Role of the major antigenic membrane protein in phytoplasma transmission by two insect vector species

New Assays for Rapid Detection of Beet Leafhopper-Associated Plant Pathogens, 'Candidatus Phytoplasma trifolii', Beet Curly Top Virus, and Spiroplasma citri

The beet leafhopper Circulifer tenellus is an important pest of agricultural crops in the United States, where it transmits beet curly top virus, beet leafhopper-transmitted virescence agent phytoplasma, and Spiroplasma citri to numerous crops, affecting yield and quality. Each of these pathogens have been linked to serious disease outbreaks within Washington State in the past century. To mitigate the risk of disease, growers target the beet leafhopper in their insect pest management programs. Knowledge of pathogen prevalence in beet leafhopper populations could help growers make better management decisions, but timely diagnostics is required. Four new assays were developed for the rapid detection of the beet leafhopper-associated pathogens. These include two assays that detect Beet leafhopper transmitted virescence agent (a PCR and a real-time PCR SYBR green assay), a duplex PCR assay that simultaneously detects beet curly top virus and Spiroplasma citri, and a multiplex real-time PCR assay for the simultaneous detection of all three pathogens. The screening of dilution series generated from plant total nucleic acid extracts with these new assays typically led to detection at levels 10- to 100-fold more sensitive than the conventional PCR assays currently used. These new tools will allow the rapid detection of beet leafhopper-associated pathogens in both plant and insect specimens and will have the potential to be used in diagnostic laboratories seeking to disseminate fast and accurate results to growers for implementation in their insect pest monitoring programs.

PhyEffector, the First Algorithm That Identifies Classical and Non-Classical Effectors in Phytoplasmas

Phytoplasmas are the causal agents of more than 100 plant diseases in economically important crops. Eleven genomes have been fully sequenced and have allowed us to gain a better understanding of the biology and evolution of phytoplasmas. Effectors are key players in pathogenicity and virulence, and their identification and description are becoming an essential practice in the description of phytoplasma genomes. This is of particular importance because effectors are possible candidates for the development of new strategies for the control of plant diseases. To date, the prediction of effectors in phytoplasmas has been a great challenge; the reliable comparison of effectoromes has been hindered because research teams have used the combination of different programs in their predictions. This is not trivial since significant differences in the results can arise, depending on the predictive pipeline used. Here, we tested different predictive pipelines to create the PhyEffector algorithm; the average value of the F1 score for PhyEffector was 0.9761 when applied to different databases or genomes, demonstrating its robustness as a predictive tool. PhyEffector can recover both classical and non-classical phytoplasma effectors, making it an invaluable tool to accelerate effectoromics in phytoplasmas.

Crossing the Gateless Barriers: Factors Involved in the Movement of Circulative Bacteria Within Their Insect Vectors

Plant bacterial pathogens transmitted by hemipteran vectors pose a large threat to the agricultural industry worldwide. Although virus-vector relationships have been widely investigated, a significant gap exists in our understanding of the molecular interactions between circulative bacteria and their insect vectors, mainly leafhoppers and psyllids. In this review, we will describe how these bacterial pathogens adhere, invade, and proliferate inside their insect vectors. We will also highlight the different transmission routes and molecular factors of phloem-limited bacteria that maintain an effective relationship with the insect host. Understanding the pathogen-vector relationship at the molecular level will help in the management of vector-borne bacterial diseases.

Competition among Flavescence Dorée Phytoplasma Strains in the Experimental Insect Vector Euscelidius variegatus

Phytoplasmas are plant pathogenic wall-less bacteria transmitted in a persistent propagative manner by hemipteran insects, mainly belonging to the suborder Auchenorrhyncha (Fulgoromorpha and Cicadomorpha). Flavescence dorée (FD) is a quarantine disease of grapevine, causing great damage to European viticulture and associated with phytoplasmas belonging to 16SrV-C (FD-C) and -D (FD-D) subgroups. FD-C and FD-D strains share similar pathogenicity, but mixed infections are rare in nature. To investigate the competition among FDp strains, specimens of the laboratory vector Euscelidius variegatus (Hemiptera: Cicadellidae) were forced to acquire both phytoplasma haplotypes upon feeding on FD-C- and FD-D-infected plants or after the injection of both strains. The pathogen colonization of insect bodies and heads was monitored with multiplex qPCR, and the efficiencies of phytoplasma transmission were estimated. Single infection, irrespective of strain type, was more frequent than expected, indicating that competition among FD strains occurs. Hypotheses of competition for resources and/or host active sites or the direct antibiosis of one strain against the other are discussed, based on the genetic complexity of FDp populations and on the high genome variability of the FD-D strain. As FD management still mainly relies on insecticides against vectors, the characterization of FDp haplotypes and the description of their epidemiology also have practical implications.

