Differential amplification of sequence heterogeneous ribosomal RNA genes and classification of the 'Fragaria multicipita' phytoplasma

Multilocus sequence typing of diverse phytoplasmas using hybridization probe-based sequence capture provides high resolution strain differentiation

Phytoplasmas are insect-vectored, difficult-to-culture bacterial pathogens that infect a wide variety of crop and non-crop plants, and are associated with diseases that can lead to significant yield losses in agricultural production worldwide. Phytoplasmas are currently grouped in the provisional genus ‘Candidatus Phytoplasma’, which includes 49 ‘Candidatus’ species. Further differentiation of phytoplasmas into ribosomal groups is based on the restriction fragment length polymorphism (RFLP) pattern of the 16S rRNA-encoding operon, with more than 36 ribosomal groups (16Sr) and over 100 subgroups reported. Since disease symptoms on plants are not associated with phytoplasma identity, accurate diagnostics is of critical importance to manage disease associated with these microorganisms. Phytoplasmas are typically detected from plant and insect tissue using PCR-based methods targeting universal taxonomic markers. Although these methods are relatively sensitive, specific and are widely used, they have limitations, since they provide limited resolution of phytoplasma strains, thus necessitating further assessment of biological properties and delaying implementation of mitigation measures. Moreover, the design of PCR primers that can target multiple loci from phytoplasmas that differ at the sequence level can be a significant challenge. To overcome these limitations, a PCR-independent, multilocus sequence typing (MLST) assay to characterize an array of phytoplasmas was developed. Hybridization probe s targeting cpn60, tuf, secA, secY, and nusA genes, as well as 16S and rp operons, were designed and used to enrich DNA extracts from phytoplasma-infected samples for DNA fragments corresponding to these markers prior to Illumina sequencing. This method was tested using different phytoplasmas including ‘Ca. P. asteris’ (16SrI-B), ‘Ca. P. pruni’ (16SrIII-A),‘Ca. P. prunorum’ (16SrX-B), ‘Ca. P. pyri’ (16SrX-C), ‘Ca. P. mali’ (16SrX-A), and ‘Ca. P. solani’ (16SrXII-A). Thousands of reads were obtained for each gene with multiple overlapping fragments, which were assembled to generate full-length (typically >2 kb), high-quality sequences. Phytoplasma groups and subgroups were accurately determined based on 16S ribosomal RNA and cpn60 gene sequences. Hybridization-based MLST facilitates the enrichment of target genes of phytoplasmas and allows the simultaneous determination of sequences corresponding to seven different markers. In this proof-of-concept study, hybridization-based MLST was demonstrated to be an efficient way to generate data regarding ‘Ca. Phytoplasma’ species/strain differentiation.

Pest categorisation of the non-EU phytoplasmas of Cydonia Mill., Fragaria L., Malus Mill., Prunus L., Pyrus L., Ribes L., Rubus L. and Vitis L

Following a request from the European Commission, the EFSA Panel on Plant Health performed a pest categorisation of nine phytoplasmas of Cydonia Mill., Fragaria L., Malus Mill., Prunus L., Pyrus L., Ribes L., Rubus L. and Vitis L. (hereafter “host plants”) known to occur only outside the EU or having a limited presence in the EU. This opinion covers the (i) reference strains of ‘Candidatus Phytoplasma australiense’, ‘Ca. P. fraxini’, ‘Ca. P. hispanicum’, ‘Ca. P. trifolii’, ‘Ca. P. ziziphi’, (ii) related strains infecting the host plants of ‘Ca. P. aurantifolia’, ‘Ca. P. pruni’, and ‘Ca. P. pyri’, and (iii) an unclassified phytoplasma causing Buckland valley grapevine yellows. Phytoplasmas can be detected by available methods and are efficiently transmitted by vegetative propagation, with plants for planting acting as a major entry pathway and a long‐distance spread mechanism. Phytoplasmas are also transmitted in a persistent and propagative manner by some insect families of the Fulgoromorpha, Cicadomorpha and Sternorrhyncha (order Hemiptera). No transovarial, pollen or seed transmission has been reported. The natural host range of the categorised phytoplasmas varies from one to more than 90 plant species, thus increasing the possible entry pathways. The host plants are widely cultivated in the EU. All the categorised phytoplasmas can enter and spread through the trade of host plants for planting, and by vectors. Establishment of these phytoplasmas is not expected to be limited by EU environmental conditions. The introduction of these phytoplasmas in the EU would have an economic impact. There are measures to reduce the risk of entry, establishment, spread and impact. Uncertainties result from limited information on distribution, biology and epidemiology. All the phytoplasmas categorised here meet the criteria evaluated by EFSA to qualify as potential Union quarantine pests, and they do not qualify as potential regulated non‐quarantine pests, because they are non‐EU phytoplasmas.

