Pseudomonas savastanoi pv. mandevillae pv. nov., a Clonal Pathogen Causing an Emerging, Devastating Disease of the Ornamental Plant Mandevilla spp

Naïve Bayes Classifiers and accompanying dataset for Pseudomonas syringae isolate characterization

The Pseudomonas syringae species complex (PSSC) is a diverse group of plant pathogens with a collective host range encompassing almost every food crop grown today. As a threat to global food security, rapid detection and characterization of epidemic and emerging pathogenic lineages is essential. However, phylogenetic identification is often complicated by an unclarified and ever-changing taxonomy, making practical use of available databases and the proper training of classifiers difficult. As such, while amplicon sequencing is a common method for routine identification of PSSC isolates, there is no efficient method for accurate classification based on this data. Here we present a suite of five Naïve bayes classifiers for PCR primer sets widely used for PSSC identification, trained on in-silico amplicon data from 2,161 published PSSC genomes using the life identification number (LIN) hierarchical clustering algorithm in place of traditional Linnaean taxonomy. Additionally, we include a dataset for translating classification results back into traditional taxonomic nomenclature (i.e. species, phylogroup, pathovar), and for predicting virulence factor repertoires.

Allelic variation in the indoleacetic acid-lysine synthase gene of the bacterial pathogen Pseudomonas savastanoi and its role in auxin production

Indole-3-acetic acid (IAA) production is a pathogenicity/virulence factor in the Pseudomonas syringae complex, including Pseudomonas savastanoi. P. savastanoi pathovars (pvs.) genomes contain the iaaL gene, encoding an enzyme that catalyzes the biosynthesis of the less biologically active compound 3-indole-acetyl-ϵ-L-lysine (IAA-Lys). Previous studies have reported the identification of IAA-Lys in culture filtrates of P. savastanoi strains isolated from oleander (pv. nerii), but the conversion of IAA into a conjugate was not detectable in olive strains (pv. savastanoi). In this paper, we show the distribution of iaaL alleles in all available P. savastanoi genomes of strains isolated from woody hosts. Most strains encode two different paralogs, except for those isolated from broom (pv. retacarpa), which contain a single allele. In addition to the three previously reported iaaL alleles (iaaL _Psv, iaaL _Psn and iaaL _Pto), we identified iaaL _Psf, an exclusive allele of strains isolated from ash (pv. fraxini). We also found that the production of IAA-Lys in P. savastanoi pv. savastanoi and pv. nerii depends on a functional iaaL _Psn allele, whereas in pv. fraxini depends on iaaL _Psf. The production of IAA-Lys was detected in cultures of an olive strain heterologously expressing IaaL_Psn-1, IaaL_Psf-1 and IaaL_Psf-3, but not when expressing IaaL_Psv-1. In addition, Arabidopsis seedlings treated with the strains overproducing the conjugate, and thus reducing the free IAA content, alleviated the root elongation inhibitory effect of IAA. IAA-Lys synthase activity assays with purified allozymes confirmed the functionality and specificity of lysine as a substrate of IaaL_Psn-1 and IaaL_Psf-3, with IaaL_Psf-3 showing the highest catalytic efficiency for both substrates. The IAA-Lys synthase activity of IaaL_Psn-1 was abolished by the insertion of two additional tyrosine residues encoded in the inactive allozyme IaaL_Psv-1. These results highlight the relevance of allelic variation in a phytohormone-related gene for the modulation of auxin production in a bacterial phytopathogen.

