Spiroplasma citri Surface Protein P89 Implicated in Adhesion to Cells of the Vector Circulifer tenellus

Integrated Information for Pathogenicity and Treatment of Spiroplasma

Spiroplasma, belonging to the class Mollicutes, is a small, helical, motile bacterium lacking a cell wall. Its host range includes insects, plants, and aquatic crustaceans. Recently, a few human cases of Spiroplasma infection have been reported. The diseases caused by Spiroplasma have brought about serious economic losses and hindered the healthy development of agriculture. The pathogenesis of Spiroplasma involves the ability to adhere, such as through the terminal structure of Spiroplasma, colonization, and invasive enzymes. However, the exact pathogenic mechanism of Spiroplasma remains a mystery. Therefore, we systematically summarize all the information about Spiroplasma in this review article. This provides a reference for future studies on virulence factors and treatment strategies of Spiroplasma.

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.

Preliminary Characterization of Phage-Like Particles from the Male-Killing Mollicute Spiroplasma poulsonii (an Endosymbiont of Drosophila)

Bacteriophages are vastly abundant, diverse, and influential, but with few exceptions (e.g. the Proteobacteria genera Wolbachia and Hamiltonella), the role of phages in heritable bacteria-arthropod interactions, which are ubiquitous and diverse, remains largely unexplored. Despite prior studies documenting phage-like particles in the mollicute Spiroplasma associated with Drosophila flies, genomic sequences of such phage are lacking, and their effects on the Spiroplasma-Drosophila interaction have not been comprehensively characterized. We used a density step gradient to isolate phage-like particles from the male-killing bacterium Spiroplasma poulsonii (strains NSRO and MSRO-Br) harbored by Drosophila melanogaster. Isolated particles were subjected to DNA sequencing, assembly, and annotation. Several lines of evidence suggest that we recovered phage-like particles of similar features (shape, size, DNA content) to those previously reported in Drosophila-associated Spiroplasma strains. We recovered three ~ 19 kb phage-like contigs (two in NSRO and one in MSRO-Br) containing 21-24 open reading frames, a read-alignment pattern consistent with circular permutation, and terminal redundancy (at least in NSRO). Although our results do not allow us to distinguish whether these phage-like contigs represent infective phage-like particles capable of transmitting their DNA to new hosts, their encoding of several typical phage genes suggests that they are at least remnants of functional phage. We also recovered two smaller non-phage-like contigs encoding a known Spiroplasma toxin (Ribosome Inactivating Protein; RIP), and an insertion element, suggesting that they are packaged into particles. Substantial homology of our particle-derived contigs was found in the genome assemblies of members of the Spiroplasma poulsonii clade.

The genome and antigen proteome analysis of Spiroplasma mirum

Spiroplasma mirum, small motile wall-less bacteria, was originally isolated from a rabbit tick and had the ability to infect newborn mice and caused cataracts. In this study, the whole genome and antigen proteins of S. mirum were comparative analyzed and investigated. Glycolysis, pentose phosphate pathway, arginine metabolism, nucleotide biosynthesis, and citrate fermentation were found in S. mirum, while trichloroacetic acid, fatty acids metabolism, phospholipid biosynthesis, terpenoid biosynthesis, lactose-specific PTS, and cofactors synthesis were completely absent. The Sec systems of S. mirum consist of SecA, SecE, SecDF, SecG, SecY, and YidC. Signal peptidase II was identified in S. mirum, but no signal peptidase I. The relative gene order in S. mirum is largely conserved. Genome analysis of available species in Mollicutes revealed that they shared only 84 proteins. S. mirum genome has 381 pseudogenes, accounting for 31.6% of total protein-coding genes. This is the evidence that spiroplasma genome is under an ongoing genome reduction. Immunoproteomics, a new scientific technique combining proteomics and immunological analytical methods, provided the direction of our research on S. mirum. We identified 49 proteins and 11 proteins (9 proteins in common) in S. mirum by anti-S. mirum serum and negative serum, respectively. Forty proteins in S. mirum were identified in relation to the virulence. All these proteins may play key roles in the pathogeny and can be used in the future for diagnoses and prevention.

Interactions of Liberibacter Species with Their Psyllid Vectors: Molecular, Biological and Behavioural Mechanisms

Liberibacter is a group of plant pathogenic bacteria, transmitted by insect vectors, psyllids (Hemiptera: Psylloidea), and has emerged as one of the most devastating pathogens which have penetrated into many parts of the world over the last 20 years. The pathogens are known to cause plant diseases, such as Huanglongbing (citrus greening disease), Zebra chip disease, and carrot yellowing, etc., threatening some very important agricultural sectors, including citrus, potato and others. Candidatus Liberibacter asiaticus (CLas), the causative agent of citrus greening disease, is one of the most important pathogens of this group. This pathogen has infected most of the citrus trees in the US, Brazil and China, causing tremendous decline in citrus productivity, and, consequently, a severely negative impact on economic and personnel associated with citrus and related industries in these countries. Like other members in this group, CLas is transmitted by the Asian citrus psyllid (ACP, Diaphorina citri) in a persistent circulative manner. An additional important member of this group is Ca. L. solanacearum (CLso), which possesses nine haplotypes and infects a variety of crops, depending on the specific haplotype and the insect vector species. Ongoing pathogen control strategies, that are mainly based on use of chemical pesticides, lack the necessary credentials of being technically feasible, and environmentally safe. For this reason, strategies based on interference with Liberibacter vector transmission have been adopted as alternative strategies for the prevention of infection by these pathogens. A significant amount of research has been conducted during the last 10-15 years to understand the aspects of transmission of these bacterial species by their psyllid vectors. These research efforts span biological, ecological, behavioural and molecular aspects of Liberibacter–psyllid interactions, and will be reviewed in this manuscript. These attempts directed towards devising new means of disease control, endeavoured to explore alternative strategies, instead of relying on using chemicals for reducing the vector populations, which is the sole strategy currently employed and which has profound negative effects on human health, beneficial organisms and the environment.

