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Pseudomonas syringae Strains Naturally Lacking the Classical P. syringae hrp/hrc Locus Are Common Leaf Colonizers Equipped with an Atypical Type III Secretion System

February 2010 , Volume 23 , Number  2
Pages  198 - 210

Christopher R. Clarke,1 Rongman Cai,1 David J. Studholme,2 David S. Guttman,3 and Boris A. Vinatzer1

1Department of Plant Pathology, Physiology, and Weed Science, Virginia Tech, Latham Hall, Ag Quad Lane, Blacksburg, 24061, U.S.A.; 2Sainsbury Laboratory, Norwich NR4 7UH, U.K.; 3Centre for the Analysis of Genome Evolution & Function, University of Toronto, Toronto, Ontario, M5S3B2 Canada

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Accepted 24 September 2009.

Pseudomonas syringae is best known as a plant pathogen that causes disease by translocating immune-suppressing effector proteins into plant cells through a type III secretion system (T3SS). However, P. syringae strains belonging to a newly described phylogenetic subgroup (group 2c) are missing the canonical P. syringae hrp/hrc cluster coding for a T3SS, flanking effector loci, and any close orthologue of known P. syringae effectors. Nonetheless, P. syringae group 2c strains are common leaf colonizers and grow on some tested plant species to population densities higher than those obtained by other P. syringae strains on nonhost species. Moreover, group 2c strains have genes necessary for the production of phytotoxins, have an ice nucleation gene, and, most interestingly, contain a novel hrp/hrc cluster, which is only distantly related to the canonical P. syringae hrp/hrc cluster. This hrp/hrc cluster appears to encode a functional T3SS although the genes hrpK and hrpS, present in the classical P. syringae hrp/hrc cluster, are missing. The genome sequence of a representative group 2c strain also revealed distant orthologues of the P. syringae effector genes avrE1 and hopM1 and the P. aeruginosa effector genes exoU and exoY. A putative life cycle for group 2c P. syringae is discussed.

© 2010 The American Phytopathological Society