Role of Recombination in the Evolution of the Model Plant Pathogen Pseudomonas syringae pv. tomato DC3000, a Very Atypical Tomato Strain

Abstract

Pseudomonas syringae pv. tomato strain DC3000 (PtoDC3000) is one of the most intensively studied bacterial plant pathogens today. Here we report a thorough investigation into PtoDC3000 and close relatives isolated from Antirrhinum majus (snapdragon), Apium graveolens (celery), and Solanaceae and Brassicaceae species. Multilocus sequence typing (MLST) was used to resolve the precise phylogenetic relationship between isolates and to determine the importance of recombination in their evolution. MLST data were correlated with an analysis of the locus coding for the type III secreted (T3S) effector AvrPto1 to investigate the role of recombination in the evolution of effector repertoires. Host range tests were performed to determine if closely related isolates from different plants have different host ranges. It was found that PtoDC3000 is located in the same phylogenetic cluster as isolates from several Brassicaceae and Solanaceae species and that these isolates have a relatively wide host range that includes tomato, Arabidopsis thaliana, and cauliflower. All other analyzed tomato isolates from three different continents form a distinct cluster and are pathogenic only on tomato. Therefore, PtoDC3000 is a very unusual tomato isolate. Several recombination breakpoints were detected within sequenced gene fragments, and population genetic tests indicate that recombination contributed more than mutation to the variation between isolates. Moreover, recombination may play an important role in the reassortment of T3S effectors between strains. The data are finally discussed from a taxonomic standpoint, and P. syringae pv. tomato is proposed to be divided into two pathovars.Pseudomonas syringae pv. tomato DC3000 (PtoDC3000) is one of the most intensively studied plant pathogen isolates today. It was completely sequenced (6), and a large part of what is known about the plant immune system has been learned by studying the interaction of PtoDC3000 with its hosts Arabidopsis thaliana and tomato (Solanum lycopersicum), as can be seen from many recent high-profile publications (see references 39 and 47 for examples). However, much less is known about how PtoDC3000 relates to other P. syringae strains. Although PtoDC3000 is a rifampin-resistant derivative of the type strain of P. syringae pv. tomato (9; D. Cuppels, personal communication), its host range (which includes tomato, cauliflower [Brassica oleracea var. botrytis], and A. thaliana) was found to be more similar to that of pathovar maculicola isolates from Brassicaceae species than to the host range of other P. syringae pv. tomato strains (which are limited to tomato) (10, 58). Also, based on physiological (10) and molecular analyses (10, 63), PtoDC3000 was suggested to be more similar to pathovar maculicola strains than to other pathovar tomato strains. However, since strains of pathovars tomato, maculicola, antirrhini (isolated from ornamental snapdragon, Antirrhinum majus), and apii (isolated from celery, Apium graveolens) were all found to be closely related (18, 25), the...

Phytobacterial Type VI Secretion System - Gene Distribution, Phylogeny, Structure and Biological Functions

“…The P. syringe T6SS-III is present only in P. syringae pv. tomato strain DC3000, and has the same gene order and high protein homology with the T6SS-II of the P. aeruginosa PA14 HS-II (data not shown), which reinforces the view of Yan and colleagues (Yan et al, 2008) that the model plant pathogen P. s. pv. tomato DC3000 is a very atypical tomato strain.…”

Contrasting recombination patterns and demographic histories of the plant pathogen Ralstonia solanacearum inferred from MLSA

“…The r/m values inferred on the whole R. solanacearum species complex (1.6 with 95% confidence interval 1.04-2.32) was higher than that obtained by Castillo and Greenberg (2007) (1.1 with 95% confidence interval 0.7-1.6), but within the range reported so far on plant pathogenic bacteria (P. syringae, P. viridiflava, X. campestris pv. campestris) (Vos and Didelot, 2008) using MLSA data (Silva et al, 2005;Yan et al, 2008;Doroghazi and Buckley, 2010;Fargier et al, 2011).…”

“…Among the recombination processes, the evolutionary importance of horizontal gene transfer has been highlighted by the tremendously increasing number of whole-genome sequences available (Martin and Beiko, 2010), and has mainly been studied on animal or human pathogenic bacteria (Achtman, 2008;Didelot and Maiden, 2010). Although some recent studies address this question on Pseudomonas syringae (Yan et al, 2008) or Xanthomonas campestris (Fargier et al, 2011), data are far less numerous on plant-associated bacteria, and more specifically plant pathogenic bacteria (Vos and Didelot, 2008).…”

The Mangotoxin Biosynthetic Operon ( mbo ) Is Specifically Distributed within Pseudomonas syringae Genomospecies 1 and Was Acquired Only Once during Evolution

“…4), which results in some uncertainty over the number of times that the mbo operon has been acquired by the strains of genomospecies 1. This type of discrepancy is commonly found in phylogenetic analyses of P. syringae when using different housekeeping genes or even different genes from an operon from the hrp cluster, particularly for very closely related taxa, and has been interpreted to be a consequence of genetic exchange and recombination (21,59,(61)(62)(63). Examination of the mbo operon insertion site offers further significant clues about its evolutionary history, in that the insertion site was the same for all the strains and in all cases was associated with the loss of a continuous 67-bp fragment that is ancestral to P. syringae.…”