Comparative analysis of the peanut witches'-broom phytoplasma genome reveals horizontal transfer of potential mobile units and effectors

Complete genome sequence of Candidatus Phytoplasma australasiaticum WF_GM2021, a plant pathogen associated with soybean witches' broom disease in Taiwan

The complete genome sequence of Candidatus Phytoplasma australasiaticum strain WF_GM2021, which consists of one 633,005-bp circular chromosome, is presented in this work. This uncultivated plant-pathogenic bacterium is associated with soybean (Glycine max) witches' broom disease in Wufeng District, Taichung City, Taiwan.

Improving the Comprehension of Pathogenicity and Phylogeny in 'Candidatus Phytoplasma meliae' through Genome Characterization

'Candidatus Phytoplasma meliae' is a pathogen associated with chinaberry yellowing disease, which has become a major phytosanitary problem for chinaberry forestry production in Argentina. Despite its economic impact, no genome information of this phytoplasma has been published, which has hindered its characterization at the genomic level. In this study, we used a metagenomics approach to analyze the draft genome of the 'Ca. P. meliae' strain ChTYXIII. The draft assembly consisted of twenty-one contigs with a total length of 751.949 bp, and annotation revealed 669 CDSs, 34 tRNAs, and 1 set of rRNA operons. The metabolic pathways analysis showed that ChTYXIII contains the complete core genes for glycolysis and a functional Sec system for protein translocation. Our phylogenomic analysis based on 133 single-copy genes and genome-to-genome metrics supports the classification as unique 'Ca. P. species' within the MPV clade. We also identified 31 putative effectors, including a homolog to SAP11 and others that have only been described in this pathogen. Our ortholog analysis revealed 37 PMU core genes in the genome of 'Ca. P. meliae' ChTYXIII, leading to the identification of 2 intact PMUs. Our work provides important genomic information for 'Ca. P. meliae' and others phytoplasmas for the 16SrXIII (MPV) group.

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

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

Complete genome sequence of "Candidatus Phytoplasma cynodontis" GY2015, a plant pathogen associated with Bermuda grass white leaf disease in Taiwan

The complete genome sequence of "Candidatus Phytoplasma cynodontis" strain GY2015, which consists of one 498,922-bp circular chromosome, is presented in this work. This uncultivated plant-pathogenic bacterium is associated with Bermuda grass white leaf disease in Taoyuan, Taiwan.

Complete genome sequence of "Candidatus Phytoplasma pruni" PR2021, an uncultivated bacterium associated with poinsettia (Euphorbia pulcherrima)

The complete genome sequence of "Candidatus Phytoplasma pruni" strain PR2021, which consists of one 705,138 bp circular chromosome and one 4,757 bp circular plasmid, is presented in this work. This bacterium is associated with poinsettia (Euphorbia pulcherrima) cultivar "Princettia Pink."

The genome of Candidatus phytoplasma ziziphi provides insights into their biological characteristics

Phytoplasmas are obligate cell wall-less prokaryotic bacteria that primarily multiply in plant phloem tissue. Jujube witches' broom (JWB) associated with phytoplasma is a destructive disease of jujube (Ziziphus jujuba Mill.). Here we report the complete 'Candidatus Phytoplasma ziziphi' chromosome of strain Hebei-2018, which is a circular genome of 764,108-base pairs with 735 predicted CDS. Notably, extra 19,825 bp (from 621,995 to 641,819 bp) compared to the previously reported one complements the genes involved in glycolysis, such as pdhA, pdhB, pdhC, pdhD, ackA, pduL and LDH. The synonymous codon usage bias (CUB) patterns by using comparative genomics analysis among the 9 phytoplasmas were similar for most codons. The ENc-GC3s analysis among the 9 phytoplasmas showed a greater effect under the selection on the CUBs of phytoplasmas genes than mutation and other factors. The genome exhibited a strongly reduced ability in metabolic synthesis, while the genes encoding transporter systems were well developed. The genes involved in sec-dependent protein translocation system were also identified.The expressions of nine FtsHs encoding membrane associated ATP-dependent Zn proteases and Mn-SodA with redox capacity in the Ca. P. ziziphi was positively correlated with the phytoplasma concentration. Taken together, the genome will not only expand the number of phytoplasma species and provide some new information about Ca. P. ziziphi, but also contribute to exploring its pathogenic mechanism.

Potential mobile units drive the horizontal transfer of phytoplasma effector phyllogen genes

Phytoplasmas are obligate intracellular plant pathogenic bacteria that can induce phyllody, which is a type of abnormal floral organ development. Phytoplasmas possess phyllogens, which are effector proteins that cause phyllody in plants. Phylogenetic comparisons of phyllogen and 16S rRNA genes have suggested that phyllogen genes undergo horizontal transfer between phytoplasma species and strains. However, the mechanisms and evolutionary implications of this horizontal gene transfer are unclear. Here, we analyzed synteny in phyllogen flanking genomic regions from 17 phytoplasma strains that were related to six 'Candidatus' species, including three strains newly sequenced in this study. Many of the phyllogens were flanked by multicopy genes within potential mobile units (PMUs), which are putative transposable elements found in phytoplasmas. The multicopy genes exhibited two distinct patterns of synteny that correlated with the linked phyllogens. The low level of sequence identities and partial truncations found among these phyllogen flanking genes indicate that the PMU sequences are deteriorating, whereas the highly conserved sequences and functions (e.g., inducing phyllody) of the phyllogens suggest that the latter are important for phytoplasma fitness. Furthermore, although their phyllogens were similar, PMUs in strains related to 'Ca. P. asteris' were often located in different regions of the genome. These findings strongly indicate that PMUs drive the horizontal transfer of phyllogens among phytoplasma species and strains. These insights improve our understanding of how symptom-determinant genes have been shared among phytoplasmas.

Detection, Identification and Molecular Characterization of the 16SrII-V Subgroup Phytoplasma Strain Associated with Pisum sativum and Parthenium hysterophorus L

Two unrelated plant species, green pea and parthenium weed, harboring typical phytoplasma symptoms, were discovered in Yunlin, Taiwan. Green pea (Pisum sativum.) and parthenium weed (Parthenium hysterophorus L.) are both herbaceous annual plants belonging to the Fabaceae and Asteraceae families, respectively. Displayed symptoms were witches' broom, phyllody and virescence, which are typical indications of phytoplasma infection. Pleomorphic phytoplasma-like bodies were observed under the transmission electron microscope in the sieve elements of symptomatic green pea and parthenium weed. The iPhyClassifier-based virtual RFLP study demonstrated that the phytoplasma associated with the diseased plants belongs to the 16SrII-V subgroup. The disease symptoms of both plants can be explained by the identification of PHYL1 and SAP11 effectors, identical to those of peanut witches' broom phytoplasma. The phytoplasma strains identified in this study present a very close phylogenetic relationship with other 16SrII-V subgroup phytoplasma strains discovered in Taiwan. These results not only convey the local status of the 16SrII-V subgroup phytoplasma strains but also encourage attention to be given to preventing the spread of this threat before it becomes pervasive.

Iodixanol density gradients as an effective phytoplasma enrichment approach to improve genome sequencing

Obtaining complete phytoplasma genomes is difficult due to the lack of a culture system for these bacteria. To improve genome assembly, a non-ionic, low- and iso-osmotic iodixanol (Optiprep™) density gradient centrifugation method was developed to enrich for phytoplasma cells and deplete plant host tissues prior to deoxyribonucleic acid (DNA) extraction and high-throughput sequencing (HTS). After density gradient enrichment, potato infected with a ‘Candidatus Phytoplasma australasia’-related strain showed a ∼14-fold increase in phytoplasma HTS reads, with a ∼1.7-fold decrease in host genomic reads compared to the DNA extracted from the same sample without density gradient centrifugation enrichment. Additionally, phytoplasma genome assemblies from libraries equalized to 5 million reads were, on average, ∼15,000 bp larger and more contiguous (N50 ∼14,800 bp larger) than assemblies from the DNA extracted from the infected potato without enrichment. The method was repeated on capsicum infected with Sweet Potato Little Leaf phytoplasma (‘Ca. Phytoplasma australasia’-related strain) with a lower phytoplasma titer than the potato. In capsicum, ∼threefold more phytoplasma reads and ∼twofold less host genomic reads were obtained, with the genome assembly size and N50 values from libraries equalized to 3.4 million reads ∼137,000 and ∼4,000 bp larger, respectively, compared to the DNA extracted from infected capsicum without enrichment. Phytoplasmas from potato and capsicum were both enriched at a density of 1.049–1.058 g/ml. Finally, we present two highly contiguous ‘Ca. Phytoplasma australasia’ phytoplasma reference genomes sequenced from naturally infected Solanaceae hosts in Australia. Obtaining high-quality phytoplasma genomes from naturally infected hosts will improve insights into phytoplasma taxonomy, which will improve their detection and disease management.

