Role of Exopolysaccharides in Sinorhizobium meliloti/Alfalfa Symbiosis.

苜蓿中华根瘤菌（Sinorhizobium meliloti）与其豆科寄主植物之间的共生相互作用是一种非常特殊的关系，涉及双方之间的代谢合作。这种结合的最终结果是共生体为了宿主的利益而固定大气中的氮。这种相互作用高度依赖于每个伴侣产生的信号分子的释放和识别。该项目的主要目标是确定这些信号分子之一，细菌产生的胞外多糖，在共生过程中如何调节和协调。S.苜蓿产生至少两种胞外多糖（琥珀多糖和EPS II），其在细菌-植物相互作用期间充当入侵信号。先前的研究已经证明，这些胞外多糖的特定低分子量形式需要由细菌产生，以使它们能够作为信号分子发挥作用。最近，研究人员在S.苜蓿属植物中，并表明这些（Sin/ExpR系统）中的至少一个在胞外多糖的生产和发酵中起着重要作用。本项目将探讨如何生物合成的共生重要胞外多糖的S。苜蓿及其活性低分子量形式的产生受到调节和协调。越来越多的证据表明，S.苜蓿根瘤菌在细菌和豆科植物之间的根瘤共生关系的发展中发挥着特殊的作用，特别是在根瘤入侵的水平上。几个有趣的问题仍然有待回答，包括：在什么样的环境条件下产生不同形式的共生活性胞外多糖，以及这些不同形式的生产是如何在共生体中调节和协调的。对这些问题和相关问题的回答将导致对共生伙伴之间的信号传导机制以及碳水化合物作为信号的使用的更深入的理解。进一步分析了S.苜蓿/紫花苜蓿共生也可能提供洞察寡糖作为信号分子在其他生物系统中的可能用途。更广泛的影响：该项目将继续其多项外展活动，其中包括中华根瘤菌项目教学实验室，少数民族学生系列讲座以及达拉斯地区科学博物馆与德克萨斯大学达拉斯分校之间的合作项目。

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.