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Applied and Environmental Microbiology, May 2008, p. 3171-3181, Vol. 74, No. 10
0099-2240/08/$08.00+0     doi:10.1128/AEM.00180-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

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

Department of Plant Pathology, Physiology, and Weed Science, Virginia Polytechnic Institute and State University, Latham Hall, Ag Quad Lane, Blacksburg, Virginia 24061,¹ CRA-Centro di Ricerca per l'Orticoltura Sede di Battipaglia, I-84091 Battipaglia (SA), Italy,² Virginia Bioinformatics Institute, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061³

Received 20 January 2008/ Accepted 19 March 2008

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.

Published ahead of print on 31 March 2008.

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