Role of the Hrp type III protein secretion system in growth of Pseudomonas syringae pv. syringae B728a on host plants in the field

hrp genes are reportedly required for pathogenicity in Pseudomonas syringae pv. syringae (Pss) and other phytopathogenic bacterial species. A subset of these genes encodes a type III secretion system through which virulence factors are thought to be delivered to plant cells. In this study, we sought...

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Bibliographic Details
Published in:Proceedings of the National Academy of Sciences - PNAS 1999-08, Vol.96 (17), p.9851-9856
Main Authors: Hirano, S.S, Charkowski, A.O, Collmer, A, Willis, D.K, Upper, C.D
Format: Article
Language:English
Subjects:
Bacteria
bacterial proteins
Bacterial Proteins - genetics
Bacterial Proteins - immunology
Bacterial Proteins - metabolism
Biological Sciences
brown spot lesions
Disease
DNA, Bacterial - metabolism
Epidemiology
experimental infections
Fabaceae - microbiology
Flowers & plants
Genes
hrp gene
infection
Leaves
Lesions
Microbiology
mutants
pathogenicity
Phaseolus vulgaris
Phytopathology
Plant diseases
Plant Diseases - microbiology
plant diseases and disorders
Plant growth
Plant interaction
Plants
Plants, Medicinal
population density
Population size
protein secretion
Proteins
Pseudomonas - genetics
Pseudomonas - metabolism
Pseudomonas - pathogenicity
Pseudomonas syringae pv. syringae
Pseudomonas syringae syringae
Secretion
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Summary:hrp genes are reportedly required for pathogenicity in Pseudomonas syringae pv. syringae (Pss) and other phytopathogenic bacterial species. A subset of these genes encodes a type III secretion system through which virulence factors are thought to be delivered to plant cells. In this study, we sought to better understand the role that hrp genes play in interactions of Pss with its host as they occur naturally under field conditions. Population sizes of hrp mutants with defects in genes that encode components of the Hrp secretion system (deltahrcC::nptII and hrpJ::omegaSpc) and a protein secreted via the system (deltahrpZ::nptII) were similar to B728a on germinating seeds. However, phyllosphere (i.e., leaf) population sizes of the hrcC and hrpJ secretion mutants, but not the hrpZ mutant, were significantly reduced relative to B728a. Thus, the Hrp type III secretion system, but not HrpZ, plays an important role in enabling Pss to flourish in the phyllosphere, but not the spermosphere. The hrcC and hrpJ mutants caused brown spot lesions on primary leaves at a low frequency when they were inoculated onto seeds at the time of planting. Pathogenic reactions also were found when the hrp secretion mutants were co-infiltrated into bean leaves with a non-lesion-forming gacS mutant of B728a. In both cases, the occurrence of disease was associated with elevated population sizes of the hrp secretion mutants. The role of the Hrp type III secretion system in pathogenicity appears to be largely mediated by its requirement for growth of Pss in the phyllosphere. Without growth, disease does not occur.
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.96.17.9851