Phytoplasma Taxonomy: Nomenclature, Classification, and Identification

Simple Summary

Phytoplasmas are vector-borne and graft-transmissible bacteria that cause various plant diseases, leading to severe economic losses. Since phytoplasmas cannot be cultured in cell-free media, their identification and taxonomy rely on molecular techniques and gene sequences. In this article, we summarize the recent advances in phytoplasma taxonomy from three different aspects, including (i) nomenclature (naming Candidatus Phytoplasma species); (ii) classification (group and subgroup assignment based on 16S rRNA gene sequences); and (iii) identification (fine differentiation of phytoplasma strains). In addition, some important issues, especially those related to recognizing new ‘Candidatus Phytoplasma’ species, are discussed. This information will be helpful for rapid diagnosis of phytoplasma diseases and accurate taxonomic identification of both emerging and known phytoplasma strains.

Abstract

Phytoplasmas are pleomorphic, wall-less intracellular bacteria that can cause devastating diseases in a wide variety of plant species. Rapid diagnosis and precise identification of phytoplasmas responsible for emerging plant diseases are crucial to preventing further spread of the diseases and reducing economic losses. Phytoplasma taxonomy (identification, nomenclature, and classification) has lagged in comparison to culturable bacteria, largely due to lack of axenic phytoplasma culture and consequent inaccessibility of phenotypic characteristics. However, the rapid expansion of molecular techniques and the advent of high throughput genome sequencing have tremendously enhanced the nucleotide sequence-based phytoplasma taxonomy. In this article, the key events and milestones that shaped the current phytoplasma taxonomy are highlighted. In addition, the distinctions and relatedness of two parallel systems of ‘Candidatus phytoplasma’ species/nomenclature system and group/subgroup classification system are clarified. Both systems are indispensable as they serve different purposes. Furthermore, some hot button issues in phytoplasma nomenclature are also discussed, especially those pertinent to the implementation of newly revised guidelines for ‘Candidatus Phytoplasma’ species description. To conclude, the challenges and future perspectives of phytoplasma taxonomy are briefly outlined.

The Diversity, Distribution and Status of Phytoplasma Diseases in China

Phytoplasmas are important prokaryotic pathogenic bacteria without cell walls, which were formerly known as mycoplasma-like organisms, and belong to the Mollicutes class, Candidatus Phytoplasma genus. They are widely distributed in plants and insects, and can cause serious diseases in important food crops, vegetables, fruit trees, ornamental plants and trees, resulting in huge economic losses. To date, more than 100 phytoplasma diseases have been reported in China, which are distributed throughout the country. Jujube witches'-broom, paulownia witches'-broom, wheat blue dwarf, banana bunchy top, sugarcane white leaf, rice orange leaf and mulberry dwarf represent the phytoplasma diseases causing the most serious damage in China. New phytoplasma diseases and their strains are being reported continuously, indicating that phytoplasmas are more diverse than previously thought. Phytoplasmas are mainly transmitted by insect vectors, such as leafhopper and planthopper, and can also be spread by grafting or Cuscuta australis (known as dodder). Mixed infections of phytoplasmas and viruses, bacteria, and spiroplasmas have also become a serious problem in several crops and are responsible for more synergistic losses. With the continuous development and improvement of technology, molecular biological detection has become the main technique for phytoplasma detection and identification. Currently, research on phytoplasma diseases in China mainly focuses on pathogen identification and classification, and insect vector and host diversity; however, there is less focus on pathogenicity, comparative genomics, and effect factors. More research attention has been paid to wheat blue dwarf phytoplasma, paulownia witches'-broom phytoplasma, jujube witches'-broom phytoplasma, and sugarcane white leaf phytoplasma. Other phytoplasma diseases have been reported; however, there have been no in-depth studies. In this paper, the history and present situation of phytoplasma research, and the status, distribution, and diversity of phytoplasma diseases are summarized, and some possible research directions of phytoplasma in the future in China are proposed.

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

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

Draft genome sequence of 'Candidatus Phytoplasma australasia', strain SS02 associated with sesame phyllody disease

'Candidatus Phytoplasma' is an uncultivated, intracellular bacterial plant pathogen transmitted by phloem-feeding insect vectors. Among the group of phytoplasmas, the Peanut Witches' Broom or 16SrII group of phytoplasmas associated with various diseases cause severe crop losses every year in India. The 'Ca. Phytoplasma sp.' strain SS02 was associated with phyllody disease of sesame plants collected from New Delhi. The genome sequence of strain SS02 was obtained using its genomic DNA enrichment and hybrid assembly of sequences generated on Illumina and Oxford Nanopore Technologies MinION platforms. The hybrid assembly strategy generated a draft genome with 60 contigs totaling 553,228 bp of length with more than 400 × depth coverage and 95.21% of the estimated completeness. The SS02 genome draft sequence contains 465 protein-coding genes, 17 tRNA genes, and 3 rRNA genes. The availability of this draft genome also provided a foundation for genome-scale genotypic analyses.

A phytoplasma effector acts as a ubiquitin-like mediator between floral MADS-box proteins and proteasome shuttle proteins

Plant pathogenic bacteria have developed effectors to manipulate host cell functions to facilitate infection. A certain number of effectors use the conserved ubiquitin-proteasome system in eukaryotic to proteolyze targets. The proteasome utilization mechanism is mainly mediated by ubiquitin interaction with target proteins destined for degradation. Phyllogens are a family of protein effectors produced by pathogenic phytoplasmas that transform flowers into leaves in diverse plants. Here, we present a noncanonical mechanism for phyllogen action that involves the proteasome and is ubiquitin-independent. Phyllogens induce proteasomal degradation of floral MADS-box transcription factors (MTFs) in the presence of RADIATION-SENSITIVE23 (RAD23) shuttle proteins, which recruit ubiquitinated proteins to the proteasome. Intracellular localization analysis revealed that phyllogen induced colocalization of MTF with RAD23. The MTF/phyllogen/RAD23 ternary protein complex was detected not only in planta but also in vitro in the absence of ubiquitin, showing that phyllogen directly mediates interaction between MTF and RAD23. A Lys-less nonubiquitinated phyllogen mutant induced degradation of MTF or a Lys-less mutant of MTF. Furthermore, the method of sequential formation of the MTF/phyllogen/RAD23 protein complex was elucidated, first by MTF/phyllogen interaction and then RAD23 recruitment. Phyllogen recognized both the evolutionarily conserved tetramerization region of MTF and the ubiquitin-associated domain of RAD23. Our findings indicate that phyllogen functionally mimics ubiquitin as a mediator between MTF and RAD23.

Detection, Identification, and Molecular Characterization of a 16SrII-V Subgroup Phytoplasma Associated with Nicotiana plumbaginifolia

Nicotiana plumbaginifolia Viviani, commonly known as curl-leaved tobacco, is an annual herbaceous plant belonging to Solanaceae family. This plant is native to Mexico, South America, and parts of the Caribbean and has been reported to be present in Taiwan since 2006. In March 2021, N. plumbaginifolia Viviani, found in Yunlin County, Taiwan, was observed to have phyllody, virescence, and witches'-broom, which is consistent with the disease symptoms caused by phytoplasma infection. Samples of the healthy and symptomatic plants were collected for analysis of the causal agent associated with the diseased N. plumbaginifolia Viviani. Under transmission electron microscopy, the phytoplasma-like pleomorphic bodies were found in the sieve tubes of the diseased plants. The 16S ribosomal RNA (rRNA)-based phylogenetic analysis and the iPhyClassifier-based virtual restriction fragment length polymorphism study demonstrated that the phytoplasma identified in this study can be classified into the 16SrII-V subgroup, which is similar to the peanut witches'-broom phytoplasma, a 'Candidatus phytoplasma aurantifolia'-related strain. Further identification of SAP54/PHYL1 and SAP11 homologs in the phytoplasma explain the disease symptoms of phyllody, virescence, and witches'-broom observed in diseased N. plumbaginifolia Viviani. The discovery of new phytoplasma plant hosts has gained scientific importance in light of the attempt to unravel an efficient strategy to fight the rapid spread of this disease, which poses a threat to the agricultural sector and food security in Taiwan.

