Definition of Plant-Pathogenic Pseudomonas Genomospecies of the Pseudomonas syringae Complex Through Multiple Comparative Approaches

Abstract

A total of 34 phytopathogenic strain genomes belonging to the Pseudomonas syringae species complex and related species, including many pathotype strains, were assessed using average nucleotide identity (ANI) analysis. Their taxonomic relationships were consistently confirmed by the tetranucleotide frequency correlation coefficient (TETRA) values, multilocus sequence typing analysis (MLSA) performed with seven housekeeping genes, using both maximum likelihood and Bayesian methods, and split consensus network analyses. The ANI, MLSA, and split consensus analyses provided consistent and identical results. We confirmed the occurrence of the well-demarcated genomospecies inferred sensu Gardan et al. using DNA-DNA hybridization and ribotyping analyses. However, some P. syringae strains of the pathovars morsprunorum and lachrymans were placed in different genomospecies in our analyses. Genomospecies 1, 2, 4, 6, and 9 resulted well demarcated, whereas strains of genomospecies 3 and 8 had ANI values between 95 and 96% in some cases, confirming that this threshold reveals very closely related species that might represent cases of splitting entities or the convergence of different species to the same ecological niche. This study confirms the robustness of the combination of genomic and phylogenetic approaches in revealing taxonomic relationships among closely related bacterial strains and provides the basis for a further reliable demarcation of the phytopathogenic Pseudomonas species. Within each species, the pathovars might represent distinct ecological units. The possibility of performing extensive and standardized host range and phenotypic tests with many strains of different pathovars can assist phytobacteriologists for better determining the boundaries of these ecological units.

Keywords: genome comparison; polyphasic taxonomy.