Detection, purification and characterisation of quorum-sensing signal molecules in plant-associated bacteria.

Quorum sensing (also called autoinduction) is a term that describes an environmental sensing system that allows bacteria to monitor their own population density. Autoinduction relies upon the interaction of a small diffusible signal molecule (the autoinducer) with a transcriptional activator protein to couple gene expression with cell population density. These signal molecules diffuse from bacterial cells and accumulate in the environment as a function of cell growth. Once a threshold concentration is reached, these signals serve as co-inducers to regulate the transcription of (a) set(s) of target genes. In Gram-negative bacteria, most autoinducers belong to the family of N-acylhomoserine lactones (AHLs). The detection of AHLs (or AHL-like activities) has been greatly facilitated by the development of sensitive bioassays that allow fast screening of microorganisms for diffusible signal molecules. AHL or diketopiperazine-mediated cell-cell signalling play roles in regulating different bacterial functions, such as antibiotic biosynthesis, production of virulence factors, exopolysaccharide biosynthesis, bacterial swarming, plasmid conjugal transfer and transition into the stationary phase. Several bacterial species that interact with plants produce AHL-like compounds. In this review, we will summarise the current knowledge about the detection, characterisation and purification of quorum-sensing molecules from plant-associated bacteria. We will also discuss some of the future prospects and biotechnological applications of autoinducers.