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Background   N -acylhomoserine lactone (AHL)-based quorum sensing (QS) systems have been described in many plant-associated Gram-negative bacteria to control certain beneficial phenotypic traits, such as production of biocontrol factors and plant growth promotion. However, the role of AHL-mediated signalling in the endophytic strains of plant-associated  Serratia  is still poorly understood. An endophytic  Serratia  sp. G3 with biocontrol potential and high levels of AHL signal production was isolated from the stems of wheat and the role of QS in this isolate was determined.    Results  Strain G3 classified as  Serratia plymuthica  based on 16S rRNA was subjected to phylogenetic analysis. Using primers to conserved sequences of  luxIR  homologues from the  Serratia  genus,  splIR  and  spsIR  from the chromosome of strain G3 were cloned and sequenced. AHL profiles from strain G3 and  Escherichia coli  DH5α expressing  splI  or  spsI  from recombinant plasmids were identified by liquid chromatography-tandem mass spectrometry. This revealed that the most abundant AHL signals produced by SplI in  E. coli  were  N -3-oxo-hexanoylhomoserine lactone (3-oxo-C6-HSL),  N -3-oxo-heptanoylhomoserine lactone (3-oxo-C7-HSL),  N -3-hydroxy-hexanoylhomoserine lactone (3-hydroxy-C6-HSL),  N -hexanoylhomoserine lactone (C6-HSL), and  N -heptanoyl homoserine lactone (C7-HSL); whereas SpsI was primarily responsible for the synthesis of  N -butyrylhomoserine lactone (C4-HSL) and  N -pentanoylhomoserine lactone (C5-HSL). Furthermore, a quorum quenching analysis by heterologous expression of the  Bacillus  A24 AiiA lactonase in strain G3 enabled the identification of the AHL-regulated biocontrol-related traits. Depletion of AHLs with this lactonase resulted in altered adhesion and biofilm formation using a microtiter plate assay and flow cells coupled with confocal laser scanning microscopy respectively. This was different from the closely related  S. plymuthica  strains HRO-C48 and RVH1, where biofilm formation for both strains is AHL-independent. In addition, QS in G3 positively regulated antifungal activity, production of exoenzymes, but negatively regulated production of indol-3-acetic acid (IAA), which is in agreement with previous reports in strain HRO-C48. However, in contrast to HRO-C48, swimming motility was not controlled by AHL-mediated QS.    Conclusions  This is the first report of the characterisation of two AHL-based quorum sensing systems in the same isolate of the genus  Serratia . Our results show that the QS network is involved in the global regulation of biocontrol-related traits in the endophytic strain G3. However, although free-living and endophytic  S. plymuthica  share some conservation on QS phenotypic regulation, the control of motility and biofilm formation seems to be strain-specific and possible linked to the life-style of this organism.

bmc microbiology

Characterisation of two quorum sensing systems in the endophytic Serratia plymuthica strain G3: differential control of motility and biofilm formation according to life-style