Comparative Genome Analysis of ‘Candidatus Phytoplasma luffae’ Reveals the Influential Roles of Potential Mobile Units in Phytoplasma Evolution

Phytoplasmas are insect-transmitted plant pathogens that cause substantial losses in agriculture. In addition to economic impact, phytoplasmas induce distinct disease symptoms in infected plants, thus attracting attention for research on molecular plant-microbe interactions and plant developmental processes. Due to the difficulty of establishing an axenic culture of these bacteria, culture-independent genome characterization is a crucial tool for phytoplasma research. However, phytoplasma genomes have strong nucleotide composition biases and are repetitive, which make it challenging to produce complete assemblies. In this study, we utilized Illumina and Oxford Nanopore sequencing technologies to obtain the complete genome sequence of ‘Candidatus Phytoplasma luffae’ strain NCHU2019 that is associated with witches’ broom disease of loofah (Luffa aegyptiaca) in Taiwan. The fully assembled circular chromosome is 769 kb in size and is the first representative genome sequence of group 16SrVIII phytoplasmas. Comparative analysis with other phytoplasmas revealed that NCHU2019 has a remarkably repetitive genome, possessing a pair of 75 kb repeats and at least 13 potential mobile units (PMUs) that account for ∼25% of its chromosome. This level of genome repetitiveness is exceptional for bacteria, particularly among obligate pathogens with reduced genomes. Our genus-level analysis of PMUs demonstrated that these phytoplasma-specific mobile genetic elements can be classified into three major types that differ in gene organization and phylogenetic distribution. Notably, PMU abundance explains nearly 80% of the variance in phytoplasma genome sizes, a finding that provides a quantitative estimate for the importance of PMUs in phytoplasma genome variability. Finally, our investigation found that in addition to horizontal gene transfer, PMUs also contribute to intra-genomic duplications of effector genes, which may provide redundancy for subfunctionalization or neofunctionalization. Taken together, this work improves the taxon sampling for phytoplasma genome research and provides novel information regarding the roles of mobile genetic elements in phytoplasma evolution.

Accelerating Complete Phytoplasma Genome Assembly by Immunoprecipitation-Based Enrichment and MinION-Based DNA Sequencing for Comparative Analyses

Phytoplasmas are uncultivated plant-pathogenic bacteria with agricultural importance. Those belonging to the 16SrII group, represented by 'Candidatus P. aurantifolia', have a wide range of plant hosts and cause significant yield losses in valuable crops, such as pear, sweet potato, peanut, and soybean. In this study, a method that combines immunoprecipitation-based enrichment and MinION long-read DNA sequencing was developed to solve the challenge of phytoplasma genome studies. This approach produced long reads with high mapping rates and high genomic coverage that can be combined with Illumina reads to produce complete genome assemblies with high accuracy. We applied this method to strain NCHU2014 and determined its complete genome sequence, which consists of one circular chromosome with 635,584 bp and one plasmid with 4,224 bp. Although 'Ca. P. aurantifolia' NCHU2014 has a small chromosome with only 471 protein-coding genes, it contains 33 transporter genes and 27 putative effector genes, which may contribute to obtaining nutrients from hosts and manipulating host developments for their survival and multiplication. Two effectors, the homologs of SAP11 and SAP54/PHYL1 identified in 'Ca. P. aurantifolia' NCHU2014, have the biochemical activities in destabilizing host transcription factors, which can explain the disease symptoms observed in infected plants. Taken together, this study provides the first complete genome available for the 16SrII phytoplasmas and contributes to the understanding of phytoplasma pathogenicity.

Genomic insights into the fast growth of paulownias and the formation of Paulownia witches' broom

Paulownias are among the fastest growing trees in the world, but they often suffer tremendous loss of wood production due to infection by Paulownia witches' broom (PaWB) phytoplasmas. In this study, we have sequenced and assembled a high-quality nuclear genome of Paulownia fortunei, a commonly cultivated paulownia species. The assembled genome of P. fortunei is 511.6 Mb in size, with 93.2% of its sequences anchored to 20 pseudo-chromosomes, and it contains 31 985 protein-coding genes. Phylogenomic analyses show that the family Paulowniaceae is sister to a clade composed of Phrymaceae and Orobanchaceae. Higher photosynthetic efficiency is achieved by integrating C3 photosynthesis and the crassulacean acid metabolism pathway, which may contribute to the extremely fast growth habit of paulownia trees. Comparative transcriptome analyses reveal modules related to cambial growth and development, photosynthesis, and defense responses. Additional genome sequencing of PaWB phytoplasma, combined with functional analyses, indicates that the effector PaWB-SAP54 interacts directly with Paulownia PfSPLa, which in turn causes the degradation of PfSPLa by the ubiquitin-mediated pathway and leads to the formation of witches' broom. Taken together, these results provide significant insights into the biology of paulownias and the regulatory mechanism for the formation of PaWB.

Parasitic modulation of host development by ubiquitin-independent protein degradation

Certain obligate parasites induce complex and substantial phenotypic changes in their hosts in ways that favor their transmission to other trophic levels. However, the mechanisms underlying these changes remain largely unknown. Here we demonstrate how SAP05 protein effectors from insect-vectored plant pathogenic phytoplasmas take control of several plant developmental processes. These effectors simultaneously prolong the host lifespan and induce witches' broom-like proliferations of leaf and sterile shoots, organs colonized by phytoplasmas and vectors. SAP05 acts by mediating the concurrent degradation of SPL and GATA developmental regulators via a process that relies on hijacking the plant ubiquitin receptor RPN10 independent of substrate ubiquitination. RPN10 is highly conserved among eukaryotes, but SAP05 does not bind insect vector RPN10. A two-amino-acid substitution within plant RPN10 generates a functional variant that is resistant to SAP05 activities. Therefore, one effector protein enables obligate parasitic phytoplasmas to induce a plethora of developmental phenotypes in their hosts.

Parasitic modulation of host development by ubiquitin-independent protein degradation

Certain obligate parasites induce complex and substantial phenotypic changes in their hosts in ways that favor their transmission to other trophic levels. However, the mechanisms underlying these changes remain largely unknown. Here we demonstrate how SAP05 protein effectors from insect-vectored plant pathogenic phytoplasmas take control of several plant developmental processes. These effectors simultaneously prolong the host lifespan and induce witches' broom-like proliferations of leaf and sterile shoots, organs colonized by phytoplasmas and vectors. SAP05 acts by mediating the concurrent degradation of SPL and GATA developmental regulators via a process that relies on hijacking the plant ubiquitin receptor RPN10 independent of substrate ubiquitination. RPN10 is highly conserved among eukaryotes, but SAP05 does not bind insect vector RPN10. A two-amino-acid substitution within plant RPN10 generates a functional variant that is resistant to SAP05 activities. Therefore, one effector protein enables obligate parasitic phytoplasmas to induce a plethora of developmental phenotypes in their hosts.

Comparison of Current Methods for Signal Peptide Prediction in Phytoplasmas

Although phytoplasma studies are still hampered by the lack of axenic cultivation methods, the availability of genome sequences allowed dramatic advances in the characterization of the virulence mechanisms deployed by phytoplasmas, and highlighted the detection of signal peptides as a crucial step to identify effectors secreted by phytoplasmas. However, various signal peptide prediction methods have been used to mine phytoplasma genomes, and no general evaluation of these methods is available so far for phytoplasma sequences. In this work, we compared the prediction performance of SignalP versions 3.0, 4.0, 4.1, 5.0 and Phobius on several sequence datasets originating from all deposited phytoplasma sequences. SignalP 4.1 with specific parameters showed the most exhaustive and consistent prediction ability. However, the configuration of SignalP 4.1 for increased sensitivity induced a much higher rate of false positives on transmembrane domains located at N-terminus. Moreover, sensitive signal peptide predictions could similarly be achieved by the transmembrane domain prediction ability of TMHMM and Phobius, due to the relatedness between signal peptides and transmembrane regions. Beyond the results presented herein, the datasets assembled in this study form a valuable benchmark to compare and evaluate signal peptide predictors in a field where experimental evidence of secretion is scarce. Additionally, this study illustrates the utility of comparative genomics to strengthen confidence in bioinformatic predictions.

