Quorum sensing in nitrogen-fixing rhizobia

Bacterial populations coordinately regulate gene expression by producing diffusible signal molecules. These signals, known as autoinducers, accumulate extracellularly and interact specifically with a receptor protein to affect changes not related to their own metabolism. Production of autoinducers typically occurs at specific stages of growth or in response to changes in the environment and induces a concerted response once a critical concentration has been reached. These diffusible signals frequently act to induce gene expression in response to bacterial cell density in a process often referred to as quorum sensing (8, 59, 60, 69, 120, 166, 178, 182, 185). Alternatively, autoinducer secretion and response may confer on the bacterium the ability to determine whether secreted molecules move away from the cell. This process, termed diffusion sensing by Rosemary Redfield, could allow the cells to regulate the secretion of effectors, such as degradative enzymes, antibiotics, surfactants, and siderophores, to minimize losses to extracellular diffusion (137). The best characterized quorum-sensing mechanism is found in gram-negative organisms and involves the use of acylated homoserine lactones (AHLs) as signal molecules (8, 59, 60, 63, 69, 120, 145, 166, 178, 182, 185). Recent publications have shown that quorum sensing plays a major role in preparing and perhaps coordinating the symbiotic nitrogen-fixing rhizobia during the establishment of their interactions with the host plant.