Transcriptome of the quorum-sensing signal-degrading Rhodococcus erythropolis responds differentially to virulent and avirulent Pectobacterium atrosepticum.

Social bacteria use chemical communication to coordinate and synchronize gene expression via the quorum-sensing (QS) regulatory pathway. In Pectobacterium, a causative agent of the blackleg and soft-rot diseases on potato plants and tubers, expression of the virulence factors is collectively controlled by the QS-signals N-acylhomoserine lactones (NAHLs). Several soil bacteria, such as the actinobacterium Rhodococcus erythropolis, are able to degrade NAHLs, hence quench the chemical communication and virulence of Pectobacterium. Here, next-generation sequencing was used to investigate structural and functional genomics of the NAHL-degrading R. erythropolis strain R138. The R. erythropolis R138 genome (6.7 Mbp) contained a single circular chromosome, one linear (250 kbp) and one circular (84 kbp) plasmid. Growth of R. erythropolis and P. atrosepticum was not altered in mixed-cultures as compared with monocultures on potato tuber slices. HiSeq-transcriptomics revealed that no R. erythropolis genes were differentially expressed when R. erythropolis was cultivated in the presence vs absence of the avirulent P. atrosepticum mutant expI, which is defective for QS-signal synthesis. By contrast 50 genes (<1% of the R. erythropolis genome) were differentially expressed when R. erythropolis was cultivated in the presence vs absence of the NAHL-producing virulent P. atrosepticum. Among them, quantitative real-time reverse-transcriptase-PCR confirmed that the expression of some alkyl-sulfatase genes decreased in the presence of a virulent P. atrosepticum, as well as deprivation of organic sulfur such as methionine, which is a key precursor in the synthesis of NAHL by P. atrosepticum.

Growth biostimulation of quorum-quenching bacteria by gammagamma-heptalactone treatment in the hydroponic rhizosphere of Solanum tuberosum.

Several bacterial plant pathogens, including Pectobacterium, use a cell-to-cell communication system called quorum sensing (QS) to synchronize and regulate expression of the virulence factors. In this study, the biomolecule gamma-heptalactone (GHL) was introduced in hydroponic culture of Solanum tuberosum to stimulate growth of the native rhizospheric bacteria which are able to degrade the QS signal, hence potentially quench the QS-regulated virulence of Pectobacterium. During two annual campaigns, GHL-treatment efficiently stimulated the growth of QS-degrading bacterial population of Rhodococcus erythropolis in the rhizosphere of potato plants. Analytical chemistry showed that GHL rapidly disappeared because it could be assimilated as a carbon source by R. erythropolis. Moreover, pyrosequencing of the rrs-amplicons revealed a strong modification of the structure and diversity of bacterial populations, when GHL-treated and untreated conditions were compared. This work highlighted a potential innovative strategy for stimulating the growth and root colonization of QS-degrading bacteria, which would act as biocontrol agents against plant QS-pathogens.

Proline antagonizes GABA-induced quenching of quorum-sensing in Agrobacterium tumefaciens.

Plants accumulate free L-proline (Pro) in response to abiotic stresses (drought and salinity) and presence of bacterial pathogens, including the tumor-inducing bacterium Agrobacterium tumefaciens. However, the function of Pro accumulation in host-pathogen interaction is still unclear. Here, we demonstrated that Pro antagonizes plant GABA-defense in the A. tumefaciens C58-induced tumor by interfering with the import of GABA and consequently the GABA-induced degradation of the bacterial quorum-sensing signal, 3-oxo-octanoylhomoserine lactone. We identified a bacterial receptor Atu2422, which is implicated in the uptake of GABA and Pro, suggesting that Pro acts as a natural antagonist of GABA-signaling. The Atu2422 amino acid sequence contains a Venus flytrap domain that is required for trapping GABA in human GABA(B) receptors. A constructed atu2422 mutant was more virulent than the wild type bacterium; moreover, transgenic plants with a low level of Pro exhibited less severe tumor symptoms than did their wild-type parents, revealing a crucial role for Venus flytrap GABA-receptor and relative abundance of GABA and Pro in host-pathogen interaction.