'Candidatus Phytoplasma asteris' subgroups display distinct disease progression dynamics during the carrot growing season

Aster Yellows phytoplasma (AYp; ‘Candidatus Phytoplasma asteris’) is an obligate bacterial pathogen that is the causative agent of multiple diseases in herbaceous plants. While this phytoplasma has been examined in depth for its disease characteristics, knowledge about the spatial and temporal dynamics of pathogen spread is lacking. The phytoplasma is found in plant’s phloem and is vectored by leafhoppers (Cicadellidae: Hemiptera), including the aster leafhopper, Macrosteles quadrilineatus Forbes. The aster leafhopper is a migratory insect pest that overwinters in the southern United States, and historical data suggest these insects migrate from southern overwintering locations to northern latitudes annually, transmitting and driving phytoplasma infection rates as they migrate. A more in-depth understanding of the spatial, temporal and genetic determinants of Aster Yellows disease progress will lead to better integrated pest management strategies for Aster Yellows disease control. Carrot, Daucus carota L., plots were established at two planting densities in central Wisconsin and monitored during the 2018 growing season for Aster Yellows disease progression. Symptomatic carrots were sampled and assayed for the presence of the Aster Yellows phytoplasma. Aster Yellows disease progression was determined to be significantly associated with calendar date, crop density, location within the field, and phytoplasma subgroup.

Functional variation in phyllogen, a phyllody-inducing phytoplasma effector family, attributable to a single amino acid polymorphism

Flower malformation represented by phyllody is a common symptom of phytoplasma infection induced by a novel family of phytoplasma effectors called phyllogens. Despite the accumulation of functional and structural phyllogen information, the molecular mechanisms of phyllody have not yet been integrated with their evolutionary aspects due to the limited data on their homologs across diverse phytoplasma lineages. Here, we developed a novel universal PCR-based approach to identify 25 phytoplasma phyllogens related to nine "Candidatus Phytoplasma" species, including four species whose phyllogens have not yet been identified. Phylogenetic analyses showed that the phyllogen family consists of four groups (phyl-A, -B, -C, and -D) and that the evolutionary relationships of phyllogens were significantly distinct from those of phytoplasmas, suggesting that phyllogens were transferred horizontally among phytoplasma strains and species. Although phyllogens belonging to the phyl-A, -C, and -D groups induced phyllody, the phyl-B group lacked the ability to induce phyllody. Comparative functional analyses of phyllogens revealed that a single amino acid polymorphism in phyl-B group phyllogens prevented interactions between phyllogens and A- and E-class MADS domain transcription factors (MTFs), resulting in the inability to degrade several MTFs and induce phyllody. Our finding of natural variation in the function of phytoplasma effectors provides new insights into molecular mechanisms underlying the aetiology of phytoplasma diseases.

Complete Genome Sequence of "Candidatus Phytoplasma asteris" RP166, a Plant Pathogen Associated with Rapeseed Phyllody Disease in Poland

The complete genome sequence of “Candidatus Phytoplasma asteris” RP166, which consists of one 829,546-bp circular chromosome, is presented in this work. This bacterium is associated with rapeseed phyllody disease in Poland and belongs to the 16SrI-B (i.e., aster yellows) group.

The complete genome sequence of “Candidatus Phytoplasma asteris” RP166, which consists of one 829,546-bp circular chromosome, is presented in this work. This bacterium is associated with rapeseed phyllody disease in Poland and belongs to the 16SrI-B (i.e., aster yellows) group.

Threeflower Tickclover (Desmodium triflorum) is a New Host for Peanut Witches' Broom Phytoplasma, a 16SrII-V Subgroup Strain in Taiwan

Three-flower Tick-clover (Desmodium triflorum) is a perennial herbaceous plant that belongs to the family of Leguminosae. Threeflower tickclover widely grows at mid-low altitude regions in Taiwan and is commonly used as a traditional herbal medicine for the treatment of dysmenorrheal, muscle spasm, cough, pain and poisoning. In March 2020, disease symptoms such as little leaf, phyllody, virescence, and witches' broom were observed on threeflower tickclover at the sansheng community park in Mailiao, Yunlin County, Taiwan. Similar disease symptoms were observed on peanut infected with peanut witches' broom (PnWB) phytoplasma grown in the same area (Liu et al. 2015). Leaf samples collected from the healthy and symptomatic threeflower tickclover were used to extract total DNA and protein for PCR and western blotting assays, respectively. Nested PCR was performed with the phytoplasma universal primer pairs P1/P7 followed by R16F2n/R16R2 for the amplification of 16S ribosomal RNA (rRNA) gene (Lee et al. 1993). A specific DNA fragment of expected size (1.2 kb) for 16S rRNA was only amplified from leaf samples of symptomatic threeflower tickclover. The nucleotide sequence of the amplified DNA fragment using primer pairs P1/P7 was deposited into the GenBank (accession no. MT452308). Blast analysis revealed that MT452308 shares 100% identity with that of GenBank accession NZ_AMWZ01000008 (complement [31109 to 32640]) of phytoplasma associated with PnWB disease (Chung et al. 2013). Based on the virtual RFLP pattern of MT452308 generated by iPhyClassifier, the phytoplasma detected in symptomatic threeflower tickclover could be classified into the 16SrII-V subgroup. For western blotting, the polyclonal antibody raised against Imp protein of purple coneflower witches' broom phytoplasma (Chien et al. 2020), which is identical with that (accession no. ADD59806) of PnWB phytoplasma, was used. An expected signal of 19 kDa specific for Imp was only detected in threeflower tickclover exhibiting disease symptoms. Subsequent assays including PCR, DNA sequencing and western blotting further confirmed that the gene encoding a SAP11-like protein (accession no. EMR14684) identified in PnWB phytoplasma was also found in samples of symptomatic threeflower tickclover, and shares 100% identity with each other. Our results indicate that threeflower tickclover, a common weed in Taiwan, may act as an alternative natural host for PnWB phytoplasma, and contributes to the spreading of phytoplasma disease.

Ixeris Chinensis is a New Host for Peanut Witches' Broom Phytoplasma, a 16SrII-V Subgroup Strain in Taiwan

Ixeris chinensis (Thunb. ex Thunb.) Nakai, a perennial herbaceous plant that belongs to the family of Asteraceae, is widely distributed at mid-low altitude regions in Taiwan. I. chinensis is commonly used as traditional herbal medicine for the treatment of inflammation, bronchitis, pneumonia, and diarrhea. In March 2020, disease symptoms such as shoot proliferation, phyllody, virescence, purple top, and witches' broom were observed on I. chinensis at the sansheng community park in Mailiao, Yunlin County, Taiwan. Totally, eight I. chinensis plants were checked and half of them were symptomatic. These disease symptoms are similar to those associated with peanut witches' broom (PnWB) disease identified in the same area (Liu et al. 2015). Three samples mixed with leaf, stem, and flower were tested including one healthy and two symptomatic I. chinensis. The total DNA of each sample was extracted and examined by nested PCR for the amplification of 16S rDNA with the phytoplasma universal primer pairs P1/P7 followed by R16F2n/R16R2 (Lee et al. 1993). A specific signal of expected size (1.2 kb) for 16S rDNA was only detected in the symptomatic I. chinensis, but not in healthy I. chinensis. The nucleotide sequence (accession no. MT416114) of the amplified DNA fragment using primer pairs P1/P7 from symptomatic I. chinensis is identical to that of GenBank accession NZ_AMWZ01000008 (complement [31109 to 32640]) of phytoplasma associated with PnWB disease (Chung et al. 2013). Analysis of the virtual RFLP pattern of MT416114 generated by iPhyClassifier revealed that the phytoplasma detected in symptomatic I. chinensis belongs to a 16SrII-V subgroup. The total protein of each sample was also extracted and examined by western blotting using the polyclonal antibody raised against Imp protein of purple coneflower witches' broom phytoplasma (Chien et al. 2020), which is identical with that (accession no. ADD59806) of PnWB phytoplasma. An expected signal of 19 kDa specific for Imp was detected in symptomatic I. chinensis, but not in healthy I. chinensis. Subsequent PCR, DNA sequencing and western blotting assays further confirmed that the gene encoding a SAP11-like protein was only detected in symptomatic I. chinensis, and shares 100% identity with that (accession no. EMR14684) of PnWB phytoplasma. Our results indicate that PnWB phytoplasma causes disease in I. chinensis, a common weed, which may act as an alternative natural host and facilitate the spreading of phytoplasma disease in Taiwan.