Citrus Stubborn Disease: Current Insights on an Enigmatic Problem Prevailing in Citrus Orchards

Citrus stubborn was initially observed in California in 1915 and was later proven as a graft-transmissible disease in 1942. In the field, diseased citrus trees have compressed and stunted appearances, and yield poor-quality fruits with little market value. The disease is caused by Spiroplasma citri, a phloem-restricted pathogenic mollicute, which belongs to the Spiroplasmataceae family (Mollicutes). S. citri has the largest genome of any Mollicutes investigated, with a genome size of roughly 1780 Kbp. It is a helical, motile mollicute that lacks a cell wall and peptidoglycan. Several quick and sensitive molecular-based and immuno-enzymatic pathogen detection technologies are available. Infected weeds are the primary source of transmission to citrus, with only a minor percentage of transmission from infected citrus to citrus. Several phloem-feeding leafhopper species (Cicadellidae, Hemiptera) support the natural spread of S. citri in a persistent, propagative manner. S. citri-free buds are used in new orchard plantings and bud certification, and indexing initiatives have been launched. Further, a quarantine system for newly introduced types has been implemented to limit citrus stubborn disease (CSD). The present state of knowledge about CSD around the world is summarized in this overview, where recent advances in S. citri detection, characterization, control and eradication were highlighted to prevent or limit disease spread through the adoption of best practices.

Genome analysis of Spiroplasma citri strains from different host plants and its leafhopper vectors

BACKGROUND

Spiroplasma citri comprises a bacterial complex that cause diseases in citrus, horseradish, carrot, sesame, and also infects a wide array of ornamental and weed species. S. citri is transmitted in a persistent propagative manner by the beet leafhopper, Neoaliturus tenellus in North America and Circulifer haematoceps in the Mediterranean region. Leafhopper transmission and the pathogen's wide host range serve as drivers of genetic diversity. This diversity was examined in silico by comparing the genome sequences of seven S. citri strains from the United States (BR12, CC-2, C5, C189, LB 319, BLH-13, and BLH-MB) collected from different hosts and times with other publicly available spiroplasmas.

RESULTS

Phylogenetic analysis using 16S rRNA sequences from 39 spiroplasmas obtained from NCBI database showed that S. citri strains, along with S. kunkelii and S. phoeniceum, two other plant pathogenic spiroplasmas, formed a monophyletic group. To refine genetic relationships among S. citri strains, phylogenetic analyses with 863 core orthologous sequences were performed. Strains that clustered together were: CC-2 and C5; C189 and R8-A2; BR12, BLH-MB, BLH-13 and LB 319. Strain GII3-3X remained in a separate branch. Sequence rearrangements were observed among S. citri strains, predominantly in the center of the chromosome. One to nine plasmids were identified in the seven S. citri strains analyzed in this study. Plasmids were most abundant in strains isolated from the beet leafhopper, followed by strains from carrot, Chinese cabbage, horseradish, and citrus, respectively. All these S. citri strains contained one plasmid with high similarity to plasmid pSci6 from S. citri strain GII3-3X which is known to confer insect transmissibility. Additionally, 17 to 25 prophage-like elements were identified in these genomes, which may promote rearrangements and contribute to repetitive regions.

CONCLUSIONS

The genome of seven S. citri strains were found to contain a single circularized chromosome, ranging from 1.58 Mbp to 1.74 Mbp and 1597-2232 protein-coding genes. These strains possessed a plasmid similar to pSci6 from the GII3-3X strain associated with leafhopper transmission. Prophage sequences found in the S. citri genomes may contribute to the extension of its host range. These findings increase our understanding of S. citri genetic diversity.

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.

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.

Infection Function of Adhesin-Like Protein ALP609 from Spiroplasma melliferum CH-1

Spiroplasma melliferum is the causative agent of spiroplasmosis in honeybees. During infection, adhesion of spiroplasmas to the host cells through adhesion factors is a crucial step. In this study, we identified an adhesin-like protein (ALP609) in S. melliferum CH-1 and investigated its role in the infection. To determine whether ALP609 is an adhesion factor, we performed indirect immunofluorescence microscopy to visualize its adhesion properties. Subsequently, an infection model of S. melliferum CH-1 was established using primary midgut cells of Apis mellifera to examine the adhesion and invasion of spiroplasma using anti-ALP609 antibodies inhibition assays and competition assays with recombinant ALP609 in vitro. We found that anti-ALP609 antibodies could inhibit the adhesion and invasion of spiroplasma to the midgut cells of A. mellifera and reduce midgut cell invasion on increased exposure to recombinant ALP609. To the best of our knowledge, this is the first report identifying adhesion-related factors in S. melliferum. Our results suggested that ALP609 is an adhesin-like protein critical for invasion of S. melliferum CH-1 into midgut cells of A. mellifera.

Identification of proteome, antigen protein and antigen membrane protein from Spiroplasma eriocheiris

Spiroplasma eriocheiris, which causes tremor disease in Chinese mitten crab Eriocheir sinensis, has led to huge economic losses in aquaculture. Immunoproteomics, a new scientific technique combining proteomics and immunological analytical methods, provided the direction of our research on S. eriocheiris. The aim of our study was to identify the proteome, antigen proteins and antigen membrane proteins of S. eriocheiris. A total of 780 S. eriocheiris proteins were identified by the LC-MS/MS technique. Based on immunoproteomics, 51 proteins and 7 proteins in S. eriocheiris were identified by anti-S. eriocheiris serum and negative serum respectively (six proteins in common). Thus, 45 antigenic proteins in S. eriocheiris were identified; among them, molecular chaperone DnaK, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), ATP synthase subunit beta and enolase can be considered as immunogenic proteins. Similarly, 32 membrane proteins and 6 membrane proteins were identified by anti-S. eriocheiris serum and negative serum respectively (two proteins in common). Thus, 30 antigenic membrane proteins in S. eriocheiris were identified; three of them have been reported as surface proteins including pyruvate kinase, enolase and GAPDH. All of these proteins may play key roles in the pathogeny and can be used in the future for diagnoses and prevention.

SIGNIFICANCE AND IMPACT OF THE STUDY

Spiroplasma eriocheiris is a novel pathogen causing the tremor disease in Chinese mitten crab Eriocheir sinensis. This is the first time LC-MS/MS was used to identify the proteome, antigen protein and antigen membrane protein of S. eriocheiris. The results can certainly provide valuable information towards the identification of virulent proteins or diagnosis of pathogenic mechanisms.

