Integration of environmental and host-derived signals with quorum sensing during plant-microbe interactions

Many plant-associated microbes use secreted autoinducer molecules, including N-acylhomoserine lactones (AHLs), to regulate diverse behaviours in association with their population density (quorum sensing). Often, these responses are affected by environmental conditions, including the presence of other AHL-producing bacterial species. In addition, plant-derived metabolites, including products that arise as a direct result of the bacterial infection, may profoundly influence AHL-regulated behaviours. These plant products can interact directly and indirectly with the quorum-sensing network and can profoundly affect the quorum-sensing behaviour. Local conditions on a microscopic scale may affect signal molecule longevity, stability and accumulation, and this could be used to give information in addition to cell density. Furthermore, in many Gram-negative bacteria, AHL signalling is subservient to an additional two-component signalling system dependent upon homologues of GacS and GacA. The signal(s) to which GacS responds are not known, but recent research suggests that a self-produced ligand may be being detected. This review will focus on two well-studied examples of AHL-regulated plant-associated behaviour, Erwinia carotovora and Agrobacterium tumefaciens, to illustrate the complexity of such signalling networks.