Flavescence dorée phytoplasma enters insect cells by a clathrin-mediated endocytosis allowing infection of its insect vector

To perform its propagative and circulative cycle into its insect vector, the flavescence dorée phytoplasma invades different cell types. Clathrin-mediated endocytosis is used by a wide range of bacteria to infect eukaryote cells. Among the insect proteins interacting with the phytoplasma adhesin VmpA, we identified the adaptor protein complex AP-1 and AP-2 suggesting that phytoplasmas could enter the insect cells via clathrin-mediated endocytosis. By infection assays of insect cells in culture, we showed that phytoplasmas entry into Drosophila S2 cells was more efficient than infection of the Euva cell line developed from the insect vector Euscelidius variegatus. Chlorpromazine, cytochalasin D and knockdown of clathrin heavy chain (chc) gene expression using RNA interference inhibited entry of phytoplasmas into S2 cells. During invasion of S2 cells, phytoplasmas were observed very closed to recombinant GFP-labelled clathrin light chain. To verify the role of clathrin in the insect colonization by phytoplasmas, RNAi was performed via artificial feeding of chc dsRNA by the vector E. variegatus. This decreased the expression of chc gene in the midgut and heads of E. variegatus. The chc lower expression correlated to a decreased of midgut and salivary gland cells colonization after the insects had ingested phytoplasmas from infected plants. In conclusion, results indicate that clathrin is important for the FD phytoplasma to enter insect cells and colonize its insect vector.

A New Perspective on the Co-Transmission of Plant Pathogens by Hemipterans

Co-infection of plants by pathogens is common in nature, and the interaction of the pathogens can affect the infection outcome. There are diverse ways in which viruses and bacteria are transmitted from infected to healthy plants, but insects are common vectors. The present review aims to highlight key findings of studies evaluating the co-transmission of plant pathogens by insects and identify challenges encountered in these studies. In this review, we evaluated whether similar pathogens might compete during co-transmission; whether the changes in the pathogen titer in the host, in particular associated with the co-infection, could influence its transmission; and finally, we discussed the pros and cons of the different approaches used to study co-transmission. At the end of the review, we highlighted areas of study that need to be addressed. This review shows that despite the recent development of techniques and methods to study the interactions between pathogens and their insect vectors, there are still gaps in the knowledge of pathogen transmission. Additional laboratory and field studies using different pathosystems will help elucidate the role of host co-infection and pathogen co-transmission in the ecology and evolution of infectious diseases.

The Complete Genome of the “Flavescence Dorée” Phytoplasma Reveals Characteristics of Low Genome Plasticity

Members of the genus ‘Candidatus Phytoplasma’ are obligate intracellular bacteria restricted to phloem sieve elements and are able to colonize several tissues and the hemolymph in their insect vectors. The current unfeasibility of axenic culture and the low complexity of genomic sequences are obstacles in assembling complete chromosomes. Here, a method combining pathogen DNA enrichment from infected insects and dual deep-sequencing technologies was used to obtain the complete genome of a phytoplasma causing Grapevine Flavescence dorée. The de novo assembly generated a circular chromosome of 654,223 bp containing 506 protein-coding genes. Quality assessment of the draft showed a high degree of completeness. Comparative analysis with other phytoplasmas revealed the absence of potential mobile units and a reduced amount of putative phage-derived segments, suggesting a low genome plasticity. Phylogenetic analyses identified Candidatus Phytoplasma ziziphi as the closest fully sequenced relative. The “Flavescence dorée” phytoplasma strain CH genome also encoded for several putative effector proteins potentially playing a role in pathogen virulence. The availability of this genome provides the basis for the study of the pathogenicity mechanisms and evolution of the Flavescence dorée phytoplasma.

