Infection and replication sites of Spiroplasma kunkelii (Class: Mollicutes) in midgut and Malpighian tubules of the leafhopper Dalbulus maidis

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.

Corn Stunt Pathosystem and Its Leafhopper Vector in Brazil

Direct and indirect injury caused by Dalbulus maidis (Hemiptera: Cicadellidae) in corn is an ever-increasing concern in Brazil and other corn-producing countries of the Americas. This highly efficient vector transmits corn stunting pathogens and is of economic concern in the Neotropics, including temperate regions where epidemic outbreaks are now common. Despite the progress made so far, Brazilian corn growers continue to struggle with this pest and its associated pathosystem. In this review, we gathered relevant and updated information on the bioecology, population dynamics, and damaging potential of D. maidis. Our goal was to better understand its intimate association and complex interactions with the host crop and transmitted pathogens. Based on available scientific literature, we identified factors which explain the recent increase in D. maidis occurrence in South America, including the cultivation of corn during multiple growing seasons, overlapping of susceptible crops, and widespread use of genetically modified hybrids. The reasons for the overall inefficiency of current suppression strategies aimed at this pest are also summarized. Finally, a management program for D. maidis and corn stunt disease is proposed, combining strategies such as eradicating volunteer corn, reducing the planting period, using tolerant hybrids, and applying chemical and/or fungal insecticides. Prospects regarding the pest's status are also outlined. Overall, the information presented here will serve as a decision-making guide within Brazilian and South American corn production systems, as well as paving the way for devising novel strategies aimed at suppressing D. maidis populations and limiting the spread of corn stunt disease.

The secret life of insect-associated microbes and how they shape insect-plant interactions

Insects are associated with a plethora of different microbes of which we are only starting to understand their role in shaping insect–plant interactions. Besides directly benefitting from symbiotic microbial metabolism, insects obtain and transmit microbes within their environment, making them ideal vectors and potential beneficiaries of plant diseases and microbes that alter plant defenses. To prevent damage, plants elicit stress-specific defenses to ward off insects and their microbiota. However, both insects and microbes harbor a wealth of adaptations that allow them to circumvent effective plant defense activation. In the past decades, it has become apparent that the enormous diversity and metabolic potential of insect-associated microbes may play a far more important role in shaping insect–plant interactions than previously anticipated. The latter may have implications for the development of sustainable pest control strategies. Therefore, this review sheds light on the current knowledge on multitrophic insect–microbe–plant interactions in a rapidly expanding field of research.

Insecticide seed treatment against corn leafhopper: helping protect grain yield in critical plant growth stages

BACKGROUND

The corn leafhopper, Dalbulus maidis (Hemiptera: Cicadellidae), spreads maize stunt pathogens and requires timely and effective crop protection. We determined the interaction between maize phenology and the vector feeding/infection period by stunt pathogens with the residual efficacy of neonicotinoid insecticidal seed treatments. Greenhouse- and field-grown maize plants, seed-treated with clothianidin or imidacloprid insecticides, were infested during seven growth stages with corn leafhoppers reared under controlled conditions on maize plants displaying infection symptoms by both spiroplasma (corn stunt spiroplasma, Spiroplasma kunkelii) and phytoplasma (maize bushy phytoplasma) pathogens.

RESULTS

In the greenhouse and field settings, seed treatment reduced the stunt disease symptoms and corn yield loss during the VE-V4 maize growth stages and caused no phytotoxicity. The neonicotinoid seed treatment reduced 20-60% of the yield losses from the corn stunt disease until the V4 growth stage. Infestation by infective corn leafhoppers in the V12 maize growth stage caused a 25-30% yield loss irrespective of seed treatment, yet no stunt disease symptom was evident. Nonetheless, corn yield losses and visual stunt symptoms as rated by a nine-category ordinal scale were strongly correlated (r = 0.79, P < 0.01).

CONCLUSION

These results reinforce that maize plants are more susceptible to leafhopper stunt disease during the VE-V4 growth stages (emergence to the fourth-leaf stage). Seed treatment helps reduce the damage in the early growth stages (VE-V2), although supplemental control measures depending on leafhopper population density may be needed from VE-V12 to protect yield losses from the maize stunt condition. © 2021 Society of Chemical Industry.

Ultrastructure of phytoplasma-infected jujube leaves with witches' broom disease

The subcellular characteristics of phytoplasma-infected jujube (Ziziphus jujuba) leaves were investigated using transmission electron microscopy. Midrib fragments of witches' broom-diseased jujube leaves were collected from abnormally small leaves at an early stage of branch clustering. The diseased jujube leaves showed multivesicular bodies (MVBs) with vesicles and tubules in the phloem parenchyma cells and sieve elements. The MVBs were connected to the plasma membrane appressed to the cell wall. There were increased callose collars at the pore-plasmodesma unit ends of the sieve elements in the diseased leaves than in control leaves. The proliferation of MVBs in the diseased jujube leaves could be associated with endoplasmic reticulum stress-dependent exosome release. The phytoplasma produced pleomorphic cells in sieve elements. Several types of putative extracellular structures were observed on the phytoplasma cells: (i) fimbriae-like threads, (ii) pili-like projections, (iii) flagella-like appendages, and (iv) tube-like structures. This study provides novel insights into intracellular obligate cell wall-less prokaryotes and host phloem structures.

