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

Complete genome sequence of "Candidatus Phytoplasma sacchari" obtained using a filter-based DNA enrichment method and Nanopore sequencing

Phytoplasmas are phloem-limited plant pathogens, such as sugarcane white leaf (SCWL) phytoplasma, which are responsible for heavy economic losses to the sugarcane industry. Characterization of phytoplasmas has been limited because they cannot be cultured in vitro. However, with the advent of genome sequencing, different aspects of phytoplasmas are being investigated. In this study, we developed a DNA enrichment method for sugarcane white leaf (SCWL) phytoplasma, evaluated the effect of DNA enrichment via Illumina sequencing technologies, and utilized Illumina and Nanopore sequencing technologies to obtain the complete genome sequence of the "Candidatus Phytoplasma sacchari" isolate SCWL1 that is associated with sugarcane white leaf in China. Illumina sequencing analysis elucidated that only 1.21% of the sequencing reads from total leaf DNA were mapped to the SCWL1 genome, whereas 40.97% of the sequencing reads from the enriched DNA were mapped to the SCWL1 genome. The genome of isolate SCWL1 consists of a 538,951 bp and 2976 bp long circular chromosome and plasmid, respectively. We identified 459 protein-encoding genes, 2 complete 5S-23S-16S rRNA gene operons, 27 tRNA genes, and an incomplete potential mobile unit (PMU) in the circular chromosome. Phylogenetic analyses and average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH) values based on the sequenced genome revealed that SCWL phytoplasma and sugarcane grassy shoot (SCGS) phytoplasma belonged to the same phytoplasma species. This study provides a genomic DNA enrichment method for phytoplasma sequencing. Moreover, we report the first complete genome of a "Ca. Phytoplasma sacchari" isolate, thus contributing to future studies on the evolutionary relationships and pathogenic mechanisms of "Ca. Phytoplasma sacchari" isolates.

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.

From sequences to species: Charting the phytoplasma classification and taxonomy in the era of taxogenomics

Phytoplasma taxonomy has been a topic of discussion for the last two and half decades. Since the Japanese scientists discovered the phytoplasma bodies in 1967, the phytoplasma taxonomy was limited to disease symptomology for a long time. The advances in DNA-based markers and sequencing improved phytoplasma classification. In 2004, the International Research Programme on Comparative Mycoplasmology (IRPCM)- Phytoplasma/Spiroplasma Working Team – Phytoplasma taxonomy group provided the description of the provisional genus ‘Candidatus Phytoplasma’ with guidelines to describe the new provisional phytoplasma species. The unintentional consequences of these guidelines led to the description of many phytoplasma species where species characterization was restricted to a partial sequence of the 16S rRNA gene alone. Additionally, the lack of a complete set of housekeeping gene sequences or genome sequences, as well as the heterogeneity among closely related phytoplasmas limited the development of a comprehensive Multi-Locus Sequence Typing (MLST) system. To address these issues, researchers tried deducing the definition of phytoplasma species using phytoplasmas genome sequences and the average nucleotide identity (ANI). In another attempts, a new phytoplasma species were described based on the Overall Genome relatedness Values (OGRI) values fetched from the genome sequences. These studies align with the attempts to standardize the classification and nomenclature of ‘Candidatus’ bacteria. With a brief historical account of phytoplasma taxonomy and recent developments, this review highlights the current issues and provides recommendations for a comprehensive system for phytoplasma taxonomy until phytoplasma retains ‘Candidatus’ status.

The function of the phytoplasma effector SWP12 depends on the properties of two key amino acids

Phytoplasmas are insect-borne bacterial pathogens capable of secreting effectors into host cells and interfering with host plant defense response processes. Previous studies have found that the Candidatus Phytoplasma tritici effector SWP12 binds to and destabilizes the wheat transcription factor TaWRKY74, increasing wheat susceptibility to phytoplasmas. Here, we used a Nicotiana benthamiana transient expression system to identify two key functional sites of SWP12 and screened a series of truncated mutants and amino acid substitution mutants to determine whether they inhibit Bax-induced cell death. Using a subcellular localization assay and online structure analysis websites, we found that structure rather than intracellular localization probably affects the function of SWP12. D33A and P85H are two inactive substitution mutants, neither of which interacts with TaWRKY74, and P85H does not inhibit Bax-induced cell death, suppress flg22-triggered reactive oxygen species (ROS) bursts, degrade TaWRKY74, or promote phytoplasma accumulation. D33A can weakly suppress Bax-induced cell death and flg22-triggered ROS bursts and degrade a portion of TaWRKY74 and weakly promote phytoplasma accumulation. S53L, CPP, and EPWB are three SWP12 homolog proteins from other phytoplasmas. Sequence analysis revealed that D33 was conserved in these proteins, and they exhibited the same polarity at P85. Transient expression in N. benthamiana showed that these proteins could inhibit Bax-induced cell death and suppress ROS bursts. Our findings clarified that P85 and D33 of SWP12 play critical and minor roles, respectively, in suppressing the plant defense response and that they play a preliminary role in determining the functions of homologous proteins.

Comparative analysis reveals distinctive genomic features of Taiwan hot-spring cyanobacterium Thermosynechococcus sp. TA-1

Thermosynechococcus is a genus of thermophilic unicellular cyanobacteria that dominates microbial mats in Asian non-acidic hot springs. These cyanobacteria are the major primary producers in their ecological niches and are promising sources of thermostable enzymes for biotechnology applications. To improve our understanding of these organisms, we conducted whole-genome sequencing of a novel strain for comparative analysis with other representatives in the same genus. This newly characterized strain, Thermosynechococcus sp. TA-1, was isolated from the Taian hot springs in Taiwan. Analyses based on average nucleotide identity (ANI) and genome-scale phylogeny suggested that TA-1 and another Taiwanese strain CL-1 belong to a novel species-level taxon. Two metagenome-assembled genomes (MAGs) originated from India represent the sister group, and Thermosynechococcus elongatus PKUAC-SCTE542 from China is the next closest lineage. All cultivated strains and MAGs from Japan form a separate monophyletic clade and could be classified into two species-level taxa. Intriguingly, although TA-1 and CL-1 share 97.0% ANI, the genome alignment identified at least 16 synteny breakpoints that are mostly associated with transposase genes, which illustrates the dynamic nature of their chromosomal evolution. Gene content comparisons identified multiple features distinct at species- or strain-level among these Thermosynechococcus representatives. Examples include genes involved in bicarbonate transportation, nitric oxide protection, urea utilization, kanamycin resistance, restriction-modification system, and chemotaxis. Moreover, we observed the insertion of type II inteins in multiple genes of the two Taiwanese strains and inferred putative horizontal transfer of an asparagine synthase gene (asnB) associated with exopolysaccharides gene cluster. Taken together, while previous work suggested that strains in this genus share a highly conserved genomic core and no clear genetic differentiation could be linked to environmental factors, we found that the overall pattern of gene content divergence is largely congruent with core genome phylogeny. However, it is difficult to distinguish between the roles of phylogenetic relatedness and geographic proximity in shaping the genetic differentiation. In conclusion, knowledge of the genomic differentiation among these strains provides valuable resources for future functional characterization.

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.

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.

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.

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.