Publications

4500

ABSTRACTIn an era of trade globalization and climate change, crop pathogens and pests are a genuine threat to food security. The detailed characterization of emerging pathogen populations is a prerequisite for managing invasive species pathways and designing sustainable disease control strategies. Huanglongbing is the disease that causes the most damage to citrus, a crop that ranks #1 worldwide in terms of fruit production. Huanglongbing can be caused by three species of the phloem‐limited alpha‐proteobacterium, “Candidatus Liberibacter,” which are transmitted by psyllids. Two of these bacteria are of highest concern, “Ca. Liberibacter asiaticus” and “Ca. Liberibacter africanus,” and have distinct thermal optima. These pathogens are unculturable, which complicates their high‐throughput genetic characterization. In the present study, we used several genotyping techniques and an extensive sample collection to characterize Ca. Liberibacter populations associated with the emergence of huanglongbing in three French outermost regions of the European Union (Guadeloupe, Martinique and Réunion). The outbreaks were primarily caused by “Ca. Liberibacter asiaticus,” as “Ca. Liberibacter africanus” was only found at a single location in Réunion. We emphasize the low diversity and high genetic relatedness between samples from Guadeloupe and Martinique, which suggests the putative movement of the pathogen between the two islands and/or the independent introduction of closely related strains. These samples were markedly different from the samples from Réunion, where the higher genetic diversity revealed by tandem‐repeat markers suggests that the disease was probably overlooked for years before being officially identified in 2015. We show that “Ca. Liberibacter asiaticus” occurs from sea level to an altitude of 950 m above sea level and lacks spatial structure. This suggests the pathogen's medium‐ to long‐distance movement. We also suggest that backyard trees acted as relays for disease spread. We discuss the implications of population biology data for surveillance and management of this threatful disease.

‘Candidatus Phytoplasma ulmi’ (16SrV-A) is the causal agent of elm yellows (EY), a lethal and/or decline disease of several Ulmus (elm) species and hybrids in North America and Europe. In this study, field observations and PCR assays were used to detect phytoplasma infections in diseased U. minor, U. pumila and U. glabra trees in southern Italy. Also, a multigene sequence analysis employing various less conserved genes was carried out to explore the genetic variation in detected strains. All the symptomatic elm trees tested were infected with ‘Ca. Phytoplasma ulmi’. No other phytoplasmas or variants could be detected. Although ‘Ca. Phytoplasma ulmi’ was already known to occur in southern Italy on European field elm, the current work expands the information on the presence, disease incidence and severity, plant host range and molecular aspects of EY phytoplasma strains occurring in southern Italy. In addition, this is the first report from Italy on the molecular characterization of EY phytoplasma strains through map, imp and groEL gene sequence and phylogenetic analyses. Among the newly detected EY phytoplasma strains, some proved distantly related to each other and to other previously characterized EY phytoplasma strains within the genes examined. This implies the presence of distinct taxonomic entities within the material examined. The occurrence of different strains was not linked to the biological traits and geographical distribution. However, the data obtained may provide a basis for further studies aimed at elucidating several other unknown aspects of the EY agent, knowledge of which is essential for effective disease management and control strategies. The results of the current work also show that the EY phytoplasma is particularly widespread in southern Italy and is of considerable economic and ecological relevance.

Dehalobacter is a genus of organohalide-respiring bacteria that is recognized for its fastidious growth using reductive dehalogenases (RDases). In the SC05 culture, however, a Dehalobacter population also mineralizes dichloromethane (DCM) produced by chloroform dechlorination using the mec cassette, just downstream of its active RDase. A closed genome of this DCM-mineralizing lineage has previously evaded assembly. Here, we present the genomes of two novel Dehalobacter strains, each of which was assembled from the metagenome of a distinct subculture from SC05. A pangenomic analysis of the Dehalobacter genus, including RDase synteny and phylogenomics, reveals at least five species of Dehalobacter based on average nucleotide identity, RDase and core gene synteny, as well as differential functional genes. An integration hotspot is also pinpointed in the Dehalobacter genome, in which many recombinase islands have accumulated. This nested recombinase island encodes the active RDase and mec cassette in both SC05 Dehalobacter genomes, indicating the transfer of key functional genes between species of Dehalobacter. Horizontal gene transfer between these two novel Dehalobacter strains has implications for the evolutionary history within the SC05 subcultures and of the Dehalobacter genus as a whole, especially regarding adaptation to anthropogenic chemicals.

The order Caryophanales, belonging to class Bacilli, is globally distributed in various ecosystems. Currently, this order comprised 12 families that show vast phenotypic, ecological and genotypic variation. The classification of Caryophanales at the family level is currently mainly based on 16S rRNA gene sequencing analysis and the presence of shared phenotypic characteristics, resulting in noticeable anomalies. Our present study revises the taxonomy of Caryophanales based on 1080 available high-quality genome sequences of type strains. The evaluated parameters included the core-genome phylogeny, pairwise average aa identity, lineage-specific core genes, physiological criteria and ecological parameters. Based on the results of this polyphasic approach, we propose that the order Caryophanales be reclassified into 41 families, which include the existing 12 families, 17 families in a recent Validation List in the IJSEM (Validation List no. 215) and 12 novel families for which we propose the names Aureibacillaceae, Cytobacillaceae, Domibacillaceae, Falsibacillaceae, Heyndrickxiaceae, Lottiidibacillaceae, Oxalophagaceae, Pradoshiaceae, Rossellomoreaceae, Schinkiaceae, Sulfoacidibacillaceae and Sutcliffiellaceae. This work represents a genomic sequence-based and systematic framework for classifying the order Caryophanales at the family level, providing new insights into its evolution.

AbstractThe poinsettia (Euphorbia pulcherrimaWilld. ex Klotzsch.) is one of Turkey’s most important potted ornamental crops. Potted poinsettia plants exhibiting smaller-sized brilliant red bracts against the green leaves with free-branching were collected from commercial florists in three provinces of Turkey. In iPhyClassifier analysis, the query 16SrDNA sequences of our isolates shared 99.58% identity with that of theCandidatusPhytoplasma pruni. Our leaf samples consistently yielded a product of 0.7 kb by PCR with primers (KFD4F/KFD4R), which was newly designed based on X-Disease group (16SrIII) of phytoplasma secY sequences available in GenBank. The partial 16S rRNA sequences were used for phylogenetic analyses covering 41 taxa from the group of 16SrIII’Candidatus Phytoplasma’ strains, including 14 strains causing disease on poinsettia worldwide. The sequences that caused the same disease in poinsettia plants in Finland, the USA, Mexico, Taiwan, Japan, Korea, and Canada, including three strains from Turkey, formed a mono-phylogenetic group with a support value of 87%.