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The Transcriptional Activator rfiA Is Quorum-Sensing Regulated by Cotranscription with the luxI Homolog pcoI and Is Essential for Plant Virulence in Pseudomonas corrugata

December 2009 , Volume 22 , Number  12
Pages  1,514 - 1,522

Grazia Licciardello,1,2 Iris Bertani,3 Laura Steindler,3 Patrizia Bella,2 Vittorio Venturi,3 and Vittoria Catara2

1Science and Technology Park of Sicily, Catania, Italy; 2Department of Phytosanitary Science and Technologies, University of Catania, Italy; and 3Bacteriology Group, International Centre for Genetic Engineering and Biotechnology, Area Science Park, Padriciano, Trieste, Italy

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Accepted 10 August 2009.

The gram-negative phytopathogen Pseudomonas corrugata has an acyl-homoserine lactone (AHL) quorum-sensing (QS) system called PcoI/PcoR that is involved in virulence on tomato. This work identifies, downstream of pcoI, a gene designated rfiA, which we demonstrate is directly linked to QS by cotranscription with pcoI. The deduced RfiA protein contains a DNA-binding domain characteristic of the LuxR family but lacks the autoinducer-binding terminus characteristic of the QS LuxR-family proteins. We also identified, downstream of rfiA, an operon designated pcoABC, encoding for the three components of a tripartite resistance nodulation-cell-division (RND) transporter system. The expression of pcoABC is regulated by RfiA. We found that lipodepsipeptide (LDP) production is cell density dependent and mutants of pcoI, pcoR, and rfiA are unable to inhibit the growth of the LDP-sensitive microorganisms Rhodotorula pilimanae and Bacillus megaterium. P. corrugata rfiA mutants were significantly reduced in their ability to cause necrosis development in tomato pith. In addition, it was established that PcoR in the absence of AHL also played a role in virulence on tomato. A model for the role of PcoI, PcoR, and RfiA in tomato pith necrosis is presented.

© 2009 The American Phytopathological Society