Spiroplasma eriocheiris Adhesin-Like Protein (ALP) Interacts with Epidermal Growth Factor (EGF) Domain Proteins to Facilitate Infection

Spiroplasma eriocheiris is a novel pathogen found in recent years, causing the tremor disease (TD) of Chinese mitten crab Eriocheir sinensis. Like Spiroplasma mirum, S. eriocheiris infects the newborn mouse (adult mice are not infected) and can cause cataract. Adhesion-related protein is an important protein involved in the interaction between pathogen and host. In this study, the Adhesin-like Protein (ALP) of S. eriocheiris was detected on its outer membrane by using immune electron microscopy, and was found to be involved in the bacterium's infection of mouse embryo fibroblasts (3T6-Swiss albino). Yeast two-hybrid analysis demonstrated that ALP interacts with a diverse group of mouse proteins. The interactions between recombinant partial fibulin7 (FBLN7; including two epidermal growth factor [EGF] domains) and ALP were confirmed by Far-western blotting and colocalization. We synthetized the domains of FBLN7 [EGF domain: amino acids 136–172 and complement control protein (CCP) domain: 81–134 amino acids], and demonstrated that only EGF domain of FBLN7 can interact with ALP. Because the EGF domain has high degree of similarity to EGF, it can activate the downstream EGFR signaling pathway, in key site amino acids. The EGFR pathway in 3T6 cells was restrained after rALP stimulation resulting from competitive binding of ALP to EGF. The unborn mouse, newborn mouse, and the adult mouse with cataract have a small amount of expressed FBLN7; however, none was detected in the brain and very little expression was seen in the eye of normal adult mice. In short, ALP as a S. eriocheiris surface protein, is critical for infection and further supports the role of ALP in S. eriocheiris infection by competitive effection of the EGF/EGFR axis of the target cells.

Stubborn Disease in Iran: Diversity of Spiroplasma citri Strains in Circulifer haematoceps Leafhoppers Collected in Sesame Fields in Fars Province

Spiroplasma citri is a bacterial pathogen responsible for the economically important citrus stubborn disease. Sesame and citrus seeds serve as hosts for both S. citri and its leafhopper vector Circulifer haematoceps. To evaluate whether sesame could act as a reservoir for citrus-infecting strains or not, the genetic diversity among S. citri strains found in leafhoppers collected in citrus and citrus-free sesame fields was investigated. Among 26 periwinkle plants exposed to the collected C. haematoceps leafhoppers, 12 plants developed typical stubborn symptoms. All symptomatic periwinkles were polymerase chain reaction positive using S. citri-specific primer pairs targeting the spiralin and P89 genes. Phylogenetic trees based on spiralin gene sequence analysis indicated that the novel field-collected strains clustered with those belonging to two formerly defined S. citri groups (groups 6 and 1). In addition, our results strongly suggest that group 1 strains could be transmitted from sesame-infected plants to citrus trees by C. haematoceps, while group 6 strains may not infect citrus trees.

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.

Immune response and survival of Circulifer haematoceps to Spiroplasma citri infection requires expression of the gene hexamerin

Spiroplasma citri is a cell wall-less bacterium that infects plants. It is transmitted by the leafhopper Circulifer haematoceps, which hosts this bacterium in the haemocel and insect tissues. Bacterial factors involved in spiroplasma colonization of the insect host have been identified, but the immune response of the leafhopper to S. citri infection remains unknown. In this study, we showed that C. haematoceps activates both humoral and cellular immune responses when challenged with bacteria. When infected by S. citri, C. haematoceps displayed a specific immune response, evidenced by activation of phagocytosis and upregulation of a gene encoding the protein hexamerin. S. citri infection also resulted in decreased phenoloxidase-like activity. Inhibition of hexamerin by RNA interference resulted in a significant reduction in phenoloxidase-like activity and increased mortality of infected leafhoppers. Therefore, the gene hexamerin is involved in S. citri control by interfering with insect phenoloxidase activity.

Occurrence and Identification of a New Vector of Rice Orange Leaf Phytoplasma in South China

Rice orange leaf disease (ROLD) is caused by rice orange leaf phytoplasma (ROLP) and occurs sporadically in rice-growing areas in countries of eastern and southeastern Asia. ROLD caused severe damage to rice production in South China in the 1980s. Although its impact subsequently declined in South China, it has reemerged as a serious threat recently. Our study showed that ROLD occurrence varies in different seasons and fields. It was more severe in summer-grown crops (from July to October) than in spring-grown crops (from March to July). In most fields, the incidence was less than 10%, and diseased plants were scattered throughout the fields. In 20% of fields, the incidence was between 10 and 30%. In some fields, over 90% of plants were affected, causing crop failure. Typical symptoms of ROLD include orange-colored leaves and poor growth. Diseased plants were determined as positive for ROLP but negative for Rice tungro bacilliform virus, Rice tungro spherical virus, and Rice transitory yellowing virus through polymerase chain reaction and reverse-transcription polymerase chain reaction. Phytoplasma bodies but not virus-like particles were observed by electron microscopy in phloem tissue of diseased leaves. The leafhopper Inazuma dorsalis, previously identified as the unique vector for ROLP, was rare in the affected fields. Another leafhopper, Nephotettix cincticeps, previously considered a nonvector for this phytoplasma, was very common. Transmission tests revealed that this insect could also transmit ROLP; therefore, it might represent a new vector responsible for the recent incidence of ROLD.

Heterologous expression and processing of the flavescence dorée phytoplasma variable membrane protein VmpA in Spiroplasma citri

Background

Flavescence dorée (FD) of grapevine is a phloem bacterial disease that threatens European vineyards. The disease is associated with a non-cultivable mollicute, a phytoplasma that is transmitted by the grapevine leafhopper Scaphoideus titanus in a persistent, propagative manner. The specificity of insect transmission is presumably mediated through interactions between the host tissues and phytoplasma surface proteins comprising the so-called variable membrane proteins (Vmps). Plant spiroplasmas and phytoplasmas share the same ecological niches, the phloem sieve elements of host plants and the hemocoel of insect vectors. Unlike phytoplasmas, however, spiroplasmas, and Spiroplasma citri in particular, can be grown in cell-free media and genetically engineered. As a new approach for studying phytoplasmas-insect cell interactions, we sought to mimic phytoplasmas through the construction of recombinant spiroplasmas exhibiting FD phytoplasma Vmps at the cell surface.

