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

A metagenomic investigation of phytoplasma diversity in Australian vegetable growing regions

In this study, metagenomic sequence data was used to investigate the phytoplasma taxonomic diversity in vegetable-growing regions across Australia. Metagenomic sequencing was performed on 195 phytoplasma-positive samples, originating either from historic collections (n=46) or during collection efforts between January 2015 and June 2022 (n=149). The sampled hosts were classified as crop (n=155), weed (n=24), ornamental (n=7), native plant (n=6), and insect (n=3) species. Most samples came from Queensland (n=78), followed by Western Australia (n=46), the Northern Territory (n=32), New South Wales (n=17), and Victoria (n=10). Of the 195 draft phytoplasma genomes, 178 met our genome criteria for comparison using an average nucleotide identity approach. Ten distinct phytoplasma species were identified and could be classified within the 16SrII, 16SrXII (PCR only), 16SrXXV, and 16SrXXXVIII phytoplasma groups, which have all previously been recorded in Australia. The most commonly detected phytoplasma taxa in this study were species and subspecies classified within the 16SrII group (n=153), followed by strains within the 16SrXXXVIII group ('Ca. Phytoplasma stylosanthis'; n=6). Several geographic- and host-range expansions were reported, as well as mixed phytoplasma infections of 16SrII taxa and 'Ca. Phytoplasma stylosanthis'. Additionally, six previously unrecorded 16SrII taxa were identified, including five putative subspecies of 'Ca. Phytoplasma australasiaticum' and a new putative 16SrII species. PCR and sequencing of the 16S rRNA gene was a suitable triage tool for preliminary phytoplasma detection. Metagenomic sequencing, however, allowed for higher-resolution identification of the phytoplasmas, including mixed infections, than was afforded by only direct Sanger sequencing of the 16S rRNA gene. Since the metagenomic approach theoretically obtains sequences of all organisms in a sample, this approach was useful to confirm the host family, genus, and/or species. In addition to improving our understanding of the phytoplasma species that affect crop production in Australia, the study also significantly expands the genomic sequence data available in public sequence repositories to contribute to phytoplasma molecular epidemiology studies, revision of taxonomy, and improved diagnostics.

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.

Phytoplasma: A plant pathogen that cannot be ignored in agricultural production-Research progress and outlook

Phytoplasmas are phloem-restricted plant-pathogenic bacteria transmitted by insects. They cause diseases in a wide range of host plants, resulting in significant economic and ecological losses worldwide. Research on phytoplasmas has a long history, with significant progress being made in the past 30 years. Notably, with the rapid development of phytoplasma research, scientists have identified the primary agents involved in phytoplasma transmission, established classification and detection systems for phytoplasmas, and 243 genomes have been sequenced and assembled completely or to draft quality. Multiple possible phytoplasma effectors have been investigated, elucidating the molecular mechanisms by which phytoplasmas manipulate their hosts. This review summarizes recent advances in phytoplasma research, including identification techniques, host range studies, whole- or draft-genome sequencing, effector pathogenesis and disease control methods. Additionally, future research directions in the field of phytoplasma research are discussed.

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 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.

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 observation of taxonomic boundaries for the 16SrII and 16SrXXV phytoplasmas using genome-based delimitation

Within the 16SrII phytoplasma group, subgroups A-X have been classified based on restriction fragment length polymorphism of their 16S rRNA gene, and two species have been described, namely 'Candidatus Phytoplasma aurantifolia' and 'Ca. Phytoplasma australasia'. Strains of 16SrII phytoplasmas are detected across a broad geographic range within Africa, Asia, Australia, Europe and North and South America. Historically, all members of the 16SrII group share ≥97.5 % nucleotide sequence identity of their 16S rRNA gene. In this study, we used whole genome sequences to identify the species boundaries within the 16SrII group. Whole genome analyses were done using 42 phytoplasma strains classified into seven 16SrII subgroups, five 16SrII taxa without official 16Sr subgroup classifications, and one 16SrXXV-A phytoplasma strain used as an outgroup taxon. Based on phylogenomic analyses as well as whole genome average nucleotide and average amino acid identity (ANI and AAI), eight distinct 16SrII taxa equivalent to species were identified, six of which are novel descriptions. Strains within the same species had ANI and AAI values of >97 %, and shared ≥80 % of their genomic segments based on the ANI analysis. Species also had distinct biological and/or ecological features. A 16SrII subgroup often represented a distinct species, e.g., the 16SrII-B subgroup members. Members classified within the 16SrII-A, 16SrII-D, and 16SrII-V subgroups as well as strains classified as sweet potato little leaf phytoplasmas fulfilled criteria to be included as members of a single species, but with subspecies-level relationships with each other. The 16SrXXV-A taxon was also described as a novel phytoplasma species and, based on criteria used for other bacterial families, provided evidence that it could be classified as a distinct genus from the 16SrII phytoplasmas. As more phytoplasma genome sequences become available, the classification system of these bacteria can be further refined at the genus, species, and subspecies taxonomic ranks.

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.

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.

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.