Corn Stunt Disease: An Ideal Insect-Microbial-Plant Pathosystem for Comprehensive Studies of Vector-Borne Plant Diseases of Corn

Over 700 plant diseases identified as vector-borne negatively impact plant health and food security globally. The pest control of vector-borne diseases in agricultural settings is in urgent need of more effective tools. Ongoing research in genetics, molecular biology, physiology, and vector behavior has begun to unravel new insights into the transmission of phytopathogens by their insect vectors. However, the intricate mechanisms involved in phytopathogen transmission for certain pathosystems warrant further investigation. In this review, we propose the corn stunt pathosystem (Zea mays-Spiroplasma kunkelii-Dalbulus maidis) as an ideal model for dissecting the molecular determinants and mechanisms underpinning the persistent transmission of a mollicute by its specialist insect vector to an economically important monocotyledonous crop. Corn stunt is the most important disease of corn in the Americas and the Caribbean, where it causes the severe stunting of corn plants and can result in up to 100% yield loss. A comprehensive study of the corn stunt disease system will pave the way for the discovery of novel molecular targets for genetic pest control targeting either the insect vector or the phytopathogen.

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.

When a Palearctic bacterium meets a Nearctic insect vector: Genetic and ecological insights into the emergence of the grapevine Flavescence dorée epidemics in Europe

Flavescence dorée (FD) is a European quarantine grapevine disease transmitted by the Deltocephalinae leafhopper Scaphoideus titanus. Whereas this vector had been introduced from North America, the possible European origin of FD phytoplasma needed to be challenged and correlated with ecological and genetic drivers of FD emergence. For that purpose, a survey of genetic diversity of these phytoplasmas in grapevines, S. titanus, black alders, alder leafhoppers and clematis were conducted in five European countries. Out of 132 map genotypes, only 11 were associated to FD outbreaks, three were detected in clematis, whereas 127 were detected in alder trees, alder leafhoppers or in grapevines out of FD outbreaks. Most of the alder trees were found infected, including 8% with FD genotypes M6, M38 and M50, also present in alders neighboring FD-free vineyards and vineyard-free areas. The Macropsinae Oncopsis alni could transmit genotypes unable to achieve transmission by S. titanus, while the Deltocephalinae Allygus spp. and Orientus ishidae transmitted M38 and M50 that proved to be compatible with S. titanus. Variability of vmpA and vmpB adhesin-like genes clearly discriminated 3 genetic clusters. Cluster Vmp-I grouped genotypes only transmitted by O. alni, while clusters Vmp-II and -III grouped genotypes transmitted by Deltocephalinae leafhoppers. Interestingly, adhesin repeated domains evolved independently in cluster Vmp-I, whereas in clusters Vmp-II and-III showed recent duplications. Latex beads coated with various ratio of VmpA of clusters II and I, showed that cluster II VmpA promoted enhanced adhesion to the Deltocephalinae Euscelidius variegatus epithelial cells and were better retained in both E. variegatus and S. titanus midguts. Our data demonstrate that most FD phytoplasmas are endemic to European alders. Their emergence as grapevine epidemic pathogens appeared restricted to some genetic variants pre-existing in alders, whose compatibility to S. titanus correlates with different vmp gene sequences and VmpA binding properties.

Immuno-Ultrastructural Localization and Putative Multiplication Sites of Huanglongbing Bacterium in Asian Citrus Psyllid Diaphorina citri

Huanglongbing, the most destructive citrus disease worldwide, is caused by the bacterium 'Candidatus Liberibacter asiaticus' (CLas) and is vectored by the Asian citrus psyllid (ACP). Very little is known about the form and distribution of CLas in infected psyllids, especially at the ultrastructural level. Here, we examined these aspects by transmission electron microscopy, combined with immunogold labeling. In CLas-exposed ACP adults, the CLas bacterial cells were found to be pleomorphic taking tubular, spherical, or flask-shaped forms, some of which seemed to divide further. Small or large aggregates of CLas were found in vacuolated cytoplasmic pockets of most ACP organs and tissues examined, including the midgut, filter chamber, hindgut, Malpighian tubules, and secretory cells of the salivary glands, in addition to fat tissues, epidermis, muscle, hemocytes, neural tissues, bacteriome, and walls of the female spermatheca and oviduct. Large aggregates of CLas were found outside the midgut within the filter chamber and between the sublayers of the basal lamina of the hindgut and Malpighian tubules. Novel intracytoplasmic structures that we hypothesized as 'putative CLas multiplication sites' were found in the cells of the midgut, salivary glands, and other tissues in CLas-exposed ACP. These structures, characterized by containing a granular matrix and closely packed bacterial cells, were unbound by membranes and were frequently associated with rough endoplasmic reticulum. Our results point to the close association between CLas and its psyllid vector, and provide support for a circulative-propagative mode of transmission.

Undetected Infection by Maize Bushy Stunt Phytoplasma Enhances Host-Plant Preference to Dalbulus maidis (Hemiptera: Cicadellidae)

Vector-borne plant pathogenic bacteria can induce changes in infected plants favoring the insect vector behavior and biology. The study aimed to determine the effect of maize bushy stunt phytoplasma (MBSP) postinoculation period on the host plant preference and transmission efficiency by the corn leafhopper, Dalbulus maidis DeLong & Wolcott, 1923 (Hemiptera: Cicadellidae). In a series of choice tests, D. maidis preference was measured as settling and oviposition on healthy maize plants versus infected maize plants showing early disease symptoms, advanced symptoms, or no symptoms. Finally, transmission efficiency of D. maidis was measured when the vector previously acquired the phytoplasma from asymptomatic source plants at different postinoculation periods. D. maidis adults preferred to settle and to oviposit on healthy than on symptomatic infected plants with advanced disease symptoms, and preferred asymptomatic plants over symptomatic ones. MBSP transmission by D. maidis was positively correlated with the postinoculation period of the source plant. Results suggest an MBSP modulation for D. maidis preference on asymptomatic infected maize plants in the early stages of the crop, allowing the pathogen an undetected transmission.

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.