Detection of 16SrVI and 16SrIX phytoplasma groups in pot marigold and tickseed plants in northeastern Iran

Pot marigold and tickseed are ornamental plants with many medicinal and cosmetic uses and for landscape, respectively. During a survey in 2018, phyllody symptoms were observed in high percentages in these plants in some regions of the Razavi Khorasan province (northeastern Iran). Total DNA was extracted from symptomatic and asymptomatic plants and polymerase chain reaction was carried on using universal phytoplasma primer pairs P1/P7 and nested primer pairs R16F2n/R16R2. The nested amplification of 1200-bp fragments confirmed the presence of phytoplasmas only in the symptomatic plants. BLAST search, phylogenetic analysis, and virtual RFLP patterns of cloned amplicons allowed to classify the pot marigold phyllody phytoplasma in the 16SrVI-A subgroup while tickseed phyllody was enclosed in the 16SrIX-I subgroup. This is the first report of the association of a 16SrVI phytoplasma with pot marigold phyllody in Iran and of the phytoplasma presence in tickseed.

Genetic Variation Among Geographically Disparate Isolates of Aster Yellows Phytoplasma in the Contiguous United States

Aster Yellows phytoplasma (AYp; Candidatus Phytoplasma asteris) is associated with diseases of herbaceous plants, including ornamentals and important commercial vegetable and grain crops. The aster leafhopper (ALH; Macrosteles quadrilineatus Forbes) is the predominant vector of these bacteria, though other leafhopper species can acquire and transmit AYp. Potentially inoculative leafhoppers are reported to overwinter in the southern United States and migrate to northern latitudes in the spring. Examining the genetic similarities and differences in AYp associated with southern and northern populations of ALH may provide insight into the role that migrating ALH play in AYp disease development. To investigate similarities among geographically distinct populations of ALH and characterize the variation in AYp associated within these populations, we identified genetic variations in subgroup designation and the relative proportions of secreted AY-WB proteins from field-collected populations of AYp isolated from ALH from select locations in the southern (Arkansas, Kansas, Oklahoma, and Texas) and the northern United States (Wisconsin) in 2016, 2017, and 2018. Isolated phytoplasma were tested for variation of AYp genotypes, numbers of potentially inoculative (AYp-positive) ALH, and presence of specific AYp virulence (effector) genes. Geographically distinct populations of ALH collected in northern and southern regions were similar in CO1 genotype but carried different proportions of AYp genotypes. While similar AYp strains were detected in geographically distinct locations, the proportion of each genotype varied over time.

Standard Detection Protocol: PCR and RFLP Analyses Based on 16S rRNA Gene

Phytoplasma detection and identification is primarily based on PCR followed by restriction fragment length polymorphism analysis. This method detects and differentiates phytoplasmas including those not yet identified. The protocol describes the application of this method for identification of phytoplasmas at 16S rRNA (16Sr) group and 16Sr subgroup levels on amplicons and also in silico on the same sequences.

