Autoinduction mediated by AHL signals has been well described in the plant pathogen P. stewartii ssp. stewartii (von Bodman et al., 2003) and has been reported recently in the pathogens P. agglomerans pv. gypsophilae and P. ananatis (Morohoshi et al., 2007; Chalupowicz et al., 2008). Based on the sequence homology to the pagRI genes of

P. agglomerans pv. gypsophilae (Chalupowicz et al., 2008; Rezzonico et al., 2009) the transcriptional regulator pagR and the AHL-synthase pagI genes (Pvag_pPag30141–Pvag_pPag30142) have been identified on plasmid Bafilomycin A1 pPag3. Using an A. tumefaciens biosensor (Shaw et al., 1997), AHL production was tested. For this purpose, the plant pathogenic strain P. ananatis LMG 2665 was added to the assay as a positive control. Pantoea vagans C9-1 has a positive autoinducer functional activity, but yields a weaker signal in the biosensor assay than P. ananatis LMG 2665 (Fig. 2). The variant P. vagans C9-1W lost this activity (Fig. 2), confirming that this strain KU-60019 clinical trial is not able to produce detectable AHLs.

Although the chromosome also contains a putative AHL synthase, located next to the sdiA gene encoding a LuxR-type transcriptional regulator (Lindsay & Ahmer, 2005; Smits et al., 2009), it can be concluded from the results of the biosensor assay that this chromosomal gene is not involved in the synthesis of the AHLs that can be detected with the A. tumefaciens biosensor. The PagRI quorum-sensing system plays a central role in the virulence of P. agglomerans pv. gypsophilae by regulating the expression of the T3SS (Chalupowicz et al., 2009). Its role in the ecological behavior of P. vagans and P. agglomerans strains that have functional pagRI genes is currently unknown (Rezzonico et al., 2009). The fact that most nonpathogenic strains lack a T3SS, however,

suggests that pagRI may have additional non-virulence-related functions in phytopathogenic pathovars. Siderophores are small molecules that bind Fe3+ with a high affinity and are synthesized by bacteria under iron starvation. The genome of P. vagans C9-1 contains biosynthetic genes for the catecholate siderophore enterobactin (ent-fep) Cell press and the hydroxamate siderophore desferrioxamine E (dfoJACS), which were reported to be produced by the strain (Feistner & Ishimaru, 1996). Siderophore biosynthesis can be an important biocontrol trait, as the strain may be able to compete with phytopathogens for the already limited supply of iron in planta. When spotted onto CAS siderophore indicator plates (Schwyn & Neilands, 1987), P. vagans C9-1 produces a large halo, indicative of siderophore synthesis, while variant C9-1W produces a small halo, just around the colony (Fig. 3). This difference can be attributed to the absence of the pPag3-encoded dfoJACS gene cluster (Pvag_pPag30339–Pvag_pPag30342), which confers the ability of desferrioxamine production to the strain. Pantoea vagans C9-1W was compared with the wild-type strain P.