First Findings in the Route of the Maize Bushy Stunt Phytoplasma Within Its Vector Dalbulus maidis (Hemiptera: Cicadellidae)

In the pathosystem of Dalbulus madis (DeLong & Wolcott) (Hemiptera: Cicadellidae), a vector of maize bushy stunt phytoplasma (MBSP), the interactions occurring during the passage, invasion, and multiplication of the phytoplasma inside the vector body have been generalized from other pathosystems, with a poor understanding of the specific interactions. With the aim to understand MBSP movement and potential specific interactions with its vector, D. maidis adults were dissected to obtain the intestine and salivary gland of both infected (acquisition access period=4 d; latent period=23 d) and noninfected individuals. The organs were processed for visualization with transmission electronic microscopy. Images of phytoplasma cells were observed in the alimentary canal, epithelium of the mesenteron, hemocele, and salivary gland of the vector, and were confirmed through observation of similar cells in maize roots with advanced disease symptoms. The study of the MBSP movement within its vector shows novel findings between the synergy of the MBSP phytoplasma and D. maidis.

Infection Density Dynamics of the Citrus Greening Bacterium "Candidatus Liberibacter asiaticus" in Field Populations of the Psyllid Diaphorina citri and Its Relevance to the Efficiency of Pathogen Transmission to Citrus Plants

Huanglongbing, or citrus greening, is a devastating disease of citrus plants recently spreading worldwide, which is caused by an uncultivable bacterial pathogen, "Candidatus Liberibacter asiaticus," and vectored by a phloem-sucking insect, Diaphorina citri. We investigated the infection density dynamics of "Ca. Liberibacter asiaticus" in field populations of D. citri with experiments using field-collected insects to address how "Ca. Liberibacter asiaticus" infection density in the vector insect is relevant to pathogen transmission to citrus plants. Of 500 insects continuously collected from "Ca. Liberibacter asiaticus"-infected citrus trees with pathological symptoms in the spring and autumn of 2009, 497 (99.4%) were "Ca. Liberibacter asiaticus" positive. The infections were systemic across head-thorax and abdomen, ranging from 10(3) to 10(7) bacteria per insect. In spring, the infection densities were low in March, at ∼ 10(3) bacteria per insect, increasing up to 10(6) to 10(7) bacteria per insect in April and May, and decreasing to 10(5) to 10(6) bacteria per insect in late May, whereas the infection densities were constantly ∼ 10(6) to 10(7) bacteria per insect in autumn. Statistical analysis suggested that several factors, such as insect sex, host trees, and collection dates, may be correlated with "Ca. Liberibacter asiaticus" infection densities in field D. citri populations. Inoculation experiments with citrus seedlings using field-collected "Ca. Liberibacter asiaticus"-infected insects suggested that (i) "Ca. Liberibacter asiaticus"-transmitting insects tend to exhibit higher infection densities than do nontransmitting insects, (ii) a threshold level (∼ 10(6) bacteria per insect) of "Ca. Liberibacter asiaticus" density in D. citri is required for successful transmission to citrus plants, and (iii) D. citri attaining the threshold infection level transmits "Ca. Liberibacter asiaticus" to citrus plants in a stochastic manner. These findings provide valuable insights into understanding, predicting, and controlling this notorious citrus pathogen.

Insect-borne plant pathogenic bacteria: getting a ride goes beyond physical contact

Plant pathogens have evolved numerous strategies that enable their movement from plant to plant. Phytopathogens use a great variety of insect species for transmission to plants, and insect transmission has evolved multiple times independently, particularly for phloem-inhabiting bacteria. Recent studies have advanced our understanding about the mechanisms of physical association between plant pathogenic bacteria and insect vectors. Furthermore, recent evidence shows that the transmission of plant pathogens goes beyond a physical association with the insect, and involves active modulation of plant processes by the bacteria to promote insect herbivore attraction, colonization and pathogen transmission.

Arthropod-Spiroplasma relationship in the genomic era

The genus Spiroplasma comprises wall-less, low-GC bacteria that establish pathogenic, mutualistic and commensal symbiotic associations with arthropods and plants. This review focuses on the symbiotic relationships between Spiroplasma bacteria and arthropod hosts in the context of the available genomic sequences. Spiroplasma genomes are reduced and some contain highly repetitive plectrovirus-related sequences. Spiroplasma's diversity in viral invasion susceptibility, virulence factors, substrate utilization, genome dynamics and symbiotic associations with arthropods make this bacterial genus a biological model that provides insights about the evolutionary traits that shape bacterial symbiotic relationships with eukaryotes.

Gross morphology and ultrastructure of salivary glands of the mute cicada Karenia caelatata Distant (Hemiptera: Cicadoidea)