Lilac Tasselflower (Emilia sonchifolia) is a New Host for Peanut Witches' Broom Phytoplasma, a 16SrII-V Subgroup Strain in Taiwan

Lilac tasselflower (Emilia sonchifolia) is an annual herbaceous plant that belongs to the family of Asteraceae. Lilac tasselflower is widely distributed at mid-low altitude regions in Taiwan, and is commonly used as traditional herbal medicine for the treatment of inflammation, rheumatism, dysentery, and analgesic. In March 2020, disease symptoms such as shoot proliferation, phyllody, and witches' broom were observed on lilac tasselflower at the sansheng community park in Mailiao, Yunlin County, Taiwan. Totally, four lilac tasselflower plants were checked and half of them were symptomatic. At the same area, similar symptoms associated with peanut witches' broom (PnWB) disease were observed (Liu et al. 2015). Samples including one healthy and two symptomatic lilac tasselflower were collected for total DNA and protein extraction used for PCR and western blotting assays, respectively. First, two sets of phytoplasma universal primer pairs P1/P7 and R16F2n/R16R2 were used to perform nested PCR for detection of 16S ribosomal RNA (rRNA) gene (Lee et al. 1993). A specific signal of expected size (1.2 kb) for 16S rRNA was only detected in samples of lilac tasselflower exhibiting disease symptoms. The amplified DNA fragment using primer pairs P1/P7 was partially sequenced (accession no. MT420682) with P1 and a nested primer (5'-GGGTCTTTACTGACGCTGAGG-3'). The 1.4 kb nucleotide sequence shares 100% identity with that of GenBank accession NZ_AMWZ01000008 (complement [31109 to 32640]) of phytoplasma associated with PnWB disease (Chung et al. 2013). Further analysis by iPhyClassifier, the virtual RFLP pattern of MT420682 confirmed that the phytoplasma detected in symptomatic lilac tasselflower could be classified into the 16SrII-V subgroup. For western blotting, total protein of each sample was examined using the polyclonal antibody raised against Imp protein of purple coneflower witches' broom phytoplasma (Chien et al. 2020), which shares 100% identity with that (accession no. ADD59806) of PnWB phytoplasma. A specific signal of expected size (19 kDa) for Imp was detected in symptomatic lilac tasselflower, but not in healthy lilac tasselflower. Subsequent PCR, DNA sequencing and western blotting assays further confirmed that the gene encoding a SAP11-like protein detected in samples of lilac tasselflower exhibiting disease symptoms is identical to that (accession no. EMR14684) of PnWB phytoplasma. Our results indicated that lilac tasselflower, which is recognized as a common weed in Taiwan, may facilitate the spreading of phytoplasma disease by acting as an alternative natural host for PnWB phytoplasma.

Species Boundaries and Molecular Markers for the Classification of 16SrI Phytoplasmas Inferred by Genome Analysis

Phytoplasmas are plant-pathogenic bacteria that impact agriculture worldwide. The commonly adopted classification system for phytoplasmas is based on the restriction fragment length polymorphism (RFLP) analysis of their 16S rRNA genes. With the increased availability of phytoplasma genome sequences, the classification system can now be refined. This work examined 11 strains in the 16SrI group within the genus ‘Candidatus Phytoplasma’ and investigated the possible species boundaries. We confirmed that the RFLP classification method is problematic due to intragenomic variation of the 16S rRNA genes and uneven weighing of different nucleotide positions. Importantly, our results based on the molecular phylogeny, differentiations in chromosomal segments and gene content, and divergence in homologous sequences, all supported that these strains may be classified into multiple operational taxonomic units (OTUs) equivalent to species. Strains assigned to the same OTU share >97% genome-wide average nucleotide identity (ANI) and >78% of their protein-coding genes. In comparison, strains assigned to different OTUs share < 94% ANI and < 75% of their genes. Reduction in homologous recombination between OTUs is one possible explanation for the discontinuity in genome similarities, and these findings supported the proposal that 95% ANI could serve as a cutoff for distinguishing species in bacteria. Additionally, critical examination of these results and the raw sequencing reads led to the identification of one genome that was presumably mis-assembled by combining two sequencing libraries built from phytoplasmas belonging to different OTUs. This finding provided a cautionary tale for working on uncultivated bacteria. Based on the new understanding of phytoplasma divergence and the current genome availability, we developed five molecular markers that could be used for multilocus sequence analysis (MLSA). By selecting markers that are short yet highly informative, and are distributed evenly across the chromosome, these markers provided a cost-effective system that is robust against recombination. Finally, examination of the effector gene distribution further confirmed the rapid gains and losses of these genes, as well as the involvement of potential mobile units (PMUs) in their molecular evolution. Future improvements on the taxon sampling of phytoplasma genomes will allow further expansions of similar analysis, and thus contribute to phytoplasma taxonomy and diagnostics.

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.

Detection, characterization and evolutionary aspects of S54LP of SP (SAP54 Like Protein of Sesame Phyllody): a phytoplasma effector molecule associated with phyllody development in sesame (Sesamum indicum L.)

SAP54, an effector protein secreted by phytoplasmas has been reported to induce phyllody. S54LP of SP (SAP54 Like Protein of Sesame Phyllody), a SAP54 ortholog from phyllody and witches' broom affected sesame (Sesamum indicum L.) was amplified, cloned and sequenced. Comparative sequence and phylogenetic analysis of diverse phytoplasma strains was carried out to delineate the evolution of S54LP of SP. The degree of polymorphism across SAP54 orthologs and the evolutionary forces acting on this effector protein were ascertained. Site-specific selection across SAP54 orthologs was estimated using Fixed Effects Likelihood (FEL) approach. Nonsynonymous substitutions were detected in the SAP54 orthologs' sequences from phytoplasmas belonging to same (sub) group. Phylogenetic analysis based on S54LP of SP grouped phytoplasmas belonging to same 16SrDNA (sub) groups into different clusters. Analysis of selection forces acting on SAP54 orthologs from nine different phytoplasma (sub)groups, affecting plant species belonging to twelve different families across ten countries showed the orthologs to be under purifying (negative) selection. One amino acid residue was found to be under pervasive diversifying (positive) selection and a total of three amino acid sites were found to be under pervasive purifying (negative) selection. The location of these amino acids in the signal peptide and mature protein was studied with an aim to understand their role in protein-protein interaction. Asparagine residues (at positions 68 and 84) were found to be under pervasive purifying selection suggesting their functional importance in the effector protein. Our study suggests lack of coevolution between SAP54 and 16SrDNA. Signal peptide appears to evolve at a rate slightly higher than the mature protein. Overall, SAP54 and its orthologs are evolving under purifying selection confirming their functional importance in phytoplasma virulence.

Comparative Genomic Analysis of a Novel Strain of Taiwan Hot-Spring Cyanobacterium Thermosynechococcus sp. CL-1

Thermosynechococcus is a genus of thermophilic unicellular cyanobacteria that are dominant in microbial mats at about 50–65°C in alkaline hot springs of eastern Asia. We used PacBio SMRT Sequencing to sequence the complete genome of a novel strain of thermophilic cyanobacterium, Thermosynechococcus sp. CL-1, isolated from the Chin-Lun hot spring (pH 9.3, 62°C) in Taiwan. Genome-scale phylogenetic analysis and average nucleotide identity (ANI) results suggested that CL-1 is a new species in the genus Thermosynechococcus. Comparative genome analysis revealed divergent genome structures of Thermosynechococcus strains. In addition, the distinct genetic differences between CL-1 and the other Thermosynechococcus strains are related to photosynthesis, transporters, signal transduction, the chaperone/usher system, nitric oxide protection, antibiotic resistance, prokaryotic immunity systems, and other physiological processes. This study suggests that Thermosynechococcus strains have actively acquired many putative horizontally transferred genes from other bacteria that enabled them to adapt to different ecological niches and stressful conditions in hot springs.