A novel 'Candidatus Phytoplasma asteris' subgroup 16SrI-(E/AI)AI associated with blueberry stunt disease in eastern Canada

Phytoplasmas ('Candidatus Phytoplasma' species) are phytopathogenic bacteria vectored by insects and are associated with crop diseases that cause severe yield losses by affecting reproductive tissue development. Infection of northern highbush blueberry plants (Vaccinium corymbosum; Ericaceae) with phytoplasma leads to yield losses by altering plant development resulting in stunting and subsequent plant death. Samples collected from symptomatic blueberry plants in two important blueberry-producing areas in Canada, in the provinces of Québec and Nova Scotia, were analysed for the presence of DNA sequences associated with phytoplasma. Analysis of the 16S rRNA gene sequences demonstrated that the plants were infected with a strain of 'Candidatus Phytoplasma asteris', which was previously identified as blueberry stunt phytoplasma (BBS; 16SrI-E). Examination of further bacterial sequences revealed that two distinct 16S rRNA-encoding gene sequences were present in each sample in combination with a single chaperonin-60 (cpn60) sequence and a single rpoperon sequence, suggesting that this strain displays 16S rRNA-encoding gene sequence heterogeneity. Two distinct rrnoperons, rrnE and the newly described rrnAI, were identified in samples analysed from all geographic locations. We propose, based on the sequences obtained, delineating the new subgroup 16SrI-(E/AI)AI, following the nomenclature proposed for heterogeneous subgroups. To our knowledge, this is the first report of a heterogeneous phytoplasma strain affecting blueberry plants and associated with blueberry stunt disease.

Identification of a 'Candidatus Phytoplasma hispanicum'-related strain, associated with yellows-type diseases, in smoke-tree sharpshooter (Homalodisca liturata Ball)

The 16SrXIII group from phytoplasma bacteria were identified in salivary glands from Homalodisca liturata, which were collected in El Comitán on the Baja California peninsula in Mexico. We were able to positively identify 15 16S rRNA gene sequences with the corresponding signature sequence of 'CandidatusPhytoplasma' (CAAGAYBATKATGTKTAGCYGGDCT) and in silico restriction fragment length polymorphism (RFLP) profiles (F value estimations) coupled with a phylogenetic analysis to confirm their relatedness to 'CandidatusPhytoplasma hispanicum', which in turn belongs to the 16SrXIII group. A restriction analysis was carried out with AluI and EcoRI to confirm that the five sequences belongs to subgroup D. The rest of the sequences did not exhibit any known RFLP profile related to a subgroup reported in the 16SrXIII group.

‘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.

The iPhyClassifier, an interactive online tool for phytoplasma classification and taxonomic assignment

The iPhyClassifier is an internet-based research tool for quick identification and classification of diverse phytoplasmas. The iPhyClassifier simulates laboratory restriction enzyme digestions and subsequent gel electrophoresis and generates virtual restriction fragment length polymorphism (RFLP) patterns. Based on RFLP pattern similarity coefficient scores, the iPhyClassifier gives instant suggestions on group and subgroup classification status of the phytoplasma strains under study. The iPhyClassifier also aligns the query sequences with that of reference strains of all previously described 'Candidatus Phytoplasma' species, -calculates sequence similarity scores, and assigns the phytoplasmas under study into respective 'Ca. Phytoplasma' species as related strains according to the guidelines set forth by the Phytoplasma Taxonomy Group of the International Research Program on Comparative Mycoplasmology. Additional functions of the iPhyClassifier include delineation of potentially new phytoplasma groups and subgroups as well as new 'Ca. Phytoplasma' species. This chapter describes the program components, the operational procedure, and the underlying principles of the iPhyClassifier operation. The chapter also provides hints on how to interpret the results.

Nested PCR and RFLP analysis based on the 16S rRNA gene

Current phytoplasma detection and identification methods are primarily based on nested polymerase chain reaction followed by restriction fragment length polymorphism analysis and gel electrophoresis. These methods can potentially detect and differentiate all phytoplasmas including those previously not described. The present protocol describes the application of this method for identification of phytoplasmas at 16S rRNA (16Sr) group and 16Sr subgroup levels.