Salivary glands of the cicada Karenia caelatata Distant were investigated using light microscopy and transmission electron microscopy. The salivary glands are paired structures and consist of principal glands and accessory glands. The principal gland is subdivided into anterior lobe and posterior lobe; the former contains about 34-39 long digitate lobules, while the latter contains approximately 30-33 long digitate lobules and 13-22 short digitate lobules. These short digitate lobules, about one fifth or sixth as long as the long digitate lobules, locate at the base of the long digitate lobules of posterior lobe. All of these digitate lobules vary in size, disposition, length and shape. The anterior lobe and the posterior lobe are connected by an anterior-posterior duct. Two efferent salivary ducts, which connect with the posterior lobe, fuse to form a common duct. The accessory gland is composed of three parts: a greatly tortuous and folded accessory salivary tube, a circlet of gular gland constituting of several acini of the same size, and a non-collapsible accessory salivary duct. The digitate lobules and gular glands possess secretory cells containing abundant secretory granules vary in size, shape, and electron density, as might indicate different materials are synthesized in different secretory regions. The anterior-posterior duct lines with a player of cuticular lining, and cells beneath the cuticular lining lack of basal infoldings, as suggests the duct serves just to transport secretions. The accessory salivary duct is lined with cuticular lining; cells of the duct have well developed basal infoldings associated with abundant mitochondria, as probably suggests the duct is a reabsorptive region of ions. The cells of the accessory salivary tube possess deep basal infoldings and well developed apical dense microvilli, indicating the cells of the tube are secretory in function. Concentric lamellar structures and a peculiar structure with abundant membrane-bound vesicles and secretory granules are observed for the first time, but their derivation and function remain unclear. The morphology and ultrastructure differences observed in the principal glands and accessory gland of the salivary glands of K. caelatata indicate that the sheath saliva was secreted by the principal glands, and the watery saliva was secreted by the accessory salivary glands. Rod-shaped microorganisms are found in the salivary glands (i.e., accessory salivary duct, gular gland, and long digitate lobule of salivary glands) for the first time, and their identity, function, and relationship to microorganisms residing in the salivary glands and/or other parts of alimentary canal of other cicadas need to be investigated further.

The repetitive domain of ScARP3d triggers entry of Spiroplasma citri into cultured cells of the vector Circulifer haematoceps

Spiroplasma citri is a plant pathogenic mollicute transmitted by the leafhopper vector Circulifer haematoceps. Successful transmission requires the spiroplasmas to cross the intestinal epithelium and salivary gland barriers through endocytosis mediated by receptor-ligand interactions. To characterize these interactions we studied the adhesion and invasion capabilities of a S. citri mutant using the Ciha-1 leafhopper cell line. S. citri GII3 wild-type contains 7 plasmids, 5 of which (pSci1 to 5) encode 8 related adhesins (ScARPs). As compared to the wild-type strain GII3, the S. citri mutant G/6 lacking pSci1 to 5 was affected in its ability to adhere and enter into the Ciha-1 cells. Proteolysis analyses, Triton X-114 partitioning and agglutination assays showed that the N-terminal part of ScARP3d, consisting of repeated sequences, was exposed to the spiroplasma surface whereas the C-terminal part was anchored into the membrane. Latex beads cytadherence assays showed the ScARP3d repeat domain (Rep3d) to be involved, and internalization of the Rep3d-coated beads to be actin-dependent. These data suggested that ScARP3d, via its Rep3d domain, was implicated in adhesion of S. citri GII3 to insect cells. Inhibition tests using anti-Rep3d antibodies and competitive assays with recombinant Rep3d both resulted in a decrease of insect cells invasion by the spiroplasmas. Unexpectedly, treatment of Ciha-1 cells with the actin polymerisation inhibitor cytochalasin D increased adhesion and consequently entry of S. citri GII3. For the ScARPs-less mutant G/6, only adhesion was enhanced though to a lesser extent following cytochalasin D treatment. All together these results strongly suggest a role of ScARPs, and particularly ScARP3d, in adhesion and invasion of the leafhopper cells by S. citri.

Exitianus obscurinervis (Hemiptera: Cicadellidae), a new experimental vector of Spiroplasma kunkelii

"Corn stunt" caused by the mollicute Spiroplasma kunkelii (Whitcomb) is potentially one of the most severe diseases affecting the corn (Zea mays L.) crop in the Americas, and the leafhopper Dalbulus maidis (DeLong & Wolcott) is considered its most important vector. However, other insects seen quite frequently in corn crops might well be its vectors in Argentina To identify any leafhoppers species other than D. maidis that can transmit S. kunkelii, transmission assays were conducted, using individuals of Exitianus obscurinervis (Stål) collected in field and reared under controlled conditions. S. kunkelii was transmitted to corn plants by E. obscurinervis. The pathogen was transmitted to seven of the 11 plants, which showed characteristic corn stunt symptoms, and the presence of the pathogen was confirmed by DAS-ELISA. The presence of S. kunkelii in the E. obscurinervis individuals used in transmission experiments was confirmed by polymerase chain reaction and electron microscopy. The current study shows the existence of a new experimental vector of S. kunkelii, the leafhopper E. obscurinervis, which acquired spiroplasmas from infected plants and inoculated it to healthy plants.

Involvement of a minimal actin-binding region of Spiroplasma citri phosphoglycerate kinase in spiroplasma transmission by its leafhopper vector

Background

Spiroplasma citri is a wall-less bacterium that colonizes phloem vessels of a large number of host plants. Leafhopper vectors transmit S. citri in a propagative and circulative manner, involving colonization and multiplication of bacteria in various insect organs. Previously we reported that phosphoglycerate kinase (PGK), the well-known glycolytic enzyme, bound to leafhopper actin and was unexpectedly implicated in the internalization process of S. citri into Circulifer haematoceps cells.

Methodology/Principal Findings

In an attempt to identify the actin-interacting regions of PGK, several overlapping PGK truncations were generated. Binding assays, using the truncations as probes on insect protein blots, revealed that the actin-binding region of PGK was located on the truncated peptide designated PGK-FL5 containing amino acids 49–154. To investigate the role of PGK-FL5-actin interaction, competitive spiroplasma attachment and internalization assays, in which His₆-tagged PGK-FL5 was added to Ciha-1 cells prior to infection with S. citri, were performed. No effect on the efficiency of attachment of S. citri to leafhopper cells was observed while internalization was drastically reduced. The in vivo effect of PGK-FL5 was confirmed by competitive experimental transmission assays as injection of PGK-FL5 into S. citri infected leafhoppers significantly affected spiroplasmal transmission.

Conclusion

These results suggest that S. citri transmission by its insect vector is correlated to PGK ability to bind actin.

