Quorum Sensing in Bacteria-Plant Interactions

Multicellular organisms rely on an accurate communication between individual cells to coordinate many aspects of physiology and development. Prokaryotic organisms, although unicellular, also express certain traits only when a critical number of bacteria has been reached. Here, the individual bacterium benefits from joint multicellular behaviour to survive, compete and persist in nature, or to colonize a particular host. Therefore, they have to communicate with each other. Fuqua et al. (1994) introduced the term “quorum sensing (QS)” to describe the process where bacterial communication is used to monitor population density and to change bacterial gene expression and behaviour accordingly (Fuqua et al. 2001; von Bodman et al. 2003a). Essentially, QS is based on production of low-mass signalling molecules, the extracellular concentration of which is related to the population density of the producing organisms. These signalling molecules can be sensed by the bacterial cells and this allows the population to initiate a concerted action once a critical concentration (“quorum”) has been reached (Whitehead et al. 2001). A wide range of (potential) low-mass signalling molecules have been identified. These include peptide-based signals in various Gram-positive organisms and the N-acyl homoserine lactone (AHL) signals found in many Gram-negative bacteria (Proteobacteria) (Fuqua et al. 2001; Whitehead et al. 2001) as well as many other signal molecules (for an overview see Visick and Fuqua 2005). However, Redfield (2002) suggested that in some cases quorum sensing might be a side effect of cells monitoring their diffusion environment instead of communicating. By this means, cells can regulate the secretion of effectors to minimize losses to extracellular diffusion. Most QS-regulated processes in plant-associated bacteria are mediated by AHL (N-acyl homoserine lactone (HSL))-based QS systems, which is the main focus of this chapter.