Electron microscopy of Mycoplasma-like bodies associated with insect and plant hosts of peach Western X-disease

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.

Multilocus Characterization, Gene Expression Analysis of Putative Immunodominant Protein Coding Regions, and Development of Recombinase Polymerase Amplification Assay for Detection of 'Candidatus Phytoplasma Pruni' in Prunus avium

Western X (WX) disease, caused by 'Candidatus Phytoplasma pruni', is a devastating disease of sweet cherry resulting in the production of small, bitter-flavored fruits that are unmarketable. Escalation of WX disease in Washington State prompted the development of a rapid detection assay based on recombinase polymerase amplification (RPA) to facilitate timely removal and replacement of diseased trees. Here, we report on a reliable RPA assay targeting putative immunodominant protein coding regions that showed comparable sensitivity to polymerase chain reaction (PCR) in detecting 'Ca. Phytoplasma pruni' from crude sap of sweet cherry tissues. Apart from the predominant strain of 'Ca. Phytoplasma pruni', the RPA assay also detected a novel strain of phytoplasma from several WX-affected trees. Multilocus sequence analyses using the immunodominant protein A (idpA), imp, rpoE, secY, and 16S ribosomal RNA regions from several 'Ca. Phytoplasma pruni' isolates from WX-affected trees showed that this novel phytoplasma strain represents a new subgroup within the 16SrIII group. Examination of high-throughput sequencing data from total RNA of WX-affected trees revealed that the imp coding region is highly expressed, and as supported by quantitative reverse transcription PCR data, it showed higher RNA transcript levels than the previously proposed idpA coding region of 'Ca. Phytoplasma pruni'.

The influence of bacteria on multitrophic interactions among plants, psyllids, and pathogen

The recent emergence of several plant diseases caused by psyllid-borne bacterial pathogens worldwide (Candidatus Liberibacter spp.) has created renewed interest on the interaction between psyllids and bacteria. In spite of these efforts to understand psyllid association with bacteria, many aspects of their interactions remain poorly understood. As more organisms are studied, subtleties on the molecular interactions as well as on the effects of the bacteria on the psyllid host are being uncovered. Additionally, psyllid-borne bacterial phytopathogens can also affect the host plant, which in turn can impact psyllid physiology and behavior. Here, we review the current literature on different aspects of the influence of bacteria on multitrophic interactions among plants, psyllids, and pathogens. We then highlight gaps that need to be addressed to advance this field, which can have significant implications for controlling these newly emergent and other plant diseases.

‘Candidatus Phytoplasma pruni’, a novel taxon associated with X-disease of stone fruits, Prunus spp.: multilocus characterization based on 16S rRNA, secY, and ribosomal protein genes

X-disease is one of the most serious diseases known in peach (Prunus persica). Based on RFLP analysis of 16S rRNA gene sequences, peach X-disease phytoplasma strains from eastern and western United States and eastern Canada were classified in 16S rRNA gene RFLP group 16SrIII, subgroup A. Phylogenetic analyses of 16S rRNA gene sequences revealed that the X-disease phytoplasma strains formed a distinct subclade within the phytoplasma clade, supporting the hypothesis that they represented a lineage distinct from those of previously described ‘Candidatus Phytoplasma ’ species. Nucleotide sequence alignments revealed that all studied X-disease phytoplasma strains shared less than 97.5 % 16S rRNA gene sequence similarity with previously described ‘Candidatus Phytoplasma ’ species. Based on unique properties of the DNA, we propose recognition of X-disease phytoplasma strain PX11CT1R as representative of a novel taxon, ‘Candidatus Phytoplasma pruni’. Results from nucleotide and phylogenetic analyses of secY and ribosomal protein (rp) gene sequences provided additional molecular markers of the ‘Ca. Phytoplasma pruni’ lineage. We propose that the term ‘Ca. Phytoplasma pruni’ be applied to phytoplasma strains whose 16S rRNA gene sequences contain the oligonucleotide sequences of unique regions that are designated in the formally published description of the taxon. Such strains include X-disease phytoplasma and - within the tolerance of a single base difference in one unique sequence - peach rosette, peach red suture, and little peach phytoplasmas. Although not employed for taxon delineation in this work, we further propose that secY, rp, and other genetic loci from the reference strain of a taxon, and where possible oligonucleotide sequences of unique regions of those genes that distinguish taxa within a given 16Sr group, be incorporated in emended descriptions and as part of future descriptions of ‘Candidatus Phytoplasma ’ taxa.

Correlation of mollicutes and their viruses with multiple sclerosis and other demyelinating diseases

To identify infectious diseases likely involved in MS, the author previously correlated the geographical distribution of MS with the global distribution of tick-borne diseases. Tick-borne infectious agents include mollicutes or mycoplasmas. The current paper reviews evidence that mollicutes, especially spiroplasmas, or their viruses could be the initial exposure that causes MS. Mollicute infections, including the effects of their toxins, can be treated or prevented with gold salts or tetracyclines. If further research recommended by this review finds a role of mycoplasmas in MS, treatment of MS with gold with tetracycline should be evaluated.