Diverse targets of phytoplasma effectors: from plant development to defense against insects

Phytoplasma research begins to bloom (75). Indeed, this review shows that substantial progress has been made with the identification of phytoplasma effectors that alter flower development, induce witches' broom, affect leaf shape, and modify plant-insect interactions. Phytoplasmas have a unique life cycle among pathogens, as they invade organisms of two distinct kingdoms, namely plants (Plantae) and insects (Animalia), and replicate intracellularly in both. Phytoplasmas release effectors into host cells of plants and insects to target host molecules, and in plants these effectors unload from the phloem to access distal tissues and alter basic developmental processes. The effectors provide phytoplasmas with a fitness advantage by modulating their plant and insect hosts. We expect that further research on the functional characterization of phytoplasma effectors will generate new knowledge that is relevant to fundamental aspects of plant sciences and entomology, and for agriculture by improving yields of crops affected by phytoplasma diseases.

Spiroplasmas and phytoplasmas: microbes associated with plant hosts

This review will focus on two distinct genera, Spiroplasma and 'Candidatus Phytoplasma,' within the class Mollicutes (which also includes the genus Mycoplasma, a concern for animal-based cell culture). As members of the Mollicutes, both are cell wall-less microbes which have a characteristic small size (1-2 microM in diameter) and small genome size (530 Kb-2220 Kb). These two genera contain microbes which have a dual host cycle in which they can replicate in their leafhopper or psyllid insect vectors as well as in the sieve tubes of their plant hosts. Major distinctions between the two genera are that most spiroplasmas are cultivable in nutrient rich media, possess a very characteristic helical morphology, and are motile, while the phytoplasmas remain recalcitrant to cultivation attempts to date and exhibit a pleiomorphic or filamentous shape. This review article will provide a historical over view of their discovery, a brief review of taxonomical characteristics, diversity, host interactions (with a focus on plant hosts), phylogeny, and current detection and elimination techniques.

Entry of Spiroplasma citri into Circulifer haematoceps cells involves interaction between spiroplasma phosphoglycerate kinase and leafhopper actin

Transmission of the phytopathogenic mollicutes, spiroplasmas, and phytoplasmas by their insect vectors mainly depends on their ability to pass through gut cells, to multiply in various tissues, and to traverse the salivary gland cells. The passage of these different barriers suggests molecular interactions between the plant mollicute and the insect vector that regulate transmission. In the present study, we focused on the interaction between Spiroplasma citri and its leafhopper vector, Circulifer haematoceps. An in vitro protein overlay assay identified five significant binding activities between S. citri proteins and insect host proteins from salivary glands. One insect protein involved in one binding activity was identified by liquid chromatography-tandem mass spectrometry (LC-MS/MS) as actin. Confocal microscopy observations of infected salivary glands revealed that spiroplasmas colocated with the host actin filaments. An S. citri actin-binding protein of 44 kDa was isolated by affinity chromatography and identified by LC-MS/MS as phosphoglycerate kinase (PGK). To investigate the role of the PGK-actin interaction, we performed competitive binding and internalization assays on leafhopper cultured cell lines (Ciha-1) in which His(6)-tagged PGK from S. citri or purified PGK from Saccharomyces cerevisiae was added prior to the addition of S. citri inoculum. The results suggested that exogenous PGK has no effect on spiroplasmal attachment to leafhopper cell surfaces but inhibits S. citri internalization, demonstrating that the process leading to internalization of S. citri in eukaryotic cells requires the presence of PGK. PGK, regardless of origin, reduced the entry of spiroplasmas into Ciha-1 cells in a dose-dependent manner.

Infection of the Circulifer haematoceps cell line Ciha-1 by Spiroplasma citri: the non-insect-transmissible strain 44 is impaired in invasion

Successful transmission of Spiroplasma citri by its leafhopper vector requires a specific interaction between the spiroplasma surface and the insect cells. With the aim of studying these interactions at the cellular and molecular levels, a cell line, named Ciha-1, was established using embryonic tissues from the eggs of the S. citri natural vector Circulifer haematoceps. This is the first report, to our knowledge, of a cell line for this leafhopper species and of its successful infection by the insect-transmissible strain S. citri GII3. Adherence of the spiroplasmas to the cultured Ciha-1 cells was studied by c.f.u. counts and by electron microscopy. Entry of the spiroplasmas into the insect cells was analysed quantitatively by gentamicin protection assays and qualitatively by double immunofluorescence microscopy. Spiroplasmas were detected within the cell cytoplasm as early as 1 h after inoculation and survived at least 2 days inside the cells. Comparing the insect-transmissible GII3 and non-insect-transmissible 44 strains revealed that adherence to and entry into Ciha-1 cells of S. citri 44 were significantly less efficient than those of S. citri GII3.

Biological diversity of prokaryotic type IV secretion systems

Type IV secretion systems (T4SS) translocate DNA and protein substrates across prokaryotic cell envelopes generally by a mechanism requiring direct contact with a target cell. Three types of T4SS have been described: (i) conjugation systems, operationally defined as machines that translocate DNA substrates intercellularly by a contact-dependent process; (ii) effector translocator systems, functioning to deliver proteins or other macromolecules to eukaryotic target cells; and (iii) DNA release/uptake systems, which translocate DNA to or from the extracellular milieu. Studies of a few paradigmatic systems, notably the conjugation systems of plasmids F, R388, RP4, and pKM101 and the Agrobacterium tumefaciens VirB/VirD4 system, have supplied important insights into the structure, function, and mechanism of action of type IV secretion machines. Information on these systems is updated, with emphasis on recent exciting structural advances. An underappreciated feature of T4SS, most notably of the conjugation subfamily, is that they are widely distributed among many species of gram-negative and -positive bacteria, wall-less bacteria, and the Archaea. Conjugation-mediated lateral gene transfer has shaped the genomes of most if not all prokaryotes over evolutionary time and also contributed in the short term to the dissemination of antibiotic resistance and other virulence traits among medically important pathogens. How have these machines adapted to function across envelopes of distantly related microorganisms? A survey of T4SS functioning in phylogenetically diverse species highlights the biological complexity of these translocation systems and identifies common mechanistic themes as well as novel adaptations for specialized purposes relating to the modulation of the donor-target cell interaction.