Results

Here, we report the expression of the FD phytoplasma VmpA in S. citri. Transformation of S. citri with plasmid vectors in which the vmpA coding sequence was under the control of the S. citri tuf gene promoter resulted in higher accumulation of VmpA than with the native promoter. Expression of VmpA at the spiroplasma surface was achieved by fusing the vmpA coding sequence to the signal peptide sequence of the S. citri adhesin ScARP3d, as revealed by direct colony immunoblotting and immunogold labelling electron microscopy. Anchoring of VmpA to the spiroplasma membrane was further demonstrated by Triton X-114 protein partitioning and Western immunoblotting. Using the same strategy, the secretion of free, functionally active β-lactamase (used as a model protein) into the culture medium by recombinant spiroplasmas was achieved.

Conclusions

Construction of recombinant spiroplasmas harbouring the FD phytoplasma variable membrane protein VmpA at their surface was achieved, which provides a new biological approach for studying interactions of phytoplasma surface proteins with host cells. Likewise, the secretion of functional β-lactamase by recombinant spiroplasmas established the considerable promise of the S. citri expression system for delivering phytoplasma effector proteins into host cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0417-5) contains supplementary material, which is available to authorized users.

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.

Onion yellow phytoplasma P38 protein plays a role in adhesion to the hosts

Adhesins are microbial surface proteins that mediate the adherence of microbial pathogens to host cell surfaces. In Mollicutes, several adhesins have been reported in mycoplasmas and spiroplasmas. Adhesins P40 of Mycoplasma agalactiae and P89 of Spiroplasma citri contain a conserved amino acid sequence known as the Mollicutes adhesin motif (MAM), whose function in the host cell adhesion remains unclear. Here, we show that phytoplasmas, which are plant-pathogenic mollicutes transmitted by insect vectors, possess an adhesion-containing MAM that was identified in a putative membrane protein, PAM289 (P38), of the 'Candidatus Phytoplasma asteris,' OY strain. P38 homologs and their MAMs were highly conserved in related phytoplasma strains. While P38 protein was expressed in OY-infected insect and plant hosts, binding assays showed that P38 interacts with insect extract, and weakly with plant extract. Interestingly, the interaction of P38 with the insect extract depended on MAM. These results suggest that P38 is a phytoplasma adhesin that interacts with the hosts. In addition, the MAM of adhesins is important for the interaction between P38 protein and hosts.

Honey bee colonies act as reservoirs for two Spiroplasma facultative symbionts and incur complex, multiyear infection dynamics

Two species of Spiroplasma (Mollicutes) bacteria were isolated from and described as pathogens of the European honey bee, Apis mellifera, ~30 years ago but recent information on them is lacking despite global concern to understand bee population declines. Here we provide a comprehensive survey for the prevalence of these two Spiroplasma species in current populations of honey bees using improved molecular diagnostic techniques to assay multiyear colony samples from North America (U.S.A.) and South America (Brazil). Significant annual and seasonal fluctuations of Spiroplasma apis and Spiroplasma melliferum prevalence in colonies from the U.S.A. (n = 616) and Brazil (n = 139) occurred during surveys from 2011 through 2013. Overall, 33% of U.S.A. colonies and 54% of Brazil colonies were infected by Spiroplasma spp., where S. melliferum predominated over S. apis in both countries (25% vs. 14% and 44% vs. 38% frequency, respectively). Colonies were co-infected by both species more frequently than expected in both countries and at a much higher rate in Brazil (52%) compared to the U.S.A. (16.5%). U.S.A. samples showed that both species were prevalent not only during spring, as expected from prior research, but also during other seasons. These findings demonstrate that the model of honey bee spiroplasmas as springtime-restricted pathogens needs to be broadened and their role as occasional pathogens considered in current contexts.

An analysis of the genomic variability of the phytopathogenic mollicute Spiroplasma kunkelii

Corn stunt disease has become a factor limiting maize production in some areas of the Americas in recent years. Although resistant maize genotypes have been developed in the past, this resistance has been unstable over time or in some geographical locations. To better understand disease components that could affect the stability of host resistance, we assessed the genome variability of the etiologic agent, Spiroplasma kunkelii. Isolates were obtained from a number of areas, and characterized molecularly by amplification of several regions of the spiroplasma chromosome and sequencing of specific gene fragments. The degree of polymorphism between isolates of different geographic origins was low, and the level of genomic variability was similar within isolates of different countries. Polymorphism among isolates was found in viral insertions and in the sequence of Skarp, a gene that encodes a membrane protein implicated in attachment to insect cells. The results suggest that the genome composition of this species is highly conserved among isolates. Hence, it is unlikely that the instability of maize resistance is due to generation of new pathotypes of S. kunkelii. Instead, other components of this complex pathosystem could account for the breakdown of resistance.

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.

The major antigenic membrane protein of "Candidatus Phytoplasma asteris" selectively interacts with ATP synthase and actin of leafhopper vectors

Phytoplasmas, uncultivable phloem-limited phytopathogenic wall-less bacteria, represent a major threat to agriculture worldwide. They are transmitted in a persistent, propagative manner by phloem-sucking Hemipteran insects. Phytoplasma membrane proteins are in direct contact with hosts and are presumably involved in determining vector specificity. Such a role has been proposed for phytoplasma transmembrane proteins encoded by circular extrachromosomal elements, at least one of which is a plasmid. Little is known about the interactions between major phytoplasma antigenic membrane protein (Amp) and insect vector proteins. The aims of our work were to identify vector proteins interacting with Amp and to investigate their role in transmission specificity. In controlled transmission experiments, four Hemipteran species were identified as vectors of “Candidatus Phytoplasma asteris”, the chrysanthemum yellows phytoplasmas (CYP) strain, and three others as non-vectors. Interactions between a labelled (recombinant) CYP Amp and insect proteins were analysed by far Western blots and affinity chromatography. Amp interacted specifically with a few proteins from vector species only. Among Amp-binding vector proteins, actin and both the α and β subunits of ATP synthase were identified by mass spectrometry and Western blots. Immunofluorescence confocal microscopy and Western blots of plasma membrane and mitochondrial fractions confirmed the localisation of ATP synthase, generally known as a mitochondrial protein, in plasma membranes of midgut and salivary gland cells in the vector Euscelidius variegatus. The vector-specific interaction between phytoplasma Amp and insect ATP synthase is demonstrated for the first time, and this work also supports the hypothesis that host actin is involved in the internalization and intracellular motility of phytoplasmas within their vectors. Phytoplasma Amp is hypothesized to play a crucial role in insect transmission specificity.