Silencing of ATP Synthase β Impairs Egg Development in the Leafhopper Scaphoideus titanus, Vector of the Phytoplasma Associated with Grapevine Flavescence Dorée

Scaphoideus titanus (Hemiptera: Cicadellidae) is the natural vector of Flavescence dorée phytoplasma, a quarantine pest of grapevine with severe impact on European viticulture. RNA interference (RNAi) machinery components are present in S. titanus transcriptome and injection of ATP synthase β dsRNAs into adults caused gene silencing, starting three days post injection (dpi) up to 20 dpi, leading to decrease cognate protein. Silencing of this gene in the closely related leafhopper Euscelidiusvariegatus previously showed female sterility and lack of mature eggs in ovaries. Here, alteration of developing egg morphology in S. titanus ovaries as well as overexpression of hexamerin transcript (amino acid storage protein) and cathepsin L protein (lysosome proteinase) were observed in dsATP-injected females. To evaluate RNAi-specificity, E.variegatus was used as dsRNA-receiving model-species. Different doses of two sets of dsRNA-constructs targeting distinct portions of ATP synthase β gene of both species induced silencing, lack of egg development, and female sterility in E. variegatus, indicating that off-target effects must be evaluated case by case. The effectiveness of RNAi in S. titanus provides a powerful tool for functional genomics of this non-model species and paves the way toward RNAi-based strategies to limit vector population, despite several technical and regulatory constraints that still need to be overcome to allow open field application.

Made for Each Other: Vector-Pathogen Interfaces in the Huanglongbing Pathosystem

Citrus greening, or huanglongbing (HLB), currently is the most destructive disease of citrus. HLB disease is putatively caused by the phloem-restricted α-proteobacterium 'Candidatus Liberibacter asiaticus'. This bacterium is transmitted primarily by the Asian citrus psyllid Diaphorina citri (Hemiptera: Liviidae). Most animal pathogens are considered pathogenic to their insect vectors, whereas the relationships between plant pathogens and their insect vectors are variable. Lately, the relationship of 'Ca. L. asiaticus' with its insect vector, D. citri, has been well investigated at the molecular, biochemical, and biological levels in many studies. Herein, the findings concerning this relationship are discussed and molecular features of the acquisition of 'Ca. L. asiaticus' from the plant host and its growth and circulation within D. citri, as well as its transmission to plants, are presented. In addition, the effects of 'Ca. L. asiaticus' on the energy metabolism (respiration, tricarboxylic acid cycle, and adenosine triphosphate production), metabolic pathways, immune system, endosymbionts, and detoxification enzymes of D. citri are discussed together with other impacts such as shorter lifespan, altered feeding behavior, and higher fecundity. Overall, although 'Ca. L. asiaticus' has significant negative effects on its insect vector, it increases its vector fitness, indicating that it develops a mutualistic relationship with its vector. This review will help in understanding the specific interactions between 'Ca. L. asiaticus' and its psyllid vector in order to design innovative management strategies.

Interactions between the flavescence dorée phytoplasma and its insect vector indicate lectin-type adhesion mediated by the adhesin VmpA

The flavescence dorée phytoplasma undergoes a propagative cycle in its insect vectors by first interacting with the insect cell surfaces, primarily in the midgut lumen and subsequently in the salivary glands. Adhesion of flavescence dorée phytoplasma to insect cells is mediated by the adhesin VmpA. We hypothesize that VmpA may have lectin-like activity, similar to several adhesins of bacteria that invade the insect gut. We first demonstrated that the luminal surface of the midgut and the basal surface of the salivary gland cells of the natural vector Scaphoideus titanus and those of the experimental vector Euscelidius variegatus were differentially glycosylated. Using ELISA, inhibition and competitive adhesion assays, and protein overlay assays in the Euva-6 insect cell line, we showed that the protein VmpA binds insect proteins in a lectin-like manner. In conclusion, the results of this study indicate that N-acetylglucosamine and mannose present on the surfaces of the midgut and salivary glands serve as recognition sites for the phytoplasma adhesin VmpA.

