New hypotheses about the origin of Pseudomonas syringae crop pathogens

Pseudomonas syringae is a common foliar plant pathogenic bacterium that causes diseases on many crop plants. We hypothesized that today's highly virulent P. syringae crop pathogens with narrow host range might have evolved after the advent of agriculture from ancestral P. syringae strains with wide host range that were adapted to mixed plant communities. The model tomato and Arabidopsis pathogen P. syringae pv. tomato (Pto) DC3000 and its close relatives isolated from crop plants were thus selected to unravel basic principles of host range evolution by applying molecular evolutionary analysis and comparative genomics approaches. Phylogenetic analysis was combined with host range tests to reconstruct the host range of the most recent common ancestor of all analyzed strains isolated from crop plants. Even though reconstruction of host range of the most recent common ancestor of all analyzed strains was not conclusive, support for this hypothesis was found in some sub-groups of strains. The focus of my studies then turned to Pto T1, which was found to represent the most common P. syringae lineage causing bacterial speck disease on tomato world-wide. Five genomes were sequenced and compared to each other. Identical genotypes were found in North America and Europe suggesting frequent pathogen movement between these continents. Moreover, the type III-secreted effector gene hopM1 was found to be under strong selection for loss of function and non-synonymous mutations in the fliC gene allowed to identify a region that triggers plant immunity. Finally, Pto T1 was compared to closely related bacteria isolated from snow pack and surface water in the French Alps. Recombination between alpine strains and crop strains was inferred and virulence gene repertoires of alpine strains and crop strains were found to overlap. Alpine strains cause disease on tomato and have relatively wider host ranges than Pto T1. The conclusion from these studies is that Pto T1 and other crop pathogens may have evolved from ancestors similar to the characterized environmental strains isolated in the French Alps by adapting their effector repertoire to individual crops becoming more virulent on these crops but losing virulence on other plants.

Name thesis:degree_name

Ph. D.

Level thesis:degree_level

doctoral

Discipline thesis:degree_discipline

Plant Pathology, Physiology, and Weed Science

Department dc:contributor.department

Plant Pathology, Physiology, and Weed Science

Grantor dc:publisher

Virginia Tech

Year dc:date.issued

2012

Author dc:creator

• Cai, Rongman

Chair dc:contributor.committeechair

• Vinatzer, Boris A.

Committee members dc:contributor.committeemember

• Kim, Inyoung
• Jelesko, John G.
• Leman, Scotland C.

• ancestral state reconstruction
• fliC
• host range evolution
• Pseudomonas syringae
• molecular evolution
• HopM1
• recombination
• phylogeography
• microevolution
• most recent common ancestor
• phylogenetic tree
• population genetics

Statement dc:rights

• In Copyright

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Dc Identifier Other

etd-05132012-234836

OAI identifier oai:identifier

oai:vtechworks.lib.vt.edu:10919/37806