Selection and characterization ofSpiroplasma citrimutants by random transposome mutagenesis

Phytopathogenic spiroplasmas can multiply in vascular plants and insects. A deeper understanding of this dual-host life could be furthered through the identification by random mutagenesis of spiroplasma genes required. The ability of the EZ::TN™ <DHFR-1> Tnp transposome™ system to create random insertional mutations in the genome of Spiroplasma citri was evaluated. The efficiency of electroporation-mediated transformation of S. citri BR3-3X averaged 28.8 CFUs/ng transposome for 109spiroplasma cells. Many transformants appearing on the selection plates were growth impaired when transferred to broth. Altering broth composition in various ways did not improve their growth. However, placing colonies into a small broth volume resulted in robust growth and successful subsequent passages of a subset of transformants. PCR using primers for the dihydrofolate reductase gene confirmed the transposon’s presence in the genomes of selected transformants. Southern blot hybridization and nucleotide sequencing suggested that insertion was random within the chromosome and usually at single sites. The insertions were stable. Growth rates of all transformants were lower than that of the wild-type S. citri, but none lost the ability to adhere to a Circulifer tenellus (CT-1) cell line. The EZ::TN™ <DHFR-1> Tnp transposome™ system represents an additional tool for genetic manipulation of the fastidious spiroplasmas.

I-SceI-mediated plasmid deletion and intra-molecular recombination in Spiroplasma citri

S. citri wild-type strain GII3 carries six plasmids (pSci1 to -6) that are thought to encode determinants involved in the transmission of the spiroplasma by its leafhopper vector. In this study we report the use of meganuclease I-SceI for plasmid deletion in S. citri. Plasmids pSci1NT-I and pSci6PT-I, pSci1 and pSci6 derivatives that contain the tetM selection marker and a unique I-SceI recognition site were first introduced into S. citri strains 44 (having no plasmid) and GII3 (carrying pSci1-6), respectively. Due to incompatibility of homologous replication regions, propagation of the S. citri GII3 transformant in selective medium resulted in the replacement of the natural pSci6 by pSci6PT-I. The spiroplasmal transformants were further transformed by an oriC plasmid carrying the I-SceI gene under the control of the spiralin gene promoter. In the S. citri 44 transformant, expression of I-SceI resulted in rapid loss of pSciNT-I showing that expression of I-SceI can be used as a counter-selection tool in spiroplasmas. In the case of the S. citri GII3 transformant carrying pSci6PT-I, expression of I-SceI resulted in the deletion of plasmid fragments comprising the I-SceI site and the tetM marker. Delineating the I-SceI generated deletions proved they had occurred though recombination between homologous sequences. To our knowledge this is the first report of I-SceI mediated intra-molecular recombination in mollicutes.

Sequences essential for transmission of Spiroplasma citri by its leafhopper vector, Circulifer haematoceps, revealed by plasmid curing and replacement based on incompatibility

Spiroplasma citri GII3 contains highly related low-copy-number plasmids pSci1 to -6. Despite the strong similarities between their replication regions, these plasmids coexist in the spiroplasma cells, indicating that they are mutually compatible. The pSci1 to -6 plasmids encode the membrane proteins known as S. citri adhesion-related proteins (ScARPs) (pSci1 to -5) and the hydrophilic protein P32 (pSci6), which had been tentatively associated with insect transmission, as they were not detected in non-insect-transmissible strains. With the aim of further investigating the role of plasmid-encoded determinants in insect transmission, we have constructed S. citri mutant strains that differ in their plasmid contents by developing a plasmid curing/replacement strategy based on the incompatibility of plasmids having identical replication regions. Experimental transmission of these S. citri plasmid mutants through injection into the leafhopper vector Circulifer haematoceps revealed that pSci6, more precisely, the pSci6_06 coding sequence, encoding a protein of unknown function, was essential for transmission. In contrast, ScARPs and P32 were dispensable for both acquisition and transmission of the spiroplasmas by the leafhopper vector, even though S. citri mutants lacking pSci1 to -5 (encoding ScARPs) were acquired and transmitted at lower efficiencies than the wild-type strain GII3.

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.

In the non-insect-transmissible line of onion yellows phytoplasma (OY-NIM), the plasmid-encoded transmembrane protein ORF3 lacks the major promoter region

‘Candidatus Phytoplasma asteris’, onion yellows strain (OY), a mildly pathogenic line (OY-M), is a phytopathogenic bacterium transmitted by Macrosteles striifrons leafhoppers. OY-M contains two types of plasmids (EcOYM and pOYM), each of which possesses a gene encoding the putative transmembrane protein, ORF3. A non-insect-transmissible line of this phytoplasma (OY-NIM) has the corresponding plasmids (EcOYNIM and pOYNIM), but pOYNIM lacks orf3. Here we show that in OY-M, orf3 is transcribed from two putative promoters and that on EcOYNIM, one of the promoter sequences is mutated and the other deleted. We also show by immunohistochemical analysis that ORF3 is not expressed in OY-NIM-infected plants. Moreover, ORF3 protein seems to be preferentially expressed in OY-M-infected insects rather than in plants. We speculate that ORF3 may play a role in the interactions of OY with its insect host.

Striking diversity of vmp1, a variable gene encoding a putative membrane protein of the stolbur phytoplasma

Studies of phytoplasma-insect vector interactions and epidemiological surveys of plant yellows associated with the stolbur phytoplasma (StolP) require the identification of relevant candidate genes and typing markers. A recent StolP genome survey identified a partial coding sequence, SR01H10, having no homologue in the "Candidatus Phytoplasma asteris" genome but sharing low similarity with a variable surface protein of animal mycoplasmas. The complete coding sequence and its genetic environment have been fully characterized by chromosome walking. The vmp1 gene encodes a protein of 557 amino acids predicted to possess a putative signal peptide and a potential C-terminal transmembrane domain. The mature 57.8-kDa VMP1 protein is likely to be anchored in the phytoplasma membrane with a large N-terminal hydrophilic part exposed to the phytoplasma cell surface. Southern blotting experiments detected multiple sequences homologous to vmp1 in the genomes of nine StolP isolates. vmp1 is variable in size, and eight different vmp1 RsaI restriction fragment length polymorphism types could be distinguished among 12 StolP isolates. Comparison of vmp1 sequences revealed that insertions in largest forms of the gene encode an additional copy of a repeated domain of 81 amino acids, while variations in 11-bp repeats led to gene disruption in two StolP isolates. vmp1 appeared to be much more variable than three housekeeping genes involved in protein translation, maturation, and secretion and may therefore be involved in phytoplasma-host interactions.

