## 1 Introduction  
_Sinorhizobium meliloti_ is a soil-dwelling \(\alpha\)-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_. In this symbiosis, bacteria invade the legume root leading to the formation of new organs called nodules, in which specialized forms of the microsymbiont fix atmospheric nitrogen that is transferred to the plant in exchange for carbohydrates and a protected niche. 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]. Nod factor perception in the plant root induces a cascade of signaling events, leading to root hair curling and cortical cell division necessary for root nodule formation. Root hair deformation entraps bacteria that initiate invasion of roots through a tube-like structure called the infection thread [6]. This process requires production of symbiotically important exopolysaccharides (EPS): succinoglycan (EPS I) or galactoglucan (EPS II) [4; 7; 8; 9; 10]. The infection threads grow towards the emergent nodules and ramify within the nodule tissue. Finally, the bacteria are released into plant cells in an endocytotic process and maintained in membrane-bound compartments called symbiosomes. Bacteria stop dividing and differentiate into morphologically new forms called bacteroids, which are able to perform the reduction of atmospheric dinitrogen into nitrogenated compounds for the plant [11; 12; 13; 14].  
The rhizosphere, i.e. the zone of soil that surrounds and is influenced by plant roots, typically provides suitable growth conditions for bacteria. Following bacterial colonization of plant roots, an increase in the bacterial population density triggers the onset of quorum sensing (QS). QS is a cell-to-cell signaling mechanism that allows rhizobia to collectively modify behaviors, some of which are crucial for the interaction with its host. Functions regulated by QS in legume-nodulating rhizobia include plasmid transfer, surface polysaccharides, motility, adaptation to stationary phase of growth and symbiotic interactions (reviewed in [4; 15; 16; 17]). Different chemical signals have been identified to participate in QS in bacteria [18; 19]. In rhizobia, QS usually relies on the synthesis and detection of \(N\)-acyl-homoserine lactones (AHL) [16; 17], although some exceptions are known, such as the unusual branched-chain isovaleryl-homoserine lactone and bradyovectin from the soybean symbiont _Bradyrhizobium japonicum_[20; 21], the cinnamoyl-homoserine lactone from stem-nodulating photosynthetic bradyrhizobia [22], or the response of _S. melilioti_ to the so-called autoinducer-2 (AI-2) [23]. Given the impact that AHLs have in several phenomena related to the development of an efficient _Rhizobium_-legume symbiosis, rhizbial AHLs can be considered symbiotically-important signals, although in some cases the contribution of AHLs are subtle and may reflect AHL-related differences in metabolism rather than direct regulatory mechanisms. Compared to other genera, rhizobia produce the greatest variety of AHLs. Here, we will review recent findings concerning QS regulation in the model bacterium _S. melilioti_ and the role played by AHLs in intercellular and interkingdom communication. The knowledge of QS-related aspects in other rhizobia will be compared.