Knockout mutations of the LuxR regulator PFL_3627 and the sRNA phrS causes pleiotropic effects on the biocontrol bacterium Pseudomonas protegens Pf-5

The Pseudomonas protegens Pf-5 (previously known as P.fluorescens Pf-5) has shown potential as a biological control agent against plant pathogenic bacteria, fungi and nematodes. The production of anti-pathogen compounds and rhizosphere colonisation are two of the key traits that determine its biocontrol competency. Genetic and biochemical studies of secondary metabolite biosynthesis combined with analysis of the genome sequence have clarified many aspects of the production of anti-pathogen compounds in Pf-5. However, there is only very limited data on the molecular factors important for rhizosphere colonisation. Successful rhizosphere colonisation is likely influenced by two features: motility and biofilm formation. In many bacteria, motility and biofilm are both regulated by quorum sensing (QS) and small RNAs (sRNA). This study focuses on a LuxR solo regulator PFL_3627 involved in QS, and a sRNA phrS. Gene knockouts of both of these genes reveal they have pleiotropic effects on both biofilm formation and motility. A microarray analysis of a PFL_3627 mutant revealed a large number of genes that are up- or down-regulated by this regulator. Generally, canonical QS regulation by LuxR is mediated by the exogenous or endogenous QS signalling molecules N-acyl-homoserine-lactone (AHL). The Pf-5 genome does not encode a LuxI homolog for AHL biosynthesis, and consistent with this no endogenous AHL production was detected using biosensor strains. An assay of exogenous addition of short or long chain AHLs suggested that the LuxR regulator PFL_3627 is not involved in sensing AHLs. Thus PFL_3627 is presumably involved in the sensing of an as yet unknown molecule, possibly of plant origin.