Characterizing the replication and stability regions of Spiroplasma citri plasmids identifies a novel replication protein and expands the genetic toolbox for plant-pathogenic spiroplasmas

Spiroplasma citri strain GII3 contains seven plasmids, pSciA and pSci1-6, that share extensive regions of sequence homology and display a mosaic gene organization. Plasmid pSci2 comprises 12 coding sequences (CDS), three of which encode polypeptides homologous to proteins Soj/ParA, involved in chromosome partitioning, and TrsE and Mob/TraG, implicated in the type IV secretion pathway. One CDS encodes the adhesin-like protein ScARP3d whereas the other eight encode polypeptides with no homology to known proteins. The pSci2 CDS pE and soj have counterparts in all seven plasmids. Through successive deletions, various pSci2 derivatives were constructed and assessed for their ability to replicate by transformation of S. citri 44, a strain which has no plasmid. The smallest functional replicon was found to contain a single CDS (pE) and its flanking intergenic regions. Shuttle (S. citri/Escherichia coli) plasmids, in which CDS pE was disrupted, failed to replicate in S. citri, suggesting that PE is the replication protein of the S. citri plasmids. Successive propagations of pSci2-derived transformed spiroplasmas, in the absence of selection pressure, revealed that only pSci2 derivatives having an intact soj gene were stably maintained, indicating that the soj-encoded polypeptide is most likely involved in plasmid partitioning. Upon transformation, pSci2 derivatives, including shuttle (S. citri/E. coli) plasmids, were shown to replicate in all S. citri strains tested regardless of whether the strain possesses endogenous plasmids, such as strain GII3, or not, such as strain R8A2. In addition, the pSci replicons were introduced efficiently into the plant-pathogenic spiroplasmas Spiroplasma kunkelii and Spiroplasma phoeniceum, the transformation of which had never, to our knowledge, been described before. These studies show that, besides their implications for the biology of S. citri, the pSci plasmids hold considerable promise as vectors of general use for genetic studies of plant-pathogenic spiroplasmas. As an example, a HA-tagged S. citri protein was expressed in S. kunkelii. Detection of pE-hybridizing sequences in various group I spiroplasma species indicated that pE replicating plasmids were not restricted to the three plant-pathogenic spiroplasmas.

Characterization of putative membrane protein genes of the 'Candidatus Phytoplasma asteris', chrysanthemum yellows isolate

To characterize potentially important surface-exposed proteins of the phytoplasma causing chrysanthemum yellows (CY), new primers were designed based on the conserved regions of 3 membrane protein genes of the completely sequenced onion yellows and aster yellows witches' broom phytoplasmas and were used to amplify CY DNA. The CY genes secY, amp, and artI, encoding the protein translocase subunit SecY, the antigenic membrane protein Amp and the arginine transporter ArtI, respectively, were cloned and completely sequenced. Alignment of CY-specific secY sequences with the corresponding genes of other phytoplasmas confirmed the 16S rDNA-based classification, while amp sequences were highly variable within the 'Candidatus Phytoplasma asteris'. Five CY partial sequences were cloned into the pRSetC expression vector, and 3 of the encoded protein fragments (Amp 64/651, Amp 64/224, ArtI 131/512) were expressed as fusion antigens for the production of CY-specific polyclonal antibodies (A416 against Amp 64/224; A407 against ArtI 131/512). A416 recognized, in Western blots, the full-length Amp from CY-infected plants (periwinkle, daisy) and insect vectors (Euscelidius variegatus, Macrosteles quadripunctulatus). A416 also reacted to European aster yellows, to primula yellows phytoplasmas, to northern Italian strains of 'Ca. Phytoplasma asteris' from lettuce and gladiolus, but it did not react to American aster yellows phytoplasma.

The abundant extrachromosomal DNA content of the Spiroplasma citri GII3-3X genome

Background

Spiroplama citri, the causal agent of citrus stubborn disease, is a bacterium of the class Mollicutes and is transmitted by phloem-feeding leafhopper vectors. In order to characterize candidate genes potentially involved in spiroplasma transmission and pathogenicity, the genome of S. citri strain GII3-3X is currently being deciphered.

Results

Assembling 20,000 sequencing reads generated seven circular contigs, none of which fit the 1.8 Mb chromosome map or carried chromosomal markers. These contigs correspond to seven plasmids: pSci1 to pSci6, with sizes ranging from 12.9 to 35.3 kbp and pSciA of 7.8 kbp. Plasmids pSci were detected as multiple copies in strain GII3-3X. Plasmid copy numbers of pSci1-6, as deduced from sequencing coverage, were estimated at 10 to 14 copies per spiroplasma cell, representing 1.6 Mb of extrachromosomal DNA. Genes encoding proteins of the TrsE-TraE, Mob, TraD-TraG, and Soj-ParA protein families were predicted in most of the pSci sequences, in addition to members of 14 protein families of unknown function. Plasmid pSci6 encodes protein P32, a marker of insect transmissibility. Plasmids pSci1-5 code for eight different S. citri adhesion-related proteins (ScARPs) that are homologous to the previously described protein P89 and the S. kunkelii SkARP1. Conserved signal peptides and C-terminal transmembrane alpha helices were predicted in all ScARPs. The predicted surface-exposed N-terminal region possesses the following elements: (i) 6 to 8 repeats of 39 to 42 amino acids each (sarpin repeats), (ii) a central conserved region of 330 amino acids followed by (iii) a more variable domain of about 110 amino acids. The C-terminus, predicted to be cytoplasmic, consists of a 27 amino acid stretch enriched in arginine and lysine (KR) and an optional 23 amino acid stretch enriched in lysine, aspartate and glutamate (KDE). Plasmids pSci mainly present a linear increase of cumulative GC skew except in regions presenting conserved hairpin structures.

