Abstract
Soil-dwelling bacteria collectively referred to as rhizobia synthesize and perceive N-acyl-homoserine lactone (AHL) signals to regulate gene expression in a population density-dependent manner. AHL-mediated signaling in these bacteria regulates several functions which are important for the establishment of nitrogen-fixing symbiosis with legume plants. Moreover, rhizobial AHL act as interkingdom signals triggering plant responses that impact the plant-bacteria interaction. Both the regulatory mechanisms that control AHL synthesis in rhizobia and the set of bacterial genes and associated traits under quorum sensing (QS) control vary greatly among the rhizobial species. In this article, we focus on the well-known QS system of the alfalfa symbiont Sinorhizobium (Ensifer) meliloti. Bacterial genes, environmental factors and transcriptional and posttranscriptional regulatory mechanisms that control AHL production in this Rhizobium, as well as the effects of the signaling molecule on bacterial phenotypes and plant responses will be reviewed. Current knowledge of S. meliloti QS will be compared with that of other rhizobia. Finally, participation of the legume host in QS by interfering with rhizobial AHL perception through the production of molecular mimics will also be addressed.
Highlights
Sinorhizobium meliloti is a soil-dwelling α-proteobacterium that can exist in a free-living state or can establish nitrogen-fixing symbiosis with legume plants belonging to the genera Medicago, Melilotus and Trigonella
Since the Cyclic diguanylate (cdG) content was higher in exponentialthan in stationary-phase S. meliloti cells [56], negative regulation of acyl-homoserine lactone (AHL) biosynthesis by cdG may serve as a fine-tuning mechanism attenuating AHL accumulation in rapidly growing cells
This finding demonstrates that a fully functional quorum sensing (QS) system is not necessary for swarming, it does not rule out a possible role for AHLs and/or population density in the regulation of swarming motility, as it has been shown for many bacteria
Summary
Sinorhizobium meliloti is a soil-dwelling α-proteobacterium that can exist in a free-living state or can establish nitrogen-fixing symbiosis with legume plants belonging to the genera Medicago, Melilotus and Trigonella. The formation of symbiotic nitrogen-fixing nodules requires a continuous molecular dialogue that co-ordinates two developmental processes: bacterial infection initiated in the epidermis and nodule organogenesis that takes place in the root cortex [1,2,3]. To initiate this interaction, legume plants secrete a variety of compounds that attract the bacteria toward their roots and activate the production of a lipochitooligosaccharide signal molecule known as the Nod factor [4,5]. The knowledge of QS-related aspects in other rhizobia will be compared
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