Phytoplasma diseases of plants: molecular diagnostics and way forward

Phytoplasmas are obligate phytopathogenic bacteria associated with devastating diseases in hundreds of crops across the world. They have been responsible for huge economic losses in many crop plants for decades now. Isolation and establishment of axenic culture of phytoplasma in complex media is a recent progress in phytoplasma research. Earlier methods for phytoplasma disease detection included symptom profiling, microscopy, serology and dodder transmission studies. With advancement in the field of molecular biology, phytoplasma diagnostics and characterisation witnessed radical improvement. Starting from PCR amplification which often necessities a nested PCR on account of low titre of phytoplasmas, to the closed tube quantitative PCR assays and then the ddPCR, an array of diagnostics have been developed for phytoplasma. The isothermal diagnostic platforms are the latest addition to this and the Loop Mediated Isothermal Amplification (LAMP) assay has been applied for the detection of phytoplasma from several hosts. The futuristic approach in phytoplasma detection will be very likely provided by an integration of nanotechnology and molecular diagnostics. Phytoplasma disease management majorly relies on early detection, vector control, use of disease free planting materials and cultivation of resistant varieties. Hence understanding the molecular mechanism of phytoplasma-host interaction is as important as timely and accurate detection, in the management of phytoplasma diseases. Further, the changing climatic scenario and global warming may lead to an upsurge in the phytoplasma diseases spread and severity across the world, making disease management even more challenging.

The agent associated with blue dwarf disease in wheat represents a new phytoplasma taxon, 'Candidatus Phytoplasma tritici'

Wheat blue dwarf (WBD) is one of the most economically damaging cereal crop diseases in northwestern PR China. The agent associated with the WBD disease is a phytoplasma affiliated with the aster yellows (AY) group, subgroup C (16SrI-C). Since phytoplasma strains within the AY group are ecologically and genetically diverse, it has been conceived that the AY phytoplasma group may consist of more than one species. This communication presents evidence to demonstrate that, while each of the two 16 rRNA genes of the WBD phytoplasma shares >97.5 % sequence similarity with that of the 'Candidatus Phytoplasma asteris' reference strain, the WBD phytoplasma clearly represents an ecologically separated lineage: the WBD phytoplasma not only has its unique transmitting vector (Psammotettix striatus) but also elicits a distinctive symptom in its predominant plant host (wheat). In addition, the WBD phytoplasma possesses molecular characteristics that further manifest its significant divergence from 'Ca. P. asteris'. Such molecular characteristics include lineage-specific antigenic membrane proteins and a lower than 95 % genome-wide average nucleotide identity score with 'Ca. P. asteris'. These ecological, molecular and genomic evidences justify the recognition of the WBD phytoplasma as a novel taxon, 'Candidatus Phytoplasma tritici'.

Silencing of ATP synthase β reduces phytoplasma multiplication in a leafhopper vector

The leafhopper Euscelidius variegatus is a natural vector of the chrysanthemum yellows phytoplasma (CYp) and a laboratory vector of the Flavescence dorée phytoplasma (FDp). Previous studies indicated a crucial role for insect ATP synthase α and β subunits during phytoplasma infection of the vector species. Gene silencing of ATP synthase β was obtained by injection of specific dsRNAs in E. variegatus. Here we present the long-lasting nature of such silencing, its effects on the small RNA profile, the significant reduction of the corresponding protein expression, and the impact on phytoplasma acquisition capability. The specific transcript expression was silenced at least up to 37 days post injection with an average reduction of 100 times in insects injected with dsRNAs targeting ATP synthase β (dsATP) compared with those injected with dsRNAs targeting green fluorescent protein (dsGFP), used as negative controls. Specific silencing of this gene was also confirmed at protein level at 15 days after the injection. Total sRNA reads mapping to dsATP and dsGFP sequences in analysed libraries showed in both cases a peak of 21 nt, a length consistent with the generation of dsRNA-derived siRNAs by RNAi pathway. Reads mapped exclusively to the fragment corresponding to the injected dsATPs, probably indicating the absence of a secondary machinery for siRNA synthesis. Insects injected either with dsATP or dsGFP successfully acquired CYp and FDp during feeding on infected plants. However, the average phytoplasma amount in dsATP insects was significantly lower than that measured in dsGFP specimens, indicating a probable reduction of the pathogen multiplication when ATP synthase β was silenced. The role of the insect ATP synthase β during phytoplasma infection process is discussed.