Flavescence Dorée Phytoplasma Has Multiple ftsH Genes that Are Differentially Expressed in Plants and Insects

Flavescence dorée (FD) is a severe epidemic disease of grapevines caused by FD phytoplasma (FDP) transmitted by the leafhopper vector Scaphoideus titanus. The recent sequencing of the 647-kbp FDP genome highlighted an unusual number of genes encoding ATP-dependent zinc proteases FtsH, which have been linked to variations in the virulence of "Candidatus Phytoplasma mali" strains. The aims of the present study were to predict the FtsH repertoire of FDP, to predict the functional domains and topologies of the encoded proteins in the phytoplasma membrane and to measure the expression profiles in different hosts. Eight complete ftsH genes have been identified in the FDP genome. In addition to ftsH6, which appeared to be the original bacterial ortholog, the other seven gene copies were clustered on a common distinct phylogenetic branch, suggesting intra-genome duplication of ftsH. The expression of these proteins, quantified in plants and insect vectors in natural and experimental pathosystems, appeared to be modulated in a host-dependent manner. Two of the eight FtsH C-tails were predicted by Phobius software to be extracellular and, therefore, in direct contact with the host cellular content. As phytoplasmas cannot synthesize amino acids, our data raised questions regarding the involvement of FtsH in the adaptation to hosts via potentially enhanced recycling of phytoplasma cellular proteins and host protein degradation.

Phytoplasma SAP11 effector destabilization of TCP transcription factors differentially impact development and defence of Arabidopsis versus maize

Phytoplasmas are insect-transmitted bacterial pathogens that colonize a wide range of plant species, including vegetable and cereal crops, and herbaceous and woody ornamentals. Phytoplasma-infected plants often show dramatic symptoms, including proliferation of shoots (witch's brooms), changes in leaf shapes and production of green sterile flowers (phyllody). Aster Yellows phytoplasma Witches' Broom (AY-WB) infects dicots and its effector, secreted AYWB protein 11 (SAP11), was shown to be responsible for the induction of shoot proliferation and leaf shape changes of plants. SAP11 acts by destabilizing TEOSINTE BRANCHED 1-CYCLOIDEA-PROLIFERATING CELL FACTOR (TCP) transcription factors, particularly the class II TCPs of the CYCLOIDEA/TEOSINTE BRANCHED 1 (CYC/TB1) and CINCINNATA (CIN)-TCP clades. SAP11 homologs are also present in phytoplasmas that cause economic yield losses in monocot crops, such as maize, wheat and coconut. Here we show that a SAP11 homolog of Maize Bushy Stunt Phytoplasma (MBSP), which has a range primarily restricted to maize, destabilizes specifically TB1/CYC TCPs. SAP11MBSP and SAP11AYWB both induce axillary branching and SAP11AYWB also alters leaf development of Arabidopsis thaliana and maize. However, only in maize, SAP11MBSP prevents female inflorescence development, phenocopying maize tb1 lines, whereas SAP11AYWB prevents male inflorescence development and induces feminization of tassels. SAP11AYWB promotes fecundity of the AY-WB leafhopper vector on A. thaliana and modulates the expression of A. thaliana leaf defence response genes that are induced by this leafhopper, in contrast to SAP11MBSP. Neither of the SAP11 effectors promote fecundity of AY-WB and MBSP leafhopper vectors on maize. These data provide evidence that class II TCPs have overlapping but also distinct roles in regulating development and defence in a dicot and a monocot plant species that is likely to shape SAP11 effector evolution depending on the phytoplasma host range.

Genomic Characterization of the Periwinkle Leaf Yellowing (PLY) Phytoplasmas in Taiwan

The periwinkle leaf yellowing (PLY) disease was first reported in Taiwan in 2005. This disease was caused by an uncultivated bacterium in the genus “Candidatus phytoplasma.” In subsequent years, this bacterium was linked to other plant diseases and caused losses in agriculture. For genomic investigation of this bacterium and its relatives, we conducted whole genome sequencing of a PLY phytoplasma from an infected periwinkle collected in Taoyuan. The de novo genome assembly produced eight contigs with a total length of 824,596 bp. The annotation contains 775 protein-coding genes, 63 pseudogenes, 32 tRNA genes, and two sets of rRNA operons. To characterize the genomic diversity across populations, a second strain that infects green onions in Yilan was collected for re-sequencing analysis. Comparison between these two strains identified 337 sequence polymorphisms and 10 structural variations. The metabolic pathway analysis indicated that the PLY phytoplasma genome contains two regions with highly conserved gene composition for carbohydrate metabolism. Intriguingly, each region contains several pseudogenes and the remaining functional genes in these two regions complement each other, suggesting a case of duplication followed by differential gene losses. Comparative analysis with other available phytoplasma genomes indicated that this PLY phytoplasma belongs to the 16SrI-B subgroup in the genus, with “Candidatus Phytoplasma asteris” that causes the onion yellowing (OY) disease in Japan as the closest known relative. For characterized effectors that these bacteria use to manipulate their plant hosts, the PLY phytoplasma has homologs for SAP11, SAP54/PHYL1, and TENGU. For genome structure comparison, we found that potential mobile unit (PMU) insertions may be the main factor that drives genome rearrangements in these bacteria. A total of 10 PMU-like regions were found in the PLY phytoplasma genome. Two of these PMUs were found to harbor one SAP11 homolog each, with one more similar to the 16SrI-B type and the other more similar to the 16SrI-A type, suggesting possible horizontal transfer. Taken together, this work provided a first look into population genomics of the PLY phytoplasmas in Taiwan, as well as identified several evolutionary processes that contributed to the genetic diversification of these plant-pathogenic bacteria.

Differentiations in Gene Content and Expression Response to Virulence Induction Between Two Agrobacterium Strains

Agrobacterium tumefaciens is important in biotechnology due to its ability to transform eukaryotic cells. Although the molecular mechanisms have been studied extensively, previous studies were focused on the model strain C58. Consequently, nearly all of the commonly used strains for biotechnology application were derived from C58 and share similar host ranges. To overcome this limitation, better understanding of the natural genetic variation could provide valuable insights. In this study, we conducted comparative analysis between C58 and 1D1609. These two strains belong to different genomospecies within the species complex and have distinct infectivity profiles. Genome comparisons revealed that each strain has >1,000 unique genes in addition to the 4,115 shared genes. Furthermore, the divergence in gene content and sequences vary among replicons. The circular chromosome is much more conserved compared to the linear chromosome. To identify the genes that may contribute to their differentiation in virulence, we compared the transcriptomes to screen for genes differentially expressed in response to the inducer acetosyringone. Based on the RNA-Seq results with three biological replicates, ∼100 differentially expressed genes were identified in each strain. Intriguingly, homologous genes with the same expression pattern account for <50% of these differentially expressed genes. This finding indicated that phenotypic variation may be partially explained by divergence in expression regulation. In summary, this study characterized the genomic and transcriptomic differences between two representative Agrobacterium strains. Moreover, the short list of differentially expressed genes are promising candidates for future characterization, which could improve our understanding of the genetic mechanisms for phenotypic divergence.

Multilocus Sequence Analysis Reveals Three Distinct Populations of "Candidatus Phytoplasma palmicola" with a Specific Geographical Distribution on the African Continent

Coconut is an important crop for both industry and small stakeholders in many intertropical countries. Phytoplasma-associated lethal yellowing-like diseases have become one of the major pests that limit coconut cultivation as they have emerged in different parts of the world. We developed a multilocus sequence typing scheme (MLST) for tracking epidemics of “Ca. Phytoplasma palmicola,” which is responsible for coconut lethal yellowing disease (CLYD) on the African continent. MLST analysis applied to diseased coconut samples collected in western and eastern African countries also showed the existence of three distinct populations of “Ca. Phytoplasma palmicola” with low intrapopulation diversity. The reasons for the observed strong geographic patterns remain to be established but could result from the lethality of CLYD and the dominance of short-distance insect-mediated transmission.

To sustain epidemiological studies on coconut lethal yellowing disease (CLYD), a devastating disease in Africa caused by a phytoplasma, we developed a multilocus sequence typing (MLST) scheme for “Candidatus Phytoplasma palmicola” based on eight housekeeping genes. At the continental level, eight different sequence types were identified among 132 “Candidatus Phytoplasma palmicola”-infected coconuts collected in Ghana, Nigeria, and Mozambique, where CLYD epidemics are still very active. “Candidatus Phytoplasma palmicola” appeared to be a bacterium that is subject to strong bottlenecks, reducing the fixation of positively selected beneficial mutations into the bacterial population. This phenomenon, as well as a limited plant host range, might explain the observed country-specific distribution of the eight haplotypes. As an alternative means to increase fitness, bacteria can also undergo genetic exchange; however, no evidence for such recombination events was found for “Candidatus Phytoplasma palmicola.” The implications for CLYD epidemiology and prophylactic control are discussed. The usefulness of seven housekeeping genes to investigate the genetic diversity in the genus “Candidatus Phytoplasma” is underlined.