Construction of an interactive online phytoplasma classification tool, iPhyClassifier, and its application in analysis of the peach X-disease phytoplasma group (16SrIII)

Phytoplasmas, the causal agents of numerous plant diseases, are insect-vector-transmitted, cell-wall-less bacteria descended from ancestral low-G+C-content Gram-positive bacteria in the Bacillus-Clostridium group. Despite their monophyletic origin, widely divergent phytoplasma lineages have evolved in adaptation to specific ecological niches. Classification and taxonomic assignment of phytoplasmas have been based primarily on molecular analysis of 16S rRNA gene sequences because of the inaccessibility of measurable phenotypic characters suitable for conventional microbial characterization. In the present study, an interactive online tool, iPhyClassifier, was developed to expand the efficacy and capacity of the current 16S rRNA gene sequence-based phytoplasma classification system. iPhyClassifier performs sequence similarity analysis, simulates laboratory restriction enzyme digestions and subsequent gel electrophoresis and generates virtual restriction fragment length polymorphism (RFLP) profiles. Based on calculated RFLP pattern similarity coefficients and overall sequence similarity scores, iPhyClassifier makes instant suggestions on tentative phytoplasma 16Sr group/subgroup classification status and 'Candidatus Phytoplasma' species assignment. Using iPhyClassifier, we revised and updated the classification of strains affiliated with the peach X-disease phytoplasma group. The online tool can be accessed at http://www.ba.ars.usda.gov/data/mppl/iPhyClassifier.html.

Roles of infection, inflammation, and the immune system in cholesterol gallstone formation

Cholesterol gallstone formation is a complex process mediated by genetic and environmental factors. Until recently, the role of the immune system in the pathogenesis of cholesterol gallstones was not considered a valid topic of research interest. This review collates and interprets an extensive body of basic literature, some of which is not customarily considered to be related to cholelithogenesis, describing the multiple facets of the immune system that appear to be involved in cholesterol cholelithogenesis. A thorough understanding of the immune interactions with biliary lipids and cholecystocytes should modify current views of the pathogenesis of cholesterol gallstones, promote further research on the pathways involved, and lead to novel diagnostic tools, treatments, and preventive measures.

Computer-simulated RFLP analysis of 16S rRNA genes: identification of ten new phytoplasma groups

Phytoplasmas are cell wall-less bacteria that cause numerous plant diseases. As no phytoplasma has been cultured in cell-free medium, phytoplasmas cannot be differentiated and classified by the traditional methods which are applied to culturable prokaryotes. Over the past decade, the establishment of a phytoplasma classification scheme based on 16S rRNA restriction fragment length polymorphism (RFLP) patterns has enabled the accurate and reliable identification and classification of a wide range of phytoplasmas. In the present study, we expanded this classification scheme through the use of computer-simulated RFLP analysis, achieving rapid differentiation and classification of phytoplasmas. Over 800 publicly available phytoplasma 16S rRNA gene sequences were aligned using the CLUSTAL_X program and the aligned 1.25 kb fragments were exported to pDRAW32 software for in silico restriction digestion and virtual gel plotting. Based on distinctive virtual RFLP patterns and calculated similarity coefficients, phytoplasma strains were classified into 28 groups. The results included the classification of hundreds of previously unclassified phytoplasmas and the delineation of 10 new phytoplasma groups representing three recently described and seven novel putative 'Candidatus Phytoplasma' taxa.

Extrachromosomal DNA isolated from tomato big bud and Candidatus Phytoplasma australiense phytoplasma strains

The nucleotide sequences of two extrachromosomal elements from tomato big bud (TBB) and one extrachromosomal element from Candidatus Phytoplasma australiense (Ca. P. australiense) phytoplasmas were determined. Both TBB plasmids (3319 and 4092 bp) contained an open reading frame ( approximately 570 bp) with homology to the rolling circle replication initiator protein (Rep). This gene was shorter than the rep genes identified from other phytoplasma plasmids, geminiviruses and bacterial plasmids. Both TBB extrachromosomal DNAs (eDNAs) encoded a putative DNA primase (dnaG) gene, a chromosomal gene required for DNA replication and which contains the conserved topoisomerase/primase domain. We speculate that the replication mechanism for the TBB phytoplasma eDNA involves the dnaG gene instead of the rep gene. The Ca. P. australiense eDNA (3773 bp) was shown to be circular and contained four open reading frames. The rep gene was encoded on ORF 1 and had homology to both plasmid (pLS1) and geminivirus-like domains.