus persicae

BACKGROUND

Myzus persicae, the green peach aphid, is a polyphagous herbivore that feeds from hundreds of species of mostly dicot crop plants. Like other phloem-feeding aphids, M. persicae rely on the endosymbiotic bacterium, Buchnera aphidicola (Buchnera Mp), for biosynthesis of essential amino acids and other nutrients that are not sufficiently abundant in their phloem sap diet. Tobacco-specialized M. persicae are typically red and somewhat distinct from other lineages of this species. To determine whether the endosymbiotic bacteria of M. persicae could play a role in tobacco adaptation, we sequenced the Buchnera Mp genomes from two tobacco-adapted and two non-tobacco M. persicae lineages.

RESULTS

With a genome size of 643.5 kb and 579 predicted genes, Buchnera Mp is the largest Buchnera genome sequenced to date. No differences in gene content were found between the four sequenced Buchnera Mp strains. Compared to Buchnera APS from the well-studied pea aphid, Acyrthosiphon pisum, Buchnera Mp has 21 additional genes. These include genes encoding five enzymes required for biosynthesis of the modified nucleoside queosine, the heme pathway enzyme uroporphyrinogen III synthase, and asparaginase. Asparaginase, which is also encoded by the genome of the aphid host, may allow Buchnera Mp to synthesize essential amino acids from asparagine, a relatively abundant phloem amino acid.

CONCLUSIONS

Together our results indicate that the obligate intracellular symbiont Buchnera aphidicola does not contribute to the adaptation of Myzus persicae to feeding on tobacco.

Plant-aphid interactions with a focus on legumes

Sap-sucking insects such as aphids cause substantial yield losses in agriculture by draining plant nutrients as well as vectoring viruses. The main method of control in agriculture is through the application of insecticides. However, aphids rapidly evolve mechanisms to detoxify these, so there is a need to develop durable plant resistance to these damaging insect pests. The focus of this review is on aphid interactions with legumes, but work on aphid interactions with other plants, particularly Arabidopsis and tomato is also discussed. This review covers advances on the plant side of the interaction, including the identification of major resistance genes and quantitative trait loci conferring aphid resistance in legumes, basal and resistance gene mediated defence signalling following aphid infestation and the role of specialised metabolites. On the aphid side of the interaction, this review covers what is known about aphid effector proteins and aphid detoxification enzymes. Recent advances in these areas have provided insight into mechanisms underlying resistance to aphids and the strategies used by aphids for successful infestations and have significant impacts for the delivery of durable resistance to aphids in legume crops.

Comparative transcriptome analysis of salivary glands of two populations of rice brown planthopper, Nilaparvata lugens, that differ in virulence

Background

The brown planthopper (BPH), Nilaparvata lugens (Stål), a destructive rice pest in Asia, can quickly overcome rice resistance by evolving new virulent populations. Herbivore saliva plays an important role in plant–herbivore interactions, including in plant defense and herbivore virulence. However, thus far little is known about BPH saliva at the molecular level, especially its role in virulence and BPH–rice interaction.

Methodology/Principal Findings

Using cDNA amplification in combination with Illumina short-read sequencing technology, we sequenced the salivary-gland transcriptomes of two BPH populations with different virulence; the populations were derived from rice variety TN1 (TN1 population) and Mudgo (M population). In total, 37,666 and 38,451 unigenes were generated from the salivary glands of these populations, respectively. When combined, a total of 43,312 unigenes were obtained, about 18 times more than the number of expressed sequence tags previously identified from these glands. Gene ontology annotations and KEGG orthology classifications indicated that genes related to metabolism, binding and transport were significantly active in the salivary glands. A total of 352 genes were predicted to encode secretory proteins, and some might play important roles in BPH feeding and BPH–rice interactions. Comparative analysis of the transcriptomes of the two populations revealed that the genes related to ‘metabolism,’ ‘digestion and absorption,’ and ‘salivary secretion’ might be associated with virulence. Moreover, 67 genes encoding putative secreted proteins were differentially expressed between the two populations, suggesting these genes may contribute to the change in virulence.

Conclusions/Significance

This study was the first to compare the salivary-gland transcriptomes of two BPH populations having different virulence traits and to find genes that may be related to this difference. Our data provide a rich molecular resource for future functional studies on salivary glands and will be useful for elucidating the molecular mechanisms underlying BPH feeding and virulence differences.

Maintenance of genetic diversity through plant-herbivore interactions

Identifying the factors governing the maintenance of genetic variation is a central challenge in evolutionary biology. New genomic data, methods and conceptual advances provide increasing evidence that balancing selection, mediated by antagonistic species interactions, maintains genome-wide functionally important genetic variation within species and natural populations. Because diverse interactions between plants and herbivorous insects dominate terrestrial communities, they provide excellent systems to address this hypothesis. Population genomic studies of Arabidopsis thaliana and its relatives suggest spatial variation in herbivory maintains adaptive genetic variation controlling defense phenotypes, both within and among populations. Conversely, inter-species variation in plant defenses promotes adaptive genetic variation in herbivores. Emerging genomic model herbivores of Arabidopsis could illuminate how genetic variation in herbivores and plants interact simultaneously.

Arabidopsis thaliana-Myzus persicae interaction: shaping the understanding of plant defense against phloem-feeding aphids

The phloem provides a unique niche for several organisms. Aphids are a large group of Hemipteran insects that utilize stylets present in their mouthparts to pierce sieve elements and drink large volumes of phloem sap. In addition, many aphids also vector viral diseases. Myzus persicae, commonly known as the green peach aphid (GPA), is an important pest of a large variety of plants that includes Arabidopsis thaliana. This review summarizes recent studies that have exploited the compatible interaction between Arabidopsis and GPA to understand the molecular and physiological mechanisms utilized by plants to control aphid infestation, as well as genes and mechanisms that contribute to susceptibility. In addition, recent efforts to identify aphid-delivered elicitors of plant defenses and novel aphid salivary components that facilitate infestation are also discussed.