Microbial Signaling: Implications for Agriculture and Managing Soil Microbiology.

Many groups of bacteria engage in chemical communication via the exchange of
diffusible pheromones, and use these chemical signals to alter their physiology in
profound ways, including changes in pathogenicity, biofilm formation, and horizontal
gene transfer.  Bacteria use these signals to estimate their population densities and at
high cell densities, to coordinate the activities of a population of cells, a phenomenon
sometimes referred to as quorum sensing.  Usually, a signal synthesis protein is
functionally paired with a signal receptor protein that is also a signal-dependent
transcriptional activator, though many variations of this pattern have been documented.
Many plant associated bacteria engage in quorum sensing, including Rhizobia, Pantoea
stewartii, Pseudomonas aureofaciens, Xanthomonas campestris and many others.  In
some cases, quorum sensing plays a central role in plant-microbe interactions.  For
example, the biocontrol agent P. aureofaciens uses quorum sensing to synthesize
antifungal compounds called phenazines, which deter host colonization by pathogenic
fungi.  The plant pathogen P. stewartii uses quorum sensing to regulate the synthesis of
stewartan polysaccharides, which block xylem vessels of maize, causing Stewart’s wilt.
Understanding quorum sensing may enable scientists to intervene in the activities of
plant-associated bacteria, blocking harmful activities, and enhancing beneficial ones.