Genetic variation, phylogenetic relationship and spatial distribution of 'Candidatus Phytoplasma ulmi' strains in Germany

A recent survey in Germany revealed the wide presence of 'Candidatus Phytoplasma ulmi' in native elm stands. Accessions were studied for their genetic variability and phylogenetic relationship based on the conserved groEL and the variable imp gene. While the groEL sequences revealed a high intraspecific homology of more than 99%, the homology of the imp gene dropped to 71% between distantly related sequences. Twenty-nine groEL and 74 imp genotypes were distinguished based on polymorphic sites. Phylogenetic analysis of the groEL gene clustered all 'Ca. P. ulmi' strains and separated them from related phytoplasmas of the 16SrV group. The inferred phylogeny of the imp gene resulted in a different tree topology and separated the 'Ca. P. ulmi' genotypes into two clusters, one closely related to the flavescence dorée phytoplasma strain FD-D (16SrV-D), the other affiliated with the flavescence dorée phytoplasma strains FD-C and FD70 and the alder yellows phytoplasma (16SrV-C). In both phylograms, 'Ca. P. ulmi' genotypes from Scots elm trees formed a coherent cluster, while genotypes from European white elms and field elms grouped less strictly. The regional distribution pattern was congruent for some of the groEL and imp genotypes, but a strict linkage for all genotypes was not apparent.

Spatiotemporal dynamics and quantitative analysis of phytoplasmas in insect vectors

Phytoplasmas are transmitted by insect vectors in a persistent propagative manner; however, detailed movements and multiplication patterns of phytoplasmas within vectors remain elusive. In this study, spatiotemporal dynamics of onion yellows (OY) phytoplasma in its vector Macrosteles striifrons were investigated by immunohistochemistry-based 3D imaging, whole-mount fluorescence staining, and real-time quantitative PCR. The results indicated that OY phytoplasmas entered the anterior midgut epithelium by seven days after acquisition start (daas), then moved to visceral muscles surrounding the midgut and to the hemocoel at 14-21 daas; finally, OY phytoplasmas entered into type III cells of salivary glands at 21-28 daas. The anterior midgut of the alimentary canal and type III cells of salivary glands were identified as the major sites of OY phytoplasma infection. Fluorescence staining further revealed that OY phytoplasmas spread along the actin-based muscle fibers of visceral muscles and accumulated on the surfaces of salivary gland cells. This accumulation would be important for phytoplasma invasion into salivary glands, and thus for successful insect transmission. This study demonstrates the spatiotemporal dynamics of phytoplasmas in insect vectors. The findings from this study will aid in understanding of the underlying mechanism of insect-borne plant pathogen transmission.

Evidence suggesting interactions between immunodominant membrane protein Imp of Flavescence dorée phytoplasma and protein extracts from distantly related insect species

AIMS

In this study, binding between the immunodominant membrane protein Imp of the 16SrV-D phytoplasma associated with Flavescence dorée disease (FD-Dp) and insect proteins of vectors and non-vectors of FD-Dp was tested.

METHODS AND RESULTS

Six Auchenorrhyncha species, from distantly related groups were selected: Scaphoideus titanus, Euscelidius variegatus, Macrosteles quadripunctulatus, Zyginidia pullula (Cicadomorpha), Ricania speculum and Metcalfa pruinosa (Fulgoromorpha). The vector status of each species was retrieved from the literature or determined by transmission trials in this study. A His-tagged partial Imp protein and a rabbit polyclonal antibody were synthesized and used for Western and Far-Western dot Blot (FWdB) experiments. Total native and membrane proteins (MP) were extracted from entire bodies and organs (gut and salivary glands) of each insect species. FWdB showed decreasing interaction intensities of Imp fusion protein with total proteins from entire bodies of S. titanus, E. variegatus (competent vectors) and M. quadripunctulatus (non-vector), while no interaction signal was detected with the other three species (non-vectors). A strong signal detected upon interaction of FD-D Imp and MP from guts of closely related insects supports the role of this organ as the first barrier to ensure successful transmission.

