In the first paper, Fray et al1 produced transgenic tobacco plants expressing a gene directing the biosynthesis of a class of microbial signaling molecules, the N-acylhomoserine lactones (AHLs). Relatively small molecules, AHLs were discovered several years ago in connection with the phenomenon of light production by certain microbes of the genus Vibrio associated with the light organs of marine fish. They were subsequently observed in several other microbes, including animal and plant pathogens. The AHLs are used by bacteria as a barometer of cell density, in a process termed 'quorum sensing'2. Typically, bacteria constitutively produce and secrete small amounts of AHLs, which appear to diffuse freely into and out of bacterial cells. If the bacterial cell density is very low, the AHL concentration in the medium is also low. However, if the cell density increases, the ambient AHL concentration rises. The bacteria have transcriptional regulator proteins that become active when the AHL concentration reaches a threshold value, leading to specific target gene expression. The nature of the target genes varies considerably, from genes leading to antibiotic synthesis in the case of saprophytic bacteria, to virulence genes in bacterial pathogens.
Quorum sensing is a clever scheme, especially for pathogens, because they can colonize a host, grow benignly on the surface, and achieve significant populations without expressing virulence factors required for internal colonization. As noted above, plants have evolved defense mechanisms that are triggered in response to virulence factors. By placing transcription of virulence factors under AHL control, the pathogen confounds plant sensing and defense is not invoked during the early stages of pathogen multiplication. However, when bacterial numbers become substantial and their AHL quorum sensing barometers trip, virulence genes are activated and the bacteria become aggressive pathogens. Because of their high numbers, a great deal of damage is done to the plant before host defense has time to be mobilized. Fray et al. worked with such a plant pathogen, Erwinia carotovora. An AHL is used by this bacterium to regulate the production of pectic enzymes, virulence factors that attack a major plant cell wall component, leading to a soft rot disease.
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