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Regulatory interactions between the Hrp type III protein secretion system and coronatine biosynthesis in Pseudomonas syringae pv. tomato DC3000

Department of Entomology and Plant Pathology, 110 Noble Research Center, Oklahoma State University, Stillwater OK 74078-3032, USA¹
Department of Plant Sciences, University of Oxford, South Parks Road, Oxford OX1 3RB, UK²
Department of Plant Pathology, Cornell University, Ithaca, NY 14853-4203, USA³

Author for correspondence: Carol L. Bender. Tel: +1 405 744 9945. Fax: +1 405 744 7373. e-mail: cbender{at}okstate.edu

In P. syringae, the co-ordinated regulation of different systems required for pathogenicity and virulence seems logical but has not been established. This question was addressed in the present study by analysing production of the phytotoxin coronatine (COR) in defined hrp/hrc mutants of P. syringae pv. tomato DC3000. COR was produced in vitro by mutants of DC3000 defective in hrcC, which encodes an outer-membrane protein required for type III-mediated secretion. When inoculated in plants, hrcC mutants produced chlorotic regions indicative of COR production, but lacked the necrotic lesions produced by the wild-type DC3000. Furthermore, a DC3000 mutant containing a polar mutation in hrcC, which inactivates hrcC, hrpT and hrpV, produced significantly higher amounts of COR than the wild-type strain in vitro. This mutant was able to produce COR earlier and at lower cell densities than the wild-type. The results indicate that the hrp/hrc secretion system is not required for COR production, but mutations in this system may have regulatory effects on the production of virulence factors such as COR.

Keywords: type III secretion system, phytotoxin, virulence

Abbreviations: COR, coronatine; CFA, coronafacic acid; CMA, coronamic acid; GUS, glucuronidase; HR, hypersensitive response; HSS medium, Hoitink–Sinden medium amended with sucrose; MG medium, mannitol-glutamate medium

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