Phytoplasmas: bacteria that manipulate plants and insects

TAXONOMY

Superkingdom Prokaryota; Kingdom Monera; Domain Bacteria; Phylum Firmicutes (low-G+C, Gram-positive eubacteria); Class Mollicutes; Candidatus (Ca.) genus Phytoplasma.

HOST RANGE

Ca. Phytoplasma comprises approximately 30 distinct clades based on 16S rRNA gene sequence analyses of approximately 200 phytoplasmas. Phytoplasmas are mostly dependent on insect transmission for their spread and survival. The phytoplasma life cycle involves replication in insects and plants. They infect the insect but are phloem-limited in plants. Members of Ca. Phytoplasma asteris (16SrI group phytoplasmas) are found in 80 monocot and dicot plant species in most parts of the world. Experimentally, they can be transmitted by approximately 30, frequently polyphagous insect species, to 200 diverse plant species.

DISEASE SYMPTOMS

In plants, phytoplasmas induce symptoms that suggest interference with plant development. Typical symptoms include: witches' broom (clustering of branches) of developing tissues; phyllody (retrograde metamorphosis of the floral organs to the condition of leaves); virescence (green coloration of non-green flower parts); bolting (growth of elongated stalks); formation of bunchy fibrous secondary roots; reddening of leaves and stems; generalized yellowing, decline and stunting of plants; and phloem necrosis. Phytoplasmas can be pathogenic to some insect hosts, but generally do not negatively affect the fitness of their major insect vector(s). In fact, phytoplasmas can increase fecundity and survival of insect vectors, and may influence flight behaviour and plant host preference of their insect hosts.

DISEASE CONTROL

The most common practices are the spraying of various insecticides to control insect vectors, and removal of symptomatic plants. Phytoplasma-resistant cultivars are not available for the vast majority of affected crops.

Characterization of a Novel Adhesin-like Gene and Design of a Real-Time PCR for Rapid, Sensitive, and Specific Detection of Spiroplasma kunkelii

Spiroplasma kunkelii, a cell wall-less bacterium, is the causal agent of corn stunt disease. The pathogen is restricted to phloem sieve cells of infected plants and is transmitted by phloem-feeding leafhoppers. Since symptoms of corn stunt disease may not appear until close to flowering time, early detection of the pathogen in disease-transmitting leafhoppers and in symptomless foliar tissues of host plants is critical to disease forecasting and outbreak management. In this study, a field-deployable real-time polymerase chain reaction (PCR) assay was developed for sensitive and specific detection of S. kunkelii. Nucleotide sequence from a previously unreported adhesin-like gene was used to design primers and a fluorogenic probe. The assay was able to detect the presence of S. kunkelii DNA as low as 5 fg, a sensitivity 100 times more than that of conventional PCR. The assay was found to be highly specific to S. kunkelii, as it did not cross-react with one of the most closely related plant pathogenic spiroplasma species, S. citri. The assay was successfully applied to rapid field detection of S. kunkelii in its plant host and insect vectors.

Plasmid pSci6 from Spiroplasma citri GII-3 confers insect transmissibility to the non-transmissible strain S. citri 44

The insect-transmissible strain GII-3 of Spiroplasma citri contains plasmids pSci1–6, five of which (pSci1–5) encode adhesin-like proteins and one (pSci6) encodes protein P32, which has been associated with insect transmissibility. In contrast, S. citri strains ASP-1 and 44, which cannot be transmitted via injection into the leafhopper vector Circulifer haematoceps, lack these proteins and also do not carry plasmids pSci1–6. To further study the apparent relationship between the presence of plasmids and insect transmissibility, plasmids from S. citri GII-3 were introduced into the insect-non-transmissible S. citri strain 44 by electrotransformation using the tetM gene as the selection marker. Tetracycline-resistant transformants were shown to carry one, two or three distinct plasmids. Plasmids pSci1–6 were all detected in the transformants, pSci1 being the most frequently found, alone or together with other plasmids. Selected S. citri 44 transformants having distinct plasmid contents were submitted, separately or in combination, to experimental transmission to periwinkle (Catharanthus roseus) plants via injection into the leafhopper vector. The occurrence of symptomatic plants indicated that, in contrast to S. citri 44, spiroplasmal transformants were transmitted to the host plant, in which they multiplied. Spiroplasma cultures isolated from these infected plants all contained pSci6, leading to the conclusion that, under the experimental conditions used, transformation by pSci6 conferred insect transmissibility to S. citri strain 44. This is believed to be the first report of a phenotypic change associated with transformation of S. citri by natural plasmids.