Conclusion

The genome of S. citri GII3-3X is characterized by abundant extrachromosomal elements. The pSci plasmids could not only be vertically inherited but also horizontally transmitted, as they encode proteins usually involved in DNA element partitioning and cell to cell DNA transfer. Because plasmids pSci1-5 encode surface proteins of the ScARP family and pSci6 was recently shown to confer insect transmissibility, diversity and abundance of S. citri plasmids may essentially aid the rapid adaptation of S. citri to more efficient transmission by different insect vectors and to various plant hosts.

Polymerase Chain Reaction-Based Detection of Spiroplasma citri Associated with Citrus Stubborn Disease

Polymerase chain reaction (PCR)-based detection of citrus stubborn disease was improved using primers based on sequences of the P89 putative adhesin gene and the P58 putative adhesin multigene of Spiroplasma citri. Real-time PCR also was developed with detection limits estimated to be between 10^-4 and 10^-4 ng by serial dilution of a recombinant S. citri plasmid into DNA extracts from healthy Madam Vinous sweet orange. PCR for the detection of S. citri by these new primers was validated by comparing culturing of the pathogen, the traditional method of diagnosis, with PCR assays from samples taken from two citrus plots in Kern County, CA. Fruit columella was collected from 384 and 377 individual trees in each of two fields, respectively; one portion was used for culturing and the other for DNA extraction and PCR. PCR results matched those of culturing 85 to 100% of the time depending on the primers used. More importantly, PCR detected S. citri from culture-negative trees in 5 to 15% of the cases, suggesting that PCR performed as well or better than culturing for detection of S. citri in field samples. Real-time PCR proved to be the best method for detection. Differential reaction of the samples to the P58 primer pairs suggested that two populations of S. citri occur in historical and present-day field isolates. Citrus stubborn disease incidence was estimated to be 58.3 and 3.7% in the two orchards. The results presented here support the use of PCR for reliable detection of S. citri in field trees.

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.

Insect injection and artificial feeding bioassays to test the vector specificity of flavescence dorée phytoplasma

ABSTRACT The specificity of vector transmission of Flavescence dorée phytoplasma (FDP) was tested by injecting FDP, extracted from laboratory-reared infective Euscelidius variegatus, into specimens of 15 other hemipteran insect species collected in European vineyards. Concentrations of viable phytoplasma extracts and latency in vectors were monitored by injection of healthy-reared E. variegatus leafhoppers. Based on these preliminary results, insects were injected by using phytoplasma extracts that ensured the highest rate of FDP acquisition and transmission by E. variegatus. Transmission into an artificial diet through a Parafilm membrane about 3 weeks after insect injection was attempted. FDP-injected insects that belonged to 15 hemipteran species were confined in cages and fed through the membrane for a 4- to 5-day inoculation access period. FDP DNA was detected by polymerase chain reaction (PCR) in the feeding buffer fed upon by Anoplotettix fuscovenosus, Aphrodes makarovi,E. variegatus, and Euscelis incisus. PCR amplification with specific primers detected FDP DNA in injected insects of all test insect species. Band intensity was positively correlated with the transmissibility of FDP. Transmission of FDP to plants by feeding was confirmed for Anoplotettix fuscovenosus, E. variegatus, and Euscelis incisus, but not for Aphrodes makarovi. Our results suggest that vector competency of FDP is restricted to specimens belonging to the family Cicadellidae, subfamily Deltocephalinae.

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.

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.

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.

Disruption of a gene predicted to encode a solute binding protein of an ABC transporter reduces transmission of Spiroplasma citri by the leafhopper Circulifer haematoceps

Spiroplasma citri is transmitted from plant to plant by phloem-feeding leafhoppers. In an attempt to identify mechanisms involved in transmission, mutants of S. citri affected in their transmission must be available. For this purpose, transposon (Tn4001) mutagenesis was used to produce mutants which have been screened for their ability to be transmitted by the leafhopper vector Circulifer haematoceps to periwinkle plants. With one mutant (G76) which multiplied in leafhoppers as efficiently as S. citri wild-type (wt) strain GII-3, the plants showed symptoms 4 to 5 weeks later than those infected with wt GII-3. Thirty to fifty percent of plants exposed to leafhoppers injected with G76 remained symptomless, whereas for wt GII-3, all plants exposed to the transmission showed severe symptoms. This suggests that the mutant G76 was injected into plants by the leafhoppers less efficiently than wt GII-3. To check this possibility, the number of spiroplasma cells injected by a leafhopper through a Parafilm membrane into SP4 medium was determined. Thirty times less mutant G76 than wt GII-3 was transmitted through the membrane. These results suggest that mutant G76 was affected either in its capacity to penetrate the salivary glands and/or to multiply within them. In mutant G76, transposon Tn4001 was shown to be inserted into a gene encoding a putative lipoprotein (Sc76) In the ABCdb database Sc76 protein was noted as a solute binding protein of an ABC transporter of the family S1_b. Functional complementation of the G76 mutant with the Sc76 gene restored the wild phenotype, showing that Sc76 protein is involved in S. citri transmission by the leafhopper vector C. haematoceps.

Spiralin is not essential for helicity, motility, or pathogenicity but is required for efficient transmission of Spiroplasma citri by its leafhopper vector Circulifer haematoceps

Spiralin is the most abundant protein at the surface of the plant pathogenic mollicute Spiroplasma citri and hence might play a role in the interactions of the spiroplasma with its host plant and/or its insect vector. To study spiralin function, mutants were produced by inactivating the spiralin gene through homologous recombination. A spiralin-green fluorescent protein (GFP) translational fusion was engineered and introduced into S. citri by using an oriC-based targeting vector. According to the strategy used, integration of the plasmid by a single-crossover recombination at the spiralin gene resulted in the expression of the spiralin-GFP fusion protein. Two distinct mutants were isolated. Western and colony immunoblot analyses showed that one mutant (GII3-9a5) did produce the spiralin-GFP fusion protein, which was found not to fluoresce, whereas the other (GII3-9a2) produced neither the fusion protein nor the wild-type spiralin. Both mutants displayed helical morphology and motility, similarly to the wild-type strain GII-3. Genomic DNA analyses revealed that GII3-9a5 was unstable and that GII3-9a2 was probably derived from GII3-9a5 by a double-crossover recombination between plasmid sequences integrated into the GII3-9a5 chromosome and free plasmid. When injected into the leafhopper vector Circulifer haematoceps, the spiralinless mutant GII3-9a2 multiplied to high titers in the insects (1.1 x 10(6) to 2.8 x 10(6) CFU/insect) but was transmitted to the host plant 100 times less efficiently than the wild-type strain. As a result, not all plants were infected, and symptom production in these plants was delayed for 2 to 4 weeks compared to that in the wild-type strain. In the infected plants however, the mutant multiplied to high titers (1.2 x 10(6) to 1.4 x 10(7) CFU/g of midribs) and produced the typical symptoms of the disease. These results indicate that spiralin is not essential for pathogenicity but is required for efficient transmission of S. citri by its insect vector.

