Role of Exopolysaccharides in Sinorhizobium meliloti/Alfalfa Symbiosis.

meliloti 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，涉及双方的代谢合作。这种联系的最终结果是共生生物为寄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伙伴产生的信号分子的释放和识别。该项目的主要目标是确定在共生过程中，细菌产生的外多糖是如何调节和协调这些信号分子之一的产生的。S. meliloti产生至少两种外多糖（琥珀聚糖和EPS II），在细菌-植物相互作用期间作为入侵信号。先前的研究表明，这些外多糖的特定低分子量形式需要由细菌产生，以使其发挥信号分子的作用。最近，研究人员在S. meliloti中发现并表征了三个群体感应系统（Mel， Sin/ExpR和Tra），并表明其中至少一个（Sin/ExpR系统）在胞外多糖的产生和结瘤中起重要作用。本项目将探讨如何调节和协调S. meliloti共生体重要外多糖的生物合成及其活性低分子量形式的产生。越来越多的证据表明，S. meliloti产生的外多糖在细菌与豆科植物根瘤共生的发展中起着特定的作用，特别是在根瘤侵袭水平上。几个有趣的问题仍有待回答，包括：在什么环境条件下产生不同形式的共生活性外多糖，以及这些不同形式的生产在共生体中是如何调节和协调的。这些问题和相关问题的答案将有助于更好地理解共生伙伴之间的信号机制以及碳水化合物作为信号的使用。进一步分析外多糖在紫花苜蓿/紫花苜蓿共生中的功能，也可能为低聚糖在其他生物系统中作为信号分子的可能用途提供见解。更广泛的影响：该项目将继续开展多项外展活动，其中包括一个中国根瘤菌项目教学实验室、一个针对少数民族学生的系列讲座，以及达拉斯地区科学博物馆和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.