Identification of a Spiroplasma citri hydrophilic protein associated with insect transmissibility

With the aim of identifyingSpiroplasma citriproteins involved in transmission by the leafhopperCirculifer haematoceps, protein maps of four transmissible and four non-transmissible strains were compared. Total cell lysates of strains were analysed by two-dimensional gel electrophoresis using commercially available immobilized pH gradients (IPGs) covering a pH range of 4–7. Approximately 530 protein spots were visualized by silver staining and the resulting protein spot patterns for the eight strains were found to be highly similar. However, comparison using PDQuest 2-D analysis software revealed two trains of protein spots that were present only in the four transmissible strains. Using MALDI-TOF (matrix-assisted laser desorption/ionization time-of-flight) mass spectrometry and a nearly completeS. citriprotein database, established during the still-ongoingS. citriGII-3-3X genome project, the sequences of both proteins were deduced. One of these proteins was identified in the general databases as adhesion-related protein (P89) involved in the attachment ofS. citrito gut cells of the insect vector. The second protein, with an apparent molecular mass of 32 kDa deduced from the electrophoretic mobility, could not be assigned to a known protein and was named P32. The P32-encoding gene (714 bp) was carried by a large plasmid of 35·3 kbp present in transmissible strains and missing in non-transmissible strains. PCR products with primers designed from thep32gene were obtained only with genomic DNA isolated from transmissible strains. Therefore, P32 has a putative role in the transmission process and it could be considered as a marker forS. citrileafhopper transmissibility. Functional complementation of a non-transmissible strain with thep32gene did not restore the transmissible phenotype, despite the expression of P32 in the complemented strain. Electron microscopic observations of salivary glands of leafhoppers infected with the complemented strain revealed a close contact between spiroplasmas and the plasmalemma of the insect cells. This further suggests that P32 protein contributes to the association ofS. citriwith host membranes.

Interaction between the membrane protein of a pathogen and insect microfilament complex determines insect-vector specificity

Many insect-transmissible pathogens are transmitted by specific insect species and not by others, even if they are closely related. The molecular mechanisms underlying such strict pathogen-insect specificity are poorly understood. Candidatus Phytoplasma asteris, OY strain, line W (OY), is a phytopathogenic bacterium transmitted from plant to plant by sap-feeding insect vectors (leafhoppers). Our study focused on an abundant cell-surface membrane protein of the phytoplasma named antigenic membrane protein (Amp), which is not homologous with any reported functional protein. Immunofluorescence microscopy of the phytoplasma-infected insect showed that OY phytoplasma was localized to the microfilaments of the visceral smooth muscle surrounding the insect's intestinal tract. The affinity column assay showed that Amp forms a complex with three insect proteins: actin, myosin heavy chain, and myosin light chain. Amp-microfilament complexes were detected in all OY-transmitting leafhopper species, but not in the non-OY-transmitting leafhoppers, suggesting that the formation of the Amp-microfilament complex is correlated with the phytoplasma-transmitting capability of leafhoppers. Although several studies have reported interactions between pathogens and mammalian microfilaments, this is an example of host-specific interactions between a bacterial surface protein and a host microfilament in insect cells. Our data also suggest that the utilization of a host microfilament may be a universal system for pathogenic bacteria infecting mammals or insects.

Absence of plasmids encoding adhesion-related proteins in non-insect-transmissible strains of Spiroplasma citri

In the plant-pathogenic mollicuteSpiroplasma citri, spiralin is the major lipoprotein at the cell surface and is thought to be one of the components involved in the interactions of the spiroplasma with its insect vector. With the aim of identifying surface proteins other than spiralin, monoclonal antibodies (mAbs) were produced by immunization of mice with the spiralin-defectiveS. citrimutant GII3-9a2. mAb 10G3 was found to react with several polypeptides of 43–47 and 80–95 kDa, all of which were detected in the detergent phase after Triton X-114 partitioning of proteins. Mass spectrometry (MALDI-TOF) analyses of the two major polypeptides P47 and P80 of GII3-9a2, reacting with mAb 10G3, revealed that P47 was a processed product and represented the C-terminal moiety of P80. Search for sequence homologies revealed that P80 shared strong similarities with theS. citriadhesion-related protein P89 (Sarp1) ofS. citriBR3, and is one (named Scarp4a) of the eight Scarps encoded by theS. citriGII-3 genome. The eightscarpgenes are carried by plasmids pSci1–5. Western immunoblotting of proteins with mAb 10G3 revealed that, in contrast to the insect-transmissibleS. citristrain GII-3, the non-insect-transmissible strains ASP-1, R8A2 and 44 did not express Scarps. Southern blot hybridization experiments indicated that these strains possessed noscarpgenes, and did not carry plasmids pSci1–5. However,S. citristrain GII3-5, lacking pSci5, was still efficiently transmitted, showing that, in the genetic background ofS. citriGII-3, the pSci5-encoded genes, and in particularscarp2b,3band5a, are not essential for insect transmission. Whether plasmid-encoded genes are involved in transmission ofS. citriby its leafhopper vector remains to be determined.

Sequence comparisons of plasmids pBJS-O of Spiroplasma citri and pSKU146 of S. kunkelii: implications for plasmid evolution

BACKGROUND

Spiroplasma citri BR3-3X and S. kunkelii CR2-3X cause serious diseases worldwide on citrus and maize species, respectively. S. citri BR3-3X harbors a plasmid, pBJS-Original (pBJS-O), that encodes the spiroplasma adhesion related protein 1 (SARP1), a protein implicated in binding of the pathogen to cells of its leafhopper vector, Circulifer tenellus. The S. kunkelii CR2-3X plasmid, pSKU146, encodes a homolog of SARP1, Sk-ARP1. Due to the close phylogenetic relationship of the two pathogens, we hypothesized that the two plasmids are closely related as well.