IMPORTANCE Coconut is an important crop for both industry and small stakeholders in many intertropical countries. Phytoplasma-associated lethal yellowing-like diseases have become one of the major pests that limit coconut cultivation as they have emerged in different parts of the world. We developed a multilocus sequence typing scheme (MLST) for tracking epidemics of “Ca. Phytoplasma palmicola,” which is responsible for coconut lethal yellowing disease (CLYD) on the African continent. MLST analysis applied to diseased coconut samples collected in western and eastern African countries also showed the existence of three distinct populations of “Ca. Phytoplasma palmicola” with low intrapopulation diversity. The reasons for the observed strong geographic patterns remain to be established but could result from the lethality of CLYD and the dominance of short-distance insect-mediated transmission.

Assembly of Phytoplasma Genome Drafts from Illumina Reads Using Phytoassembly

Genome drafts for the phytoplasmas may be rapidly and efficiently assembled from NGS sequence data alone exploiting the proper bioinformatic tools and starting from properly collected samples. Here, we describe the use of the Phytoassembly pipeline ( https://github.com/cpolano/phytoassembly ), a fully automated tool that accepts as input row Illumina data from two samples (a phytoplasma infected sample and a healthy reference sample) to produce a phytoplasma genome draft, using the healthy plant host genome as a filter and profiting from the difference in reads coverage between the genome of the pathogen and that of the host. For phytoplasma infected samples containing >2% of pathogen DNA and an isogenic healthy reference sequence the resulting assemblies span the almost entire genomes.

Presence of an Agrobacterium-Type Tumor-Inducing Plasmid in Neorhizobium sp. NCHU2750 and the Link to Phytopathogenicity

The genus Agrobacterium contains a group of plant-pathogenic bacteria that have been developed into an important tool for genetic transformation of eukaryotes. To further improve this biotechnology application, a better understanding of the natural genetic variation is critical. During the process of isolation and characterization of wild-type strains, we found a novel strain (i.e., NCHU2750) that resembles Agrobacterium phenotypically but exhibits high sequence divergence in several marker genes. For more comprehensive characterization of this strain, we determined its complete genome sequence for comparative analysis and performed pathogenicity assays on plants. The results demonstrated that this strain is closely related to Neorhizobium in chromosomal organization, gene content, and molecular phylogeny. However, unlike the characterized species within Neorhizobium, which all form root nodules with legume hosts and are potentially nitrogen-fixing mutualists, NCHU2750 is a gall-forming pathogen capable of infecting plant hosts across multiple families. Intriguingly, this pathogenicity phenotype could be attributed to the presence of an Agrobacterium-type tumor-inducing plasmid in the genome of NCHU2750. These findings suggest that these different lineages within the family Rhizobiaceae are capable of transitioning between ecological niches by having novel combinations of replicons. In summary, this work expanded the genomic resources available within Rhizobiaceae and provided a strong foundation for future studies of this novel lineage. With an infectivity profile that is different from several representative Agrobacterium strains, this strain may be useful for comparative analysis to better investigate the genetic determinants of host range among these bacteria.

Comparative genome analysis of jujube witches'-broom Phytoplasma, an obligate pathogen that causes jujube witches'-broom disease

BACKGROUND

JWB phytoplasma is a kind of insect-transmitted and uncultivable bacterial plant pathogen causeing a destructive Jujube disease. To date, no genome information about JWB phytoplasma has been published, which hindered its characterization at genomic level. To understand its pathogenicity and ecology, the genome of a JWB phytoplasma isolate jwb-nky was sequenced and compared with other phytoplasmas enabled us to explore the mechanisms of genomic rearrangement.

RESULTS

The complete genome sequence of JWB phytoplasma (jwb-nky) was determined, which consisting of one circular chromosome of 750,803 bp with a GC content of 23.3%. 694 protein-encoding genes, 2 operons for rRNA genes and 31 tRNA genes as well as 4 potential mobile units (PMUs) containing clusters of DNA repeats were identified. Based on PHIbaes analysis, a large number of genes were genome-specific and approximately 13% of JWB phytoplasma genes were predicted to be associated with virulence. Although transporters for maltose, dipeptides/oligopeptides, spermidine/putrescine, cobalt, Mn/Zn and methionine were identified, KEGG pathway analysis revealed the reduced metabolic capabilities of JWB phytoplasma. Comparative genome analyses between JWB phytoplasma and other phytoplasmas shows the occurrence of large-scale gene rearrangements. The low synteny with other phytoplasmas indicated that the expansion of multiple gene families/duplication probably occurred separately after differentiation.

CONCLUSIONS

In this study, the complete genome sequence of a JWB phytoplasma isolate jwb-nky that causing JWB disease was reported for the first time and a number of species-specific genes were identified in the genome. The study enhanced our understandings about genomic basis and the pathogenicity mechanism of this pathogen, which will aid in the development of improved strategies for efficient management of JWB diseases.

An Effective Pipeline Based on Relative Coverage for the Genome Assembly of Phytoplasmas and Other Fastidious Prokaryotes

BACKGROUND

For the plant pathogenic phytoplasmas, as well as for several fastidious prokaryotes, axenic cultivation is extremely difficult or not possible yet; therefore, even with second generation sequencing methods, obtaining the sequence of their genomes is challenging due to host sequence contamination.

OBJECTIVE

With the Phytoassembly pipeline here presented, we aim to provide a method to obtain high quality genome drafts for the phytoplasmas and other uncultivable plant pathogens, by exploiting the coverage differential in the ILLUMINA sequences from the pathogen and the host, and using the sequencing of a healthy, isogenic plant as a filter.

VALIDATION

The pipeline has been benchmarked using simulated and real ILLUMINA runs from phytoplasmas whose genome is known, and it was then used to obtain high quality drafts for three new phytoplasma genomes.

CONCLUSION

For phytoplasma infected samples containing >2-4% of pathogen DNA and an isogenic reference healthy sample, the resulting assemblies can be next to complete. The Phytoassembly source code is available on GitHub at https://github.com/cpolano/phytoassembly.

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

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

Convergent Evolution among Ruminant-Pathogenic Mycoplasma Involved Extensive Gene Content Changes

Convergent evolution, a process by which organisms evolved independently to have similar traits, provides opportunities to understand adaptation. The bacterial genus Mycoplasma contains multiple species that evolved independently to become ruminant pathogens, which represents an interesting study system for investigating the process. In this work, we determined the genome sequences of 11 Entomoplasma/Mesoplasma species. This new data set, together with the other available Mollicutes genomes, provided comprehensive taxon sampling for inferring the gene content evolution that led to the emergence of Mycoplasma Mycoides cluster. Our results indicated that the most recent common ancestor (MRCA) of the Mycoides-Entomoplasmataceae clade lost ∼15% of the core genes when it diverged from the Spiroplasma Apis clade. After this initial wave of genome reduction, relatively few gene gains or losses were inferred until the emergence of the Mycoides cluster. Compared with those Entomoplasmataceae lineages that maintained the association with insects, the MRCA of the Mycoides cluster experienced a second wave of gene losses, as well as acquiring >100 novel genes through horizontal gene transfer. These gene acquisitions involved many with the Mycoplasma Hominis/Pneumoniae lineages as the putative donors, suggesting that gene exchanges among these vertebrate symbionts with distinct phylogenetic affiliations may be important in the emergence of the Mycoides cluster. These findings demonstrated that the gene content of bacterial genomes could be exceedingly dynamic, even for those symbionts with highly reduced genomes. Moreover, the emergence of novel pathogens may involve extensive remodeling of gene content, rather than acquisition of few virulence genes.

Complete Genome Sequence of Agrobacterium tumefaciens 1D1609

Agrobacterium tumefaciens 1D1609 is a highly virulent strain isolated from a crown gall tumor of alfalfa (Medicago sativa L.). Compared to other well-characterized A. tumefaciens strains, such as C58 and Ach5, 1D1609 has a distinctive host range. Here, we report its complete genome sequence to facilitate future studies.

