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

May 2010 , Volume 23 , Number  5
Pages  682 - 701

Inmaculada Ortiz-Martín,1 Richard Thwaites,2 John W. Mansfield,3 and Carmen R. Beuzón1

1Instituto de Hortofruticultura Subtropical y Mediterranea, Universidad de Málaga-Consejo Superior de Investigaciones Científicas (IHSM-UMA-CSIC), Depto. Biología Celular, Genética y Fisiología, Campus de Teatinos, Málaga E-29071, Spain; 2Food and Environment Research Agency, Sand Hutton, YO41 1LZ, York, U.K.; 3Faculty of Natural Science, Division of Biology, Sir Alexander Fleming Bd. Imperial College London, South Kensington, SW7 2AZ, London

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Accepted 12 January 2010.

Many plant-pathogenic bacteria require type III secretion systems (T3SS) to cause disease in compatible hosts and to induce the hypersensitive response in resistant plants. T3SS gene expression is induced within the plant and responds to host and environmental factors. In Pseudomonas syringae, expression is downregulated by the Lon protease in rich medium and by HrpV under inducing conditions. HrpV acts as an anti-activator by binding HrpS. HrpG, which can also bind HrpV, has been reported to act as an anti-anti-activator. Previous studies have used mostly in vitro inducing conditions, different pathovars, and methodology. We have used single and double lon and hrpV mutants of P. syringae pv. phaseolicola 1448a, as well as strains ectopically expressing the regulators, to examine their role in coordinating expression of the T3SS. We applied real-time polymerase chain reaction to analyze gene expression both in vitro and in planta, and assessed bacterial fitness using competitive indices. Our results indicate that i) Lon downregulates expression of the hrp/hrc genes in all conditions, probably by constitutively degrading naturally unstable HrpR; ii) HrpV and HrpT downregulate expression of the hrp/hrc genes in all conditions; and iii) HrpG has an additional, HrpV-independent role, regulating expression of the hrpC operon.

© 2010 The American Phytopathological Society