Positive Regulation of the Hrp Type III Secretion System in Pseudomonas syringae pv. phaseolicola

• Published in: Molecular plant-microbe interactions Vol. 23; no. 5; pp. 665 - 681
• Main Authors: Ortiz-Martín, Inmaculada, Thwaites, Richard, Macho, Alberto P, Mansfield, John W, Beuzón, Carmen R
• Format: Journal Article
• Language: English
• Published: United States The American Phytopathological Society 01.05.2010
• Subjects: Bacterial Proteins - genetics; Bacterial Proteins - metabolism; beans; biochemical pathways; Conserved Sequence; Culture Media - metabolism; Down-Regulation - genetics; environmental factors; Fabaceae - microbiology; field crops; Flagella - metabolism; flagellum; gene expression; gene expression regulation; Gene Expression Regulation, Bacterial; Genes, Bacterial - genetics; host plants; hypersensitive response; microbial colonization; microbial genetics; mutants; Mutation - genetics; Nicotiana - microbiology; operon; pathogenesis; pathotypes; Phaseolus vulgaris; Plant Leaves - microbiology; plant pathogenic bacteria; polymerase chain reaction; Pseudomonas syringae; Pseudomonas syringae - genetics; Pseudomonas syringae - pathogenicity; Pseudomonas syringae pv. phaseolicola; Reverse Transcriptase Polymerase Chain Reaction; transcription (genetics); Type III secretion system; Up-Regulation - genetics; vigor; virulence; Virulence Factors - metabolism
• ISSN: 0894-0282; 1943-7706
• DOI: 10.1094/mpmi-23-5-0665

Disease in compatible hosts and induction of the hypersensitive response in resistant plants by most plant-pathogenic bacteria require a functional type III secretion system (T3SS). Expression of T3SS genes responds to host and environmental factors and is induced within the plant. In Pseudomonas syringae, expression of the T3SS requires HrpL, which is transcriptionally upregulated by HrpR and HrpS. In some pathovars, expression of the hrpRS genes is upregulated by the GacA/S two-component system. Additionally, HrpA, the major component of the T3SS pilus, has also been linked to the regulation of the hrpRS gene expression. Previous studies concerning regulation of hypersensitive response and pathogenesis/hypersensitive response conserved (hrp/hrc) gene expression have used mostly in vitro inducing conditions, different pathovars, and methodology. Here, we analyze the roles of HrpL, GacA, and HrpA in the bean pathogen, using single, double, and triple mutants as well as strains ectopically expressing the regulators. We use real-time polymerase chain reaction analysis in vitro and in planta to quantify gene expression and competitive indices and other assays to assess bacterial fitness. Our results indicate that i) HrpL acts as a general virulence regulator that upregulates non-T3SS virulence determinants and downregulates flagellar function; ii) GacA modulates the expression of hrpL, and its contribution to virulence is entirely HrpL dependent; iii) there is a basal HrpL-independent expression of the T3SS genes in rich medium that is important for full activation of the system, maybe by keeping the system primed for rapid activation upon contact with the plant; and iv) HrpA upregulates expression of the T3SS genes and is essential to activate expression of the hrpZ operon upon contact with the plant.