Detection, Identification, and Molecular Characterization of the 16SrII-D Phytoplasmas Infecting Vegetable and Field Crops in Oman

Typical symptoms of phytoplasma infection were observed on 11 important crops in Oman that included alfalfa, sesame, chickpea, eggplant, tomato, spinach, rocket, carrot, squash, field pea, and faba bean. To identify the phytoplasmas in these crops, samples from infected and asymptomatic plants were collected, followed by amplifying and sequencing of the 16S ribosomal RNA, secA, tuf, imp, and SAP11 genes. We found that these sequences share >99% similarity with the peanut witches' broom subgroup (16SrII-D). Whereas some sequence variation was found in the five genes among 11 phytoplasma isolates of different crops, all sequences grouped into one clade along with those of other phytoplasmas belonging to the 16SrII-D group. Thus, 16SrII-D phytoplasmas infect a diverse range of crops in Oman. Phytoplasmas in this group have not been reported to occur in carrot, spinach, rocket, and field pea previously. Within Oman, this is the first report of the presence of 16SrII-D phytoplasmas in tomato, spinach, rocket, carrot, squash, field pea, and faba bean. Sequences of the five genes enabled for better distinction of the 16SrII-D phytoplasmas that occur in Oman.

Draft genome sequence of the New Jersey aster yellows strain of 'Candidatus Phytoplasma asteris'

The NJAY (New Jersey aster yellows) strain of ‘Candidatus Phytoplasma asteris’ is a significant plant pathogen responsible for causing severe lettuce yellows in the U.S. state of New Jersey. A draft genome sequence was prepared for this organism. A total of 177,847 reads were assembled into 75 contigs > 518 bp with a total base value of 652,092 and an overall [G+C] content of 27.1%. A total of 733 protein coding genes were identified. This Whole Genome Shotgun project has been deposited at DDBJ/ENA/GenBank under the accession MAPF00000000. This draft genome was used for genome- and gene-based comparative phylogenetic analyses with other phytoplasmas, including the closely related ‘Ca. Phytoplasma asteris’ strain, aster yellows witches’- broom (AY-WB). NJAY and AY-WB exhibit approximately 0.5% dissimilarity at the nucleotide level among their shared genomic segments. Evidence indicated that NJAY harbors four plasmids homologous to those known to encode pathogenicity determinants in AY-WB, as well as a chromosome-encoded mobile unit. Apparent NJAY orthologs to the important AY-WB virulence factors, SAP11 and SAP54, were identified. A number of secreted proteins, both membrane-bound and soluble, were encoded, with many bearing similarity to known AY-WB effector molecules and others representing possible secreted proteins that may be novel to the NJAY lineage.

Phytoplasma-conserved phyllogen proteins induce phyllody across the Plantae by degrading floral MADS domain proteins

ABCE-class MADS domain transcription factors (MTFs) are key regulators of floral organ development in angiosperms. Aberrant expression of these genes can result in abnormal floral traits such as phyllody. Phyllogen is a virulence factor conserved in phytoplasmas, plant pathogenic bacteria of the class Mollicutes. It triggers phyllody in Arabidopsis thaliana by inducing degradation of A- and E-class MTFs. However, it is still unknown whether phyllogen can induce phyllody in plants other than A. thaliana, although phytoplasma-associated phyllody symptoms are observed in a broad range of angiosperms. In this study, phyllogen was shown to cause phyllody phenotypes in several eudicot species belonging to three different families. Moreover, phyllogen can interact with MTFs of not only angiosperm species including eudicots and monocots but also gymnosperms and a fern, and induce their degradation. These results suggest that phyllogen induces phyllody in angiosperms and inhibits MTF function in diverse plant species.

Phytoplasmas-The "Crouching Tiger" Threat of Australian Plant Pathology

Phytoplasmas are insect-vectored bacteria that cause disease in a wide range of plant species. The increasing availability of molecular DNA analyses, expertise and additional methods in recent years has led to a proliferation of discoveries of phytoplasma-plant host associations and in the numbers of taxonomic groupings for phytoplasmas. The widespread use of common names based on the diseases with which they are associated, as well as separate phenetic and taxonomic systems for classifying phytoplasmas based on variation at the 16S rRNA-encoding gene, complicates interpretation of the literature. We explore this issue and related trends through a focus on Australian pathosystems, providing the first comprehensive compilation of information for this continent, covering the phytoplasmas, host plants, vectors and diseases. Of the 33 16Sr groups reported internationally, only groups I, II, III, X, XI and XII have been recorded in Australia and this highlights the need for ongoing biosecurity measures to prevent the introduction of additional pathogen groups. Many of the phytoplasmas reported in Australia have not been sufficiently well studied to assign them to 16Sr groups so it is likely that unrecognized groups and sub-groups are present. Wide host plant ranges are apparent among well studied phytoplasmas, with multiple crop and non-crop species infected by some. Disease management is further complicated by the fact that putative vectors have been identified for few phytoplasmas, especially in Australia. Despite rapid progress in recent years using molecular approaches, phytoplasmas remain the least well studied group of plant pathogens, making them a "crouching tiger" disease threat.

Identification of putative effector genes and their transcripts in three strains related to 'Candidatus Phytoplasma aurantifolia'

Molecular mechanisms underlying phytoplasma interactions with host plants are largely unknown. In this study attempts were made to identify effectors of three phytoplasma strains related to 'Ca. P. aurantifolia', crotalaria phyllody (CrP), faba bean phyllody (FBP), and witches' broom disease of lime (WBDL), using information from draft genome of peanut witches' broom phytoplasma. Seven putative effectors were identified in WBDL genome (SAP11, SAP21, Eff64, Eff115, Eff197, Eff211 and EffSAP67), five (SAP11, SAP21, Eff64, Eff99 and Eff197) in CrP and two (SAP11, Eff64) in FBP. No homologs to Eff64, Eff197 and Eff211 in phytoplasmas of other phylogenetic groups were found. SAP11 and Eff64 homologs of 'Ca. P. aurantifolia' strains shared at least 95.9% identity and were detected in the three phytoplasmas, supporting their role within the group. Five of the putative effectors (SAP11, SAP21, Eff64, Eff115, and Eff99) were transcribed from total RNA extracts of periwinkle plants infected with these phytoplasmas. Transcription profiles of selected putative effectors of CrP, FBP and WBDL indicated that SAP11 transcripts were the most abundant in the three phytoplasmas. SAP21 transcript levels were comparable to those of SAP11 for CrP and not measurable for the other phytoplasmas. Eff64 had the lowest transcription level irrespective of sampling date and phytoplasma isolate. Eff115 transcript levels were the highest in WBDL infected plants. This work reports the first sequence information for 14 putative effectors in three strains related to 'Ca. P. aurantifolia', and offers novel insight into the transcription profile of five of them during infection of periwinkle.

A few sequence polymorphisms among isolates of Maize bushy stunt phytoplasma associate with organ proliferation symptoms of infected maize plants

Background and Aims

Maize bushy stunt phytoplasma (MBSP) is a bacterial pathogen of maize ( Zea mays L.) across Latin America. MBSP belongs to the 16SrI-B sub-group within the genus ' Candidatus Phytoplasma'. MBSP and its insect vector Dalbulus maidis (Hemiptera: Cicadellidae) are restricted to maize; both are thought to have coevolved with maize during its domestication from a teosinte-like ancestor. MBSP-infected maize plants show a diversity of symptoms. and it is likely that MBSP is under strong selection for increased virulence and insect transmission on maize hybrids that are widely grown in Brazil. In this study it was investigated whether the differences in genome sequences of MBSP isolates from two maize-growing regions in South-east Brazil explain variations in symptom severity of the MBSP isolates on various maize genotypes.

Methods

MBSP isolates were collected from maize production fields in Guaíra and Piracicaba in South-east Brazil for infection assays. One representative isolate was chosen for de novo whole-genome assembly and for the alignment of sequence reads from the genomes of other phytoplasma isolates to detect polymorphisms. Statistical methods were applied to investigate the correlation between variations in disease symptoms of infected maize plants and MBSP sequence polymorphisms.

Key Results

MBSP isolates contributed consistently to organ proliferation symptoms and maize genotype to leaf necrosis, reddening and yellowing of infected maize plants. The symptom differences are associated with polymorphisms in a phase-variable lipoprotein, which is a candidate effector, and an ATP-dependent lipoprotein ABC export protein, whereas no polymorphisms were observed in other candidate effector genes. Lipoproteins and ABC export proteins activate host defence responses, regulate pathogen attachment to host cells and activate effector secretion systems in other pathogens.

Conclusions

Polymorphisms in two putative virulence genes among MBSP isolates from maize-growing regions in South-east Brazil are associated with variations in organ proliferation symptoms of MBSP-infected maize plants.