CONCLUSIONS

Our results showed that specific Imp binding, correlated with vector status, is involved in interactions between FD-Dp and insect proteins.

SIGNIFICANCE AND IMPACT OF THE STUDY

Integrating knowledge on host-pathogen protein-protein interactions and on insect phylogeny would help to identify the actual range of vectors of phytoplasma strains of economic importance.

Immunofluorescence Assay to Study Early Events of Vector Salivary Gland Colonization by Phytoplasmas

To visualize phytoplasmas at early stages of vector infection, an immunofluorescence assay was developed. The chapter provides experimental details on dissection of salivary glands, incubation of the dissected organs with phytoplasma suspension, fixation, embedding, sectioning, labeling, and final visualization with confocal microscopy. All the procedure will be described for the leafhopper Euscelidius variegatus, natural vector of "Candidatus phytoplasma asteris" and laboratory vector of Flavescence dorée phytoplasma.

RNA interference of muscle actin and ATP synthase beta increases mortality of the phytoplasma vector Euscelidius variegatus

BACKGROUND

RNA interference (RNAi) techniques have emerged as powerful tools to develop novel management strategies for the control of insect pests. The leafhopper Euscelidius variegatus is a natural vector of chrysanthemum yellows phytoplasma and a laboratory vector of Flavescence dorée phytoplasma. Phytoplasmas are insect-borne bacterial plant pathogens that cause economically relevant crop losses worldwide.

RESULTS

In this study, we demonstrated that microinjection of muscle actin and ATP synthase β double-stranded (ds)RNAs into adult insects caused an exponential reduction in the expression of both genes, which began within 72 h of dsRNA administration and lasted for 14 days, leading to almost complete silencing of the target genes. Such silencing effects on muscle actin expression appeared to be both time- and dose-dependent. Our results also showed that the knockdown of both genes caused a significant decrease in survival rates in comparison with green fluorescent protein (GFP) dsRNA-injected control insects.

CONCLUSION

The effectiveness of RNAi-based gene silencing in E. variegatus guarantees the availability of a powerful reverse genetic tool for the functional annotation of its genes and the identification of those potentially involved in the interaction with phytoplasmas. In addition, this study demonstrated that muscle actin and ATP synthase β may represent candidate genes for RNAi-based control of E. variegatus. © 2018 Society of Chemical Industry.

Phytoplasma Transmission: Insect Rearing and Infection Protocols

Phytoplasmas are obligate pathogens and thus they can be studied only in association with their plants or insect hosts. In this chapter, we present protocols for rearing some phytoplasma insect vectors, to obtain infected insects and plants under controlled environmental conditions. We focus on Euscelidius variegatus and Macrosteles quadripunctulatus that can infect Arabidopsis thaliana, and Hyalesthes obsoletus and Scaphoideus titanus, that can infect grapevine.

A multi-layered mechanistic modelling approach to understand how effector genes extend beyond phytoplasma to modulate plant hosts, insect vectors and the environment

Members of the Candidatus genus Phytoplasma are small bacterial pathogens that hijack their plant hosts via the secretion of virulence proteins (effectors) leading to a fascinating array of plant phenotypes, such as witch's brooms (stem proliferations) and phyllody (retrograde development of flowers into vegetative tissues). Phytoplasma depend on insect vectors for transmission, and interestingly, these insect vectors were found to be (in)directly attracted to plants with these phenotypes. Therefore, phytoplasma effectors appear to reprogram plant development and defence to lure insect vectors, similarly to social engineering malware, which employs tricks to lure people to infected computers and webpages. A multi-layered mechanistic modelling approach will enable a better understanding of how phytoplasma effector-mediated modulations of plant host development and insect vector behaviour contribute to phytoplasma spread, and ultimately to predict the long reach of phytoplasma effector genes.