Identification of Mulberry Dwarf Phytoplasmas in the Genital Organs and Eggs of Leafhopper Hishimonoides sellatiformis

ABSTRACT The presence of mulberry dwarf (MD) phytoplasmas in organs of the inoculative vector insects Hishimonoides sellatiformis and Hishimonus sellatus was determined by means of electron microscopy (EM) and polymerase chain reaction (PCR) assays. Many MD phytoplasmas were detected in genital organs as well as in the intestines, salivary glands, brains, fat bodies, and thoracic ganglia of Hishimonoides sellatiformis, but only in the intestine and salivary glands of Hishimonus sellatus. Many phytoplasmas with characteristic morphology were observed via EM in ovaries, seminal receptacles, and testes, and they were further identified by PCR assays with group I-specific primers. In addition, the organisms were detected by direct or nested PCR assays in eggs (head pigmentation stage of embryos) laid on mulberry shoots by inoculative leafhoppers and in the newly hatched nymphs from these eggs. These findings indicate that transovarial transmission of MD phytoplasmas occurs in Hishimonoides sellatiformis.

Isolation and characterization of full-length chromosomes from non-culturable plant-pathogenic Mycoplasma-like organisms

We describe the isolation and characterization of full-length chromosomes from non-culturable plant-pathogenic, mycoplasma-like organisms (MLOs). MLO chromosomes are circular and their sizes (640 to 1185 kbp) are heterogeneous. Divergence in the range of chromosome sizes is apparent between MLOs in the two major MLO disease groups, and chromosome size polymorphism occurs among some related agents. MLO chromosome sizes overlap those of culturable mycoplasmas; consequently, small genome size alone cannot explain MLO non-culturability. Hybridization with cloned MLO-specific chromosomal and 16S rRNA probes detected two separate chromosomes in some MLO 'type' strains. Large DNA molecules that appear to be MLO megaplasmids were also demonstrated. The ability to characterize full-length chromosomes from virtually any non-culturable prokaryote should greatly facilitate the molecular and genetic analysis of these difficult bacteria.

Identification and analysis of a genomic strain cluster of mycoplasmalike organisms associated with Canadian peach (eastern) X disease, western X disease, and clover yellow edge

Genetic interrelatedness among 13 strains of mycoplasmalike organisms (MLOs) from various sources was evaluated by dot hybridization and restriction fragment length polymorphism analyses using cloned DNA probes derived from Canadian peach X (CX) and western X (WX) MLOs. Dot hybridization analysis indicated that CX, WX, and clover yellow edge MLOs are closely related and form a distinct strain cluster that is only distantly related to the 10 other MLOs. Similarity coefficients derived from restriction fragment length polymorphism analysis revealed that CX, WX, and clover yellow edge MLOs represent three distinct genomic types.

Identification and characterization of plasmids from the western aster yellows mycoplasmalike organism

Supercoiled double-stranded DNA molecules (plasmids) were isolated from plants infected with three laboratory strains of western aster yellows mycoplasma-like organism (AY-MLO) by using cesium chloride-ethidium bromide density gradients. Southern blot analysis, using plasmids from the severe strain of AY-MLO (SAY-MLO) as the probe, identified at least four plasmids in celery, aster, and periwinkle plants and in Macrosteles severini leafhopper vectors infected with either the dwarf AY-MLO, Tulelake AY-MLO, or SAY-MLO strain. Plasmids were also detected in two California field isolates of AY-MLO but not in plants infected with the beet leafhopper-transmitted virescence agent, western X, or elm yellows MLOs. SAY-MLO plasmids were 5.2, 4.9, 3.4, and 1.7 kilobase pairs in size. Plasmids isolated from dwarf AY- and Tulelake AY-MLOs were 7.4, 5.1, 3.5, and 1.7 kilobase pairs in size. No evidence was obtained for integration of SAY-MLO plasmids into the MLO chromosome.

Cloning and Detection of DNA from a Nonculturable Plant Pathogenic Mycoplasma-like Organism

The ability to detect, quantify, and differentiate nonculturable mycoplasma-like organisms (MLOs) would greatly facilitate epidemiological and taxonomical studies of this unique group of plant and insect pathogens. DNA isolated from extracts of insects infected with the Western X-disease MLO was cloned in Escherichia coli. X-disease-specific clones, when labeled and used as probes, readily detected X-disease MLOs in infected plants and insects but did not hybridize with DNA from healthy plants or insects, or from several other plant pathogenic MLOs or spiroplasmas. These methods provide both a sensitive diagnostic tool and a basis for genetically differentiating MLOs.

Pathogenicity of mollicutes for insects: possible use in biological control

Acholeplasmas, spiroplasmas and other non-helical sterol-requiring mycoplasmas of unknown phylogenetic affinity inhabit insects. Of these, only spiroplasmas are known to be pathogenic. Group I-2 spiroplasmas, or Spiroplasma apis, especially in combination with other organisms, reduce honey-bee longevity. Plant pathogenic mycoplasma-like organisms are often found intracellularly in insects. Spiroplasmas are found predominantly in the gut lumen or haemolymph (or both) of their insect hosts. Pathogenicity of mycoplasmas is usually altered by extended passage in unusual hosts, in only one of two alternate hosts, or in culture media. Enhancement of experimental pathogenicity may occur with extended cultural passages, but maintenance of natural pathogenicity must be accomplished by continuous exposure to the usual host. Recent data provide new information on the ecology of pathogenicity. Spiroplasmas from unique habitats also tend to be unique. Spiroplasmas isolated from flowers appear to be adapted to insect species that frequent floral surfaces. Group IV spiroplasmas have been isolated from members of 4 holometabolous insect orders (including Lepidoptera), all of which visit flowers. Social or predatory insects, or insects with an "aggregation" phase in their life histories, also appear to be prone to spiroplasma infection. Some insect species which harbor spiroplasmas also carry infections of other mollicutes, some of which involve the haemolymph. Appearance of spiroplasmas in adult insects in nature is strongly affected by seasonality. Extensive tests of the host ranges of the new insect mollicutes will be required before their suitability for biological control can be evaluated.