Role of Exopolysaccharides in Sinorhizobium meliloti/Alfalfa Symbiosis.

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The symbiotic interaction between the bacterium Sinorhizobium meliloti and its leguminous host plant is a very specific relationship involving metabolic co-operation between the two partners. The end result of this association is the fixation of atmospheric nitrogen by the symbiont for the benefit of the host. This interaction is highly dependent on the release and recognition of signal molecules produced by each partner. The main goal of this project is to determine how production of one of these signal molecules, the bacterially produced exopolysaccharides, is regulated and coordinated during the symbiotic process. S. meliloti produces at least two exopolysaccharides (succinoglycan and EPS II) that serve as invasion signals during the bacterial-plant interaction. Previous research has demonstrated that specific low molecular weight forms of these exopolysaccharides need to be generated by the bacterium to enable their role as signal molecules. More recently the researchers discovered and characterized three quorum-sensing systems (Mel, Sin/ExpR and Tra) in S. meliloti and showed that at least one of these (the Sin/ExpR system) plays an important role in exopolysaccharide production and nodulation. This project will explore how the biosynthesis of the symbiotically important exopolysaccharides of S. meliloti and the production of their active low molecular weight forms are regulated and coordinated. It is increasingly evident that the exopolysaccharides produced by S. meliloti play specific roles in the development of the root nodule symbiosis between the bacteria and legumes, particularly at the level of nodule invasion. Several interesting questions remain to be answered, including: under what environmental conditions are the different forms of the symbiotically active exopolysaccharides produced and how the production of these different forms are regulated and coordinated in the symbiont. The answers to these and related questions will lead to a greater understanding of the signaling mechanisms between symbiotic partners and the use of carbohydrates as signals. Further analyses of the functions of exopolysaccharides in the S. meliloti/Medicago sativa symbiosis may also provide insight into the possible uses of oligosaccharides as signaling molecules in other biological systems.Broader Impacts: The project will continue its multiple outreach activities, including among others, a Sinorhizobium project teaching laboratory, a lecture series to minority students and a collaboration project between the Dallas area science museums and the University of Texas at Dallas.