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

Summary 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.