CAREER: Role of Exopolysaccharides in Rhizobium meliloti/ alfalfa Symbiosis

9733532紫花苜蓿根瘤菌与其寄主紫花苜蓿之间建立固氮共生需要冈萨雷斯细菌胞外多糖，但胞外多糖在这一相互作用中的确切作用尚未很好地确定。紫花苜蓿根瘤菌1021菌株的蔗糖聚糖是有效结瘤所必需的酸性硫氟胞外多糖。不能合成琥珀多糖的苜蓿突变体形成空瘤，没有细菌和类杆菌，也不能固定氮。RM1021还具有产生第二胞外多糖(EPS II)的隐蔽能力，该能力抑制缺乏琥珀多糖的菌株对紫花苜蓿根瘤入侵的共生缺陷。从这些研究中得出的一个共同主题是，少量的低分子量胞外多糖足以介导不能产生胞外多糖的苜蓿突变体的成功入侵，这意味着低分子量胞外多糖可能在这一过程中发挥信号分子的作用。由15-18个EPS II二糖亚基组成的低分子EPS II组分，当低至每株7皮摩尔时，有效地允许非侵染菌株入侵根瘤，这表明低分子EPS II可能在侵染过程中起到共生信号的作用。本项目将对苜蓿红曲霉产生胞外多糖进行遗传和生化分析，并鉴定参与产生具有生物活性的低分子形式的EPS II的基因。本项目还将研究琥珀多糖和EPS II在共生中的作用。这些结果将为鉴定具有生物活性的根瘤菌胞外多糖的潜在植物受体及其在共生中的作用奠定基础。在植物和哺乳动物的病原体中，多糖日益被认为是毒力的重要决定因素。葡聚糖是最具特性的细菌多糖之一，已经加深了对包括人类病原体在内的其他细菌中多糖的合成和生物学作用的了解。多糖生物合成的基础研究可能最终有助于设计合成复杂碳水化合物的酶步骤，以控制连接、特异性和立体化学。这些研究可能导致对寡糖在生物系统中作为信号的可能更广泛使用的洞察，而不是通常所认识到的。这些研究有望更好地理解植物-细菌共生关系，这是生物固氮所必需的。

9733532 Gonzalez Bacterial exopolysaccharide is required for establishment of the nitrogen fixing symbiosis between Rhizobium meliloti and its host plant Medicago sativa (alfalfa), but the precise role of exopolysaccharides in this interaction is not well defined. Succinoglycan, the acidic Calcofluor-binding exopolysaccharide of Rhizobium meliloti strain 1021 is required for effective nodulation. R. meliloti mutants that fail to synthesize succinoglycan form empty nodules that are devoid of bacteria and bacteroids and unable to fix nitrogen. Rm l021 has also the cryptic ability to produce a second exopolysaccharide (EPS II) that suppresses the symbiotic defects succinoglycan-deficient strains on alfalfa nodule invasion. A common theme emerging from these studies is that small quantities of low molecular weight exopolysaccharides are sufficient to mediate successful invasion by R. meliloti mutants which fail to produce exopolysaccharides, implying that low molecular weight exopolysaccharide may act as a signaling molecule during this process. A low molecular weight EPS II fraction consisting of 15-18 EPS II disaccharide subunits efficiently allowed nodule invasion by non-infective strains when present in amounts as low as 7 picomoles per plant, suggesting that low molecular weight EPS II may act as a symbiotic signal during infection. This project will perform genetic and biochemical analyses of exopolysaccharide production by R. meliloti and identify the genes involved in the production of the biologically-active low molecular weight form of EPS II. This project will also investigate the roles of succinoglycan and EPS II in symbiosis. The results will set the stage for the identification of the putative plant receptors for the biologically-active Rhizobium exopolysaccharides and their role in symbiosis. Polysaccharides are increasingly being recognized as important determinants of virulence in both plant and mammalian pathogens. Succingoglycan is one of the best-characterized bacterial polysaccharides and has already advanced understanding of the synthesis and biological roles of polysaccharides in other bacteria, including human pathogens. Basic studies of polysaccharide biosynthesis might eventually contribute to the design of enzymatic steps in the synthesis of complex carbohydrates to control linkage specificity and stereochemistry. These studies may lead to insights into the possible broader use of oligosaccharides as signals in biological systems than has generally been recognized. These studies promise to lead to a better understanding of plant-bacterial symbiosis which is necessary for biological nitrogen fixation.