Molecular and Genomic Characterization of the Pseudomonas syringae Phylogroup 4: An Emerging Pathogen of Arabidopsis thaliana and Nicotiana benthamiana

Environmental fluctuations such as increased temperature, water availability, and air CO2 concentration triggered by climate change influence plant disease dynamics by affecting hosts, pathogens, and their interactions. Here, we describe a newly discovered Pseudomonas syringae strain found in a natural population of Arabidopsis thaliana collected from the southwest of France. This strain, called Psy RAYR-BL, is highly virulent on natural Arabidopsis accessions, Arabidopsis model accession Columbia 0, and tobacco plants. Despite the severe disease phenotype caused by the Psy RAYR-BL strain, we identified a reduced repertoire of putative Type III virulence effectors by genomic sequencing compared to P. syringae pv tomato (Pst) DC3000. Furthermore, hopBJ1Psy is found exclusively on the Psy RAYR-BL genome but not in the Pst DC3000 genome. The plant expression of HopBJ1Psy induces ROS accumulation and cell death. In addition, HopBJ1Psy participates as a virulence factor in this plant-pathogen interaction, likely explaining the severity of the disease symptoms. This research describes the characterization of a newly discovered plant pathogen strain and possible virulence mechanisms underlying the infection process shaped by natural and changing environmental conditions.

Complete genome sequence of the kiwifruit bacterial canker pathogen Pseudomonas savastanoi strain MHT1

BACKGROUND

Pseudomonas savastanoi is an important plant pathogen that infects and causes symptoms in a variety of economically important crops, causing considerable loss of yield and quality. Because there has been no research reported to date on bacterial canker of kiwifruit (Actinidia chinensis) plants caused by P. savastanoi and, in particular, no in-depth studies of the complete genome sequence or pathogenic mechanism, long-lasting and environmentally friendly control measures against this pathogen in kiwifruit are lacking. This study therefore has both theoretical value and practical significance.

RESULTS

We report the complete genome sequence of P. savastanoi strain MHT1, which was first reported as the pathogen causing bacterial canker in kiwifruit plants. The genome consists of a 6.00-Mb chromosome with 58.5% GC content and 5008 predicted genes. Comparative genome analysis of four sequenced genomes of representative P. savastanoi strains revealed that 230 genes are unique to the MHT1 strain and that these genes are enriched in antibiotic metabolic processes and metabolic pathways, which may be associated with the drug resistance and host range observed in this strain. MHT1 showed high syntenic relationships with different P. savastanoi strains. Furthermore, MHT1 has eight conserved effectors that are highly homologous to effectors from P. syringae, Pseudomonas amygdali, and Ralstonia solanacearum strains. The MHT1 genome contains six genomic islands and two prophage sequences. In addition, 380 genes were annotated as antibiotic resistance genes and another 734 as encoding carbohydrate-active enzymes.

CONCLUSION

The whole-genome sequence of this kiwifruit bacterial canker pathogen extends our knowledge of the P. savastanoi genome, sets the stage for further studies of the interaction between kiwifruit and P. savastanoi, and provides an important theoretical foundation for the prevention and control of bacterial canker.

HrpL Regulon of Bacterial Pathogen of Woody Host Pseudomonas savastanoi pv. savastanoi NCPPB 3335

The Pseudomonas savastanoi species comprises a group of phytopathogenic bacteria that cause symptoms of disease in woody hosts. This is mediated by the rapid activation of a pool of virulence factors that suppress host defences and hijack the host’s metabolism to the pathogen’s benefit. The hrpL gene encodes an essential transcriptional regulator of virulence functions, including the type III secretion system (T3SS), in pathogenic bacteria. Here, we analyzed the contribution of HrpL to the virulence of four pathovars (pv.) of P. savastanoi isolated from different woody hosts (oleander, ash, broom, and dipladenia) and characterized the HrpL regulon of P. savastanoi pv. savastanoi NCPPB 3335 using two approaches: whole transcriptome sequencing (RNA-seq) and the bioinformatic prediction of candidate genes containing an hrp-box. Pathogenicity tests carried out for the P. savastanoi pvs. showed that HrpL was essential for symptom development in both non-host and host plants. The RNA-seq analysis of the HrpL regulon in P. savastanoi revealed a total of 53 deregulated genes, 49 of which were downregulated in the ΔhrpL mutant. Bioinformatic prediction resulted in the identification of 50 putative genes containing an hrp-box, 16 of which were shared with genes previously identified by RNA-seq. Although most of the genes regulated by HrpL belonged to the T3SS, we also identified some genes regulated by HrpL that could encode potential virulence factors in P. savastanoi.