Chalupowicz, L., Department of Plant Pathology and Weed Research, ARO, Volcani Center, Bet Dagan, 50250, Israel, Department of Plant Sciences, Maiman Institute for Proteome Research, Tel-Aviv University, Tel-Aviv 69978, Israel
Manulis-Sasson, S., Department of Plant Pathology and Weed Research, ARO, Volcani Center, Bet Dagan, 50250, Israel
Itkin, M., Department of Plant Sciences, Maiman Institute for Proteome Research, Tel-Aviv University, Tel-Aviv 69978, Israel
Sacher, A., Proteomics Laboratories, Maiman Institute for Proteome Research, Tel-Aviv University, Tel-Aviv 69978, Israel
Sessa, G., Department of Plant Sciences, Maiman Institute for Proteome Research, Tel-Aviv University, Tel-Aviv 69978, Israel
Barash, I., Department of Plant Sciences, Maiman Institute for Proteome Research, Tel-Aviv University, Tel-Aviv 69978, Israel

The quorum-sensing (QS) regulatory system of the gall-forming Pantoea agglomerans pv. gypsophilae was identified. Mass spectral analysis, together with signal-specific biosensors, demonstrated that P. agglomerans pv. gypsophilae produced N-butanoyl-L-homoserine lactone (C4-HSL) as a major and N-hexanoyl-L-homoserine lactone (C6-HSL) as a minor QS signal. Homologs of luxI and luxR regulatory genes, pagI and pagR, were characterized in strain P. agglomerans pv. gypsophilae Pag824-1 and shown to be convergently transcribed and separated by 14 bp. The deduced PagI (23.8 kDa) and PagR (26.9 kDa) show high similarity with SmaI (41% identity) and SmaR (43% identity), respectively, of Serratia sp. American Type Culture Collection 39006. PagR possesses characteristic autoinducer binding and a helix-turn-helix DNA-binding domain. Gall formation by P. agglomerans pv. gypsophilae depends on a plasmid-borne hrp/hrc gene cluster, type III effectors, and phytohormones. Disruption of pagI, pagR, or both genes simultaneously in Pag824-1 reduced gall size in gypsophila cuttings by 50 to 55% when plants were inoculated with 106 CFU/ml. Higher reductions in gall size (70 to 90%) were achieved by overexpression of pagI or addition of exogenous C4-HSL. Expression of the hrp/hrc regulatory gene hrpL and the type III effector pthG in the pagI mutant, as measured with quantitative reverse-transcriptase polymerase chain reaction, was reduced by 5.8 and 6.6, respectively, compared with the wild type, suggesting an effect of the QS system on the Hrp regulon. © 2008 The American Phytopathological Society.