Two Phytoplasmas Elicit Different Responses in the Insect Vector Euscelidius variegatus Kirschbaum

Phytoplasmas are plant-pathogenic bacteria transmitted by hemipteran insects. The leafhopper Euscelidius variegatus is a natural vector of chrysanthemum yellows phytoplasma (CYp) and a laboratory vector of flavescence dorée phytoplasma (FDp). The two phytoplasmas induce different effects on this species: CYp slightly improves whereas FDp negatively affects insect fitness. To investigate the molecular bases of these different responses, transcriptome sequencing (RNA-seq) analysis of E. variegatus infected with either CYp or FDp was performed. The sequencing provided the first de novo transcriptome assembly for a phytoplasma vector and a starting point for further analyses on differentially regulated genes, mainly related to immune system and energy metabolism. Insect phenoloxidase activity, immunocompetence, and body pigmentation were measured to investigate the immune response, while respiration and movement rates were quantified to confirm the effects on energy metabolism. The activation of the insect immune response upon infection with FDp, which is not naturally transmitted by E. variegatus, confirmed that this bacterium is mostly perceived as a potential pathogen. Conversely, the acquisition of CYp, which is naturally transmitted by E. variegatus, seems to increase the insect fitness by inducing a prompt response to stress. This long-term relationship is likely to improve survival and dispersal of the infected insect, thus enhancing the opportunity of phytoplasma transmission.

Variable Membrane Protein A of Flavescence Dorée Phytoplasma Binds the Midgut Perimicrovillar Membrane of Euscelidius variegatus and Promotes Adhesion to Its Epithelial Cells

Phytoplasmas are uncultivated plant pathogens and cell wall-less bacteria and are transmitted from plant to plant by hemipteran insects. The phytoplasma's circulative propagative cycle in insects requires the crossing of the midgut and salivary glands, and primary adhesion to cells is an initial step toward the invasion process. The flavescence dorée (FD) phytoplasma possesses a set of variable membrane proteins (Vmps) exposed on its surface, and this pathogen is suspected to interact with insect cells. The results showed that VmpA is expressed by the flavescence dorée phytoplasma present in the midgut and salivary glands. Phytoplasmas cannot be cultivated at present, and no mutant can be produced to investigate the putative role of Vmps in the adhesion of phytoplasma to insect cells. To overcome this difficulty, we engineered the Spiroplasma citri mutant G/6, which lacks the ScARP adhesins, for VmpA expression and used VmpA-coated fluorescent beads to determine if VmpA acts as an adhesin in ex vivo adhesion assays and in vivo ingestion assays. VmpA specifically interacted with Euscelidius variegatus insect cells in culture and promoted the retention of VmpA-coated beads to the midgut of E. variegatus In this latest case, VmpA-coated fluorescent beads were localized and embedded in the perimicrovillar membrane of the insect midgut. Thus, VmpA functions as an adhesin that could be essential in the colonization of the insect by the FD phytoplasmas.IMPORTANCE Phytoplasmas infect a wide variety of plants, ranging from wild plants to cultivated species, and are transmitted by different leafhoppers, planthoppers, and psyllids. The specificity of the phytoplasma-insect vector interaction has a major impact on the phytoplasma plant host range. As entry into insect cells is an obligate process for phytoplasma transmission, the bacterial adhesion to insect cells is a key step. Thus, studying surface-exposed proteins of phytoplasma will help to identify the adhesins implicated in the specific recognition of insect vectors. In this study, it is shown that the membrane protein VmpA of the flavescence dorée (FD) phytoplasma acts as an adhesin that is able to interact with cells of Euscelidius variegatus, the experimental vector of the FD phytoplasma.

The enemy within: phloem-limited pathogens

The growing impact of phloem-limited pathogens on high-value crops has led to a renewed interest in understanding how they cause disease. Although these pathogens cause substantial crop losses, many are poorly characterized. In this review, we present examples of phloem-limited pathogens that include intracellular bacteria with and without cell walls, and viruses. Phloem-limited pathogens have small genomes and lack many genes required for core metabolic processes, which is, in part, an adaptation to the unique phloem environment. For each pathogen class, we present multiple case studies to highlight aspects of disease caused by phloem-limited pathogens. The pathogens presented include Candidatus Liberibacter asiaticus (citrus greening), Arsenophonus bacteria, Serratia marcescens (cucurbit yellow vine disease), Candidatus Phytoplasma asteris (Aster Yellows Witches' Broom), Spiroplasma kunkelii, Potato leafroll virus and Citrus tristeza virus. We focus on commonalities in the virulence strategies of these pathogens, and aim to stimulate new discussions in the hope that widely applicable disease management strategies can be found.