Spiroplasma citri, a plant pathogenic molligute: relationships with its two hosts, the plant and the leafhopper vector

Spiroplasma citri, the type species of the genus Spiroplasma (Spiroplasmataceae, Mollicutes), is restricted to the phloem sieve tubes and transmitted by phloem sap-feeding insects, as is characteristic of the phytopathogenic mollicutes. The spiroplasmas are the only mollicutes showing motility and helical morphology, apparently mediated by a contractile fibrillar cytoskeleton bound to the inner surface of the spiroplasmal membrane. MreB genes, which are involved in cell-shape determination, have been identified in S. citri. Identified genes of other functional groups are those involved in the transmission of S. citri by the leafhoppers and genes coding for lipoproteins, including spiralin, bound to the outer surface of the spiroplasma membrane. S. citri mutants that are unable to use fructose induce only mild and delayed symptoms. Fructose utilization by the sieve tube-restricted wild-type spiroplasmas is postulated to deprive the companion cells of fructose, thereby impairing sucrose loading into the sieve tubes.

Characterization of homologues of the apple proliferation immunodominant membrane protein gene from three related phytoplasmas

SUMMARY Homologues of the immunodominant membrane protein gene from apple proliferation (AP) phytoplasma have been cloned and sequenced for three further members of the AP subclade, namely European stone fruit yellows, peach yellow leaf roll and a European isolate of pear decline (PD). The putative translation products of all three were similar in size to that of AP and all had a transmembrane region towards the N-terminus and a large C-terminal hydrophilic domain, probably held on the outside of the cell membrane in vivo. Sequence similarities for the putative proteins were compared with interrelationships of the phytoplasmas as measured by rRNA gene sequence similarity. The proteins from AP and PD were more similar (57% identical in the major hydrophilic domain) than those for any other pair (31-34%), but these two phytoplasmas were not more closely related by rRNA gene sequences than other pairs. The possibility that the relative similarities of these proteins is related to the host is discussed. It is suggested that the similarity of the AP and PD proteins may reflect the fact that these two proteins have narrow plant host ranges in two closely related genera in the tribe Maloideae (family Rosaceae), whilst the other two have broader host ranges, mainly in the tribe Prunoideae.

Immunodominant membrane proteins from two phytoplasmas in the aster yellows clade (chlorante aster yellows and clover phyllody) are highly divergent in the major hydrophilic region

The mechanisms by which phytoplasmas interact with their hosts are not understood. Mollicute membrane proteins may play a role in such interactions and therefore the amp genes encoding immunodominant proteins from two phytoplasmas, aster yellows and clover phyllody, which fall within the largest taxonomic subclade of the phytoplasmas, have been cloned and characterized. The putative translation products, antigenic membrane proteins (Amps), of these genes have properties which are typical for bacterial membrane proteins, and which suggest that each has a single large extracellular hydrophilic domain held by a transmembrane region near the C-terminus, with only a short C-terminal intracellular sequence. Both of the Amps characterized here have bacterial leader sequences which are cleaved during maturation. Whilst the signal peptide and transmembrane regions of the two proteins are very similar, the major hydrophilic domains are highly divergent in both size and sequence. The Amps from the two phytoplasmas are also different in structure and sequence from the immunodominant membrane proteins of three other phytoplasmas whose genes have been cloned previously.

Characterization of Spiroplasma citri adhesion related protein SARP1, which contains a domain of a novel family designated sarpin

Transmission of the plant pathogen Spiroplasma citri by its leafhopper vector, Circulifer tenellus, involves adherence to and invasion of insect host cells. The S. citri adhesion related protein P89 (SARP1) was purified by immunoprecipitation using anti-SARP1 monoclonal antibodies. The protein's N-terminal amino acid sequence was determined and used to design a degenerate oligonucleotide. The labeled oligonucleotide hybridized to a 3.5 kb MboI fragment from S. citri DNA, which was then cloned and sequenced. Additionally, a 1.9 kb RsaI fragment of S. citri DNA, partially overlapping the MboI fragment, was isolated and characterized. Sequence analysis of the two clones revealed four open reading frames. ORF1 (675 bp) encodes the C-terminal part of a Soj-like protein. ORFs 1 and 2 were separated from ORFs 3 and 4 by a putative transcription termination site, indicated by a hairpin structure. ORF3 encodes an amphiphilic 798 amino acid long protein with a cleavable signal peptide and a predicted transmembrane helix near the C-terminus. The mature protein of 85.96 kDa has a calculated pI value of 5.5 and has an N-terminal amino acid sequence consistent with that determined from the purified SARP1. At the N-terminus of this protein is a region consisting of six repeats, each 39-42 amino acids, a motif belonging to a previously unrecognized family of repeats found in a variety of bacterial proteins. The taxonomically spotty presence of this 'sarpin' domain and the relationship of the repeats to each other suggests a convergent evolution in multiple lineages.

A genomic approach to the understanding of Xylella fastidiosa pathogenicity

Xylella fastidiosa is a fastidious, xylem-limited bacterium that causes several economically important plant diseases, including citrus variegated chlorosis (CVC). X. fastidiosa is the first plant pathogen to have its genome completely sequenced. In addition, it is probably the least previously studied of any organism for which the complete genome sequence is available. Several pathogenicity-related genes have been identified in the X. fastidiosa genome by similarity with other bacterial genes involved in pathogenesis in plants, as well as in animals. The X. fastidiosa genome encodes different classes of proteins directly or indirectly involved in cell-cell interactions, degradation of plant cell walls, iron homeostasis, anti-oxidant responses, synthesis of toxins, and regulation of pathogenicity. Neither genes encoding members of the type III protein secretion system nor avirulence-like genes have been identified in X. fastidiosa.