Pectobacterium carotovorum subsp carotovorum strains show diversity in production of and response to N-acyl homoserine lactones

Pectobacterium carotovorum subsp.carotovorum (Pcc) is a plant pathogen, which can cause soft rot in a wide range of plants. A regulatory network controls the synthesis of virulence factors, mainly plant cell wall degrading enzymes, in a cell density dependent manner. Small signalling molecules, N-acyl homoserine lactones (AHLs), activate the synthesis of exoenzymes when cell density exceeds a certain threshold value (quorum sensing). Thin layer chromatography in conjunction with AHL indicator strains were used for characterization of AHL molecules produced by different wild-type (WT) Pcc strains isolated from various hosts. Two groups of Pcc strains were found: one group with N-(3-oxo-hexanoyl)-L-HSL (OHHL) molecule as a dominant (e.g. 71) and another group two containing N-(3-oxo-octanoyl)-L-HSL (OOHL) as the dominant AHL, but also producing N-(octanoyl)-L-HSL (OHL) and OHHL molecules (e.g. SCC3193). Pcc hsl-negative mutants and their parental WT strains (71 and SCC3193), were used to study the response to single synthetic AHL analogues or AHLs extracted from cultures of different bacteria when co-inoculated into wells in potato slices. Mutants varied in their response to the addition of single AHL analogues or AHLs extracted from the cultures of different bacteria. OHHL restored the ability to macerate tuber tissue of both mutants and synthetic OHL of SCC3193 mutant. Symptoms developed on leaves of in vitro grown potato plants inoculated with the WT strains but not with the mutants, even not with added AHL homologues. However, plant stress determined by chlorophyll fluorescence imaging showed a response to the mutants when AHLs were supplemented, that also restored maceration ability in the potato tuber slice assay.