Quantitative proteome-level analysis of paulownia witches' broom disease with methyl methane sulfonate assistance reveals diverse metabolic changes during the infection and recovery processes

Paulownia witches' broom (PaWB) disease caused by phytoplasma is a fatal disease that leads to considerable economic losses. Although there are a few reports describing studies of PaWB pathogenesis, the molecular mechanisms underlying phytoplasma pathogenicity in Paulownia trees remain uncharacterized. In this study, after building a transcriptome database containing 67,177 sequences, we used isobaric tags for relative and absolute quantification (iTRAQ) to quantify and analyze the proteome-level changes among healthy P. fortunei (PF), PaWB-infected P. fortunei (PFI), and PaWB-infected P. fortunei treated with 20 mg L^-1 or 60 mg L^-1 methyl methane sulfonate (MMS) (PFI-20 and PFI-60, respectively). A total of 2,358 proteins were identified. We investigated the proteins profiles in PF vs. PFI (infected process) and PFI-20 vs. PFI-60 (recovered process), and further found that many of the MMS-response proteins mapped to "photosynthesis" and "ribosome" pathways. Based on our comparison scheme, 36 PaWB-related proteins were revealed. Among them, 32 proteins were classified into three functional groups: (1) carbohydrate and energy metabolism, (2) protein synthesis and degradation, and (3) stress resistance. We then investigated the PaWB-related proteins involved in the infected and recovered processes, and discovered that carbohydrate and energy metabolism was inhibited, and protein synthesis and degradation decreased, as the plant responded to PaWB. Our observations may be useful for characterizing the proteome-level changes that occur at different stages of PaWB disease. The data generated in this study may serve as a valuable resource for elucidating the pathogenesis of PaWB disease during phytoplasma infection and recovery stages.

Genome sequence, prevalence and quantification of the first iflavirus identified in a phytoplasma insect vector

The leafhopper Euscelidius variegatus is a natural vector of chrysanthemum yellows phytoplasma (CY) and an efficient vector of flavescence dorée phytoplasma (FD) under laboratory conditions. During a transcriptome sequencing (RNA-seq) project aimed at investigating the interactions between the insect and the two phytoplasmas, a 10,616-nucleotide-long contig with high sequence similarity to known picorna-like viruses was identified among the assembled insect transcripts. The discovery came totally unexpected, because insects from the laboratory colony did not show any evident symptom that could be related to the presence of a virus. The amino acid sequence, the shape and size of viral particles, and the results of phylogenetic analysis suggest that this virus, named Euscelidius variegatus virus 1 (EVV-1), can be considered a new member of a new species in the genus Iflavirus. EVV-1 was detected in all of the tested insects from the laboratory colony used for RNA-seq, both in phytoplasma-exposed and in non-exposed insects, but the viral load measured in FD-exposed samples was significantly lower than that in non-exposed insects. This result suggests the possible existence of an intriguing cross-talk among insects, endogenous bacteria, and viruses. The identification of two other E. variegatus laboratory colonies that were free of EVV-1 could represent the key to addressing some basic virological issues, e.g., viral replication and transmission mechanisms, and offer the opportunity to use infectious clones to express heterologous genes in the leafhopper and manipulate the expression of endogenous genes by promoting virus-induced gene silencing.

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.

Immunodominant membrane proteins of phytoplasmas

Phytoplasmas are plant-pathogenic, phloem-colonizing, cell wall-less microorganisms that are primarily dependent on insect transmission for their spread and survival. The life cycle of phytoplasmas involves replication in insects and host plants. Until recently, phytoplasmas have resisted all attempts at cultivation in cell-free media, making these pathogens poorly characterized on a physiological and biochemical basis. However, host-pathogen relationships can be studied by investigating immunodominant membrane proteins (IDPs), which are located on the exterior surfaces of phytoplasma cells and are the most abundant proteins of the cell membrane. These membrane proteins come in direct contact with both insect and plant hosts and are thought to play a crucial role in phytoplasma spread both within the plant and by insect vectors. Therefore, there is great interest in studying this class of proteins. We summarize and discuss important investigations about these membrane proteins, which have already provided a better understanding of the host-phytoplasma relationship.