RESULTS

The nucleotide sequence of pBJS-O was determined and compared to the sequences of a plasmid from BR3-T (pBJS-T), which is a multiply passaged leafhopper transmissible derivative of BR3-3X, and to known plasmid sequences including that of pSKU146. In addition to arp1, the 13,374 bp pBJS-O sequence putatively contains nine genes, recognized as open reading frames (ORFs). Several pBJS-O ORFs have homologs on pSKU146. However, the sequences flanking soj-like genes on both plasmids were found to be more distant from one another than sequences in any other region. Further, unlike pSKU146, pBJS-O lacks the conserved oriT region characteristic of the IncP group of bacterial plasmids. We were unable to identify a region in pBJS-O resembling a known plasmid origin of transfer. In regions where sequence was available for the plasmid from both BR3-3X and BR3-T, the pBJS-T sequence had a 0.4 kb deletion relative to its progenitor, pBJS-O. Southern blot hybridization of extrachromosomal DNA from various S. citri strains and spiroplasma species to an arp-specific probe and a probe made from the entire plasmid DNA of BR3-3X revealed limited conservation of both sequences in the genus Spiroplasma. Finally, we also report the presence on the BR3-3X chromosome of arp2, an S. citri homolog of arp1 that encodes the predicted protein SARP2. The C-terminal domain of SARP2 is homologous to that of SARP1, but its N-terminal domain is distinct.

CONCLUSION

Our data suggest that pBJS is a novel S. citri plasmid that does not belong to any known plasmid incompatibility group. The differences between pBJS-O and pSKU146 suggest that one or more events of recombination have contributed to the divergence of the plasmids of the two sister Spiroplasma species; the plasmid from S. citri itself has diverged slightly during the derivation of S. citri BR3-T from BR3-3X. Our data also show that pBJS-O encodes the putative adhesin SARP1. The presence of traE and mob on pBJS-O suggests a role for the plasmid in spiroplasmal conjugation.

Cryptic plasmid pSKU146 from the wall-less plant pathogen Spiroplasma kunkelii encodes an adhesin and components of a type IV translocation-related conjugation system

A cryptic plasmid of the wall-less plant pathogenic mollicute, Spiroplasma kunkelii CR2-3X, was cloned and its sequence analyzed. The 14,615 bp plasmid, designated pSKU146, has a nucleotide content of 28 mol% G + C, and contains 18 potential protein-coding regions (open reading frames, ORFs), of which six encode proteins that exhibit similarity to virulence-associated proteins involved in cell-to-cell adhesion or conjugal DNA transfer. One ORF encodes a 96 kDa protein, SkARP1, that is highly similar to SARP1 adhesin involved in attachment of Spiroplasma citri to insect vector gut membrane. Five ORFs encode proteins similar to TraE and Mob in walled bacteria, and to ORFs found in the integrative, conjugative element (ICEF) of Mycoplasma fermentans, respectively. Presence of domains similar to proteins of the Type IV secretion system in pathogenic bacteria suggests that spiroplasma possesses a related translocation system. Plasmid pSKU146 also contains two identical oriT regions each containing a nick sequence characteristic of the IncP conjugative plasmid family, as well as a 58 bp palindromic sequence, palSK1. Features in pSKU146 suggest that the plasmid functions as a mobile genetic element in conjugative transmission of spiroplasma pathogenicity-related genes.

Spiroplasma citri Spiralin Acts In Vitro as a Lectin Binding to Glycoproteins from Its Insect Vector Circulifer haematoceps

ABSTRACT In order to understand the molecular mechanisms underlying transmission of Spiroplasma citri by the leafhopper Circulifer haematoceps, we screened leafhopper proteins as putative S. citri-binding molecules using a spiroplasma overlay assay of protein blots (Far-western assay). Insect proteins were separated by one- or two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis, blotted, and probed with S. citri proteins. In this in vitro assay, we found that spiroplasma proteins exhibited affinity for seven leafhopper proteins. The interactions between S. citri proteins and insect proteins with molecular masses of 50 and 60 kDa were found to be sugar sensitive. These insect proteins were identified as high mannose N-glycoproteins, which support an interaction of glycoprotein-lectin type with S. citri proteins. Lectin detection in S. citri has revealed only one protein of 24 kDa. Using a leafhopper protein overlay assay on an S. citri protein blot, one spiroplasma protein with a similar molecular mass of 24 kDa was shown to display an insect protein-binding capacity. This protein was identified as the spiralin, which is the most abundant membrane protein of S. citri. Far-western experiments performed with purified spiralin and insect glycoproteins confirmed the binding of spiralin to the insect glycoproteins of 50 and 60 kDa. Thus, the spiralin could play a key role in the transmission of S. citri by mediating spiroplasma adherence to epithelial cells of insect vector gut or salivary gland.

Identification and characterization of traE genes of Spiroplasma kunkelii

Four traE homologs, designated traE1, traE2, traE3 and traE4, were identified and amplified from the genome of the leafhopper-transmitted corn stunt pathogen Spiroplasma kunkelii and were predicted to encode membrane-bound adenine tri-phosphatases (ATPases). Deduced proteins of all traE genes have 62.3% to 89.9% similarity to the conserved VirB4 domain that is frequently a component of type IV secretory pathways involved in intracellular trafficking and secretion of DNA and proteins. In phylogenetic analysis, TraE homologs of S. kunkelii, Mycoplasma pulmonis and Mycoplasma fermentans cluster together and are more similar to TraE proteins of Gram-positive bacteria than to those of Gram-negative bacteria, thereby resembling the 16S rRNA phylogeny. Gene traE2 was most conserved whereas the presence of the three other traE genes varied among S. kunkelii strains, M2, CS-2B, FL-80 and PU8-17. Further, traE1 and traE2 appeared to be located on the chromosome, and traE3 and traE4 genes on plasmids of S. kunkelii strain M2. Transcripts of the spiralin gene and traE2 genes were detected on Northern blots containing total ribonucleic acids (RNA) of S. kunkelii cultures and S. kunkelii-infected plants and insects, in which traE2 appeared to be of a larger transcription unit. Full-length expression products of the other traE genes were not detected. S. kunkelii traE genes could be involved in S. kunkelii cell morphogenesis, adhesion and DNA recombination.