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The Tat Pathway of the Plant Pathogen Pseudomonas syringae is Required for Optimal Virulence

February 2006 , Volume 19 , Number  2
Pages  200 - 212

Isabelle Caldelari , 1 Stefan Mann , 1 Casey Crooks , 2 and Tracy Palmer , 1 , 3

1Department of Molecular Microbiology, John Innes Centre, Colney lane, Norwich, NR4 7UH, U.K.; 2Department of Disease and Stress Biology, John Innes Centre, Colney lane, Norwich, NR4 7UH, U.K.; 3School of Biological Sciences, University of East Anglia, Norwich, NR4 7TJ, U.K.

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Accepted 25 October 2005.

Pseudomonas syringae is a gram-negative bacterium that infects a number of agriculturally important plant species. The ability of the organism to deliver virulence factors across the plant cell wall is a key to its pathogenicity. Deletion mutants in the twin arginine translocation (Tat) pathway of two pathovars of P. syringae, pvs. tomato DC3000 and maculicola ES4326, displayed a range of pleiotropic phenotypic changes, such as defects in fluorescent siderophore production, a decrease in sodium dodecyl sulfate and copper resistance, and a significant loss in fitness using Arabidopsis thaliana or tomato as plant hosts. The genome sequence of P. syringae pv. tomato DC3000 encodes a number of potential virulence factors that are predicted to be translocated via the Tat pathway, including several proteins involved in iron scavenging (two siderophore receptors, PSPTO3474 and PSPTO3294, and an aminotransferase, PSPTO2155, involved in siderophore biosynthesis). Further candidates for Tat-dependent pathogenicity determinants include the homologs of a cell wall amidase (PSPTO5528), an enzyme involved in periplasmic glucans biosynthesis (PSPTO5542), and two putative phospholipases (PSPTO3648 and PSPTOB0005). Translocation of the putative amidase, aminotransferase, glucans biosynthetic enzyme, and the two phospholipases, but not the two siderophore receptors, is shown to be dependent on the Tat pathway. Strains deleted for the genes encoding the probable aminotransferase and amidase enzymes are significantly less infectious than the wild type. We conclude that the incremental effects due to the failure to correctly localize at least two, and possibly more, Tat substrates gives rise to the attenuated fitness phenotype of the P. syringae pv. tomato DC3000 tat strain.

Additional keywords: iron acquisition, plant pathogen, twin-arginine signal peptide.

© 2006 The American Phytopathological Society