根瘤菌结瘤因子的结构和功能

结瘤因子是根瘤菌分泌的寡糖,它作为外在信号,诱发宿主植物根部各种生理反应。引起根毛变形,诱导皮层细胞分裂,形成根瘤原基,作者主要就这一早期结瘤过程中结瘤因子的结构和功能作一综述。     

根瘤菌的结瘤基因与结瘤因子

根瘤菌的结瘤基因与结瘤因子郭先武（华中农业大学农业部农业微生物重点实验室武汉４３００７０）根瘤菌侵染豆科植物形成根瘤,并合成ＮＨ３供植物利用,其自身也在植物环境中得以有效延续。这就是根瘤菌与宿主植物的共生关系。形成共生关系的基因分成三类［７］,一类是...     

根瘤菌结瘤基因研究的新进展

简要综述了目前根瘤菌结癌基因研究的3个热点方向,即结瘤因子、nodlD基因的调控和结瘤基因系统发育分析的新进展。结瘤因子的骨架核心是结瘤基因中的共同性基因nodABC表达的产物,宿主专一性基因则进行骨架结构的修饰,所形成的特异性结瘤因子是根瘤苗宿主范围的主要决定因素。结瘤调控基因nodD的作用方式与其存在的拷贝数目和产物NodD蛋白活性有关,同时NodD的敏感性还影响到根瘤菌的宿主范围。结瘤基因的系统发育揭示出根瘤菌宿主范围与共同性结瘤基因间比其它基荫的相关性更高。结瘤基因与豆科宿主之间存在一定的共进化关系。     

根瘤菌结瘤因子的研究进展

根瘤菌结瘤因子的研究进展靖元孝（华南师范大学生物系,广州５１０６３１）关键词根瘤菌结瘤因子Ｒｈｉｚｏｂｉｕｍ、Ｂｒａｄｙｒｈｉｚｏｂｉｕｍ和Ａｚｏｒｈｉ－ｚｏｂｉｕｍ三类细菌能侵染豆科植物并形成根瘤。在根瘤形成过程中,共生伙伴之间首先进行信号物质交换...     

根瘤菌转录因子RluD基因突变抑制大豆共生结瘤

[目的]研究根瘤菌自身转录因子RluD在大豆共生结瘤中的作用,[方法]利用三亲杂交对根瘤菌RluD突变体进行构建,并利用PCR和qRT-PCR对突变体进行验证。利用大豆结瘤实验,测定RluD突变对根瘤菌结瘤能力的影响。最后,利用转录组对RluD突变所影响的信号通路进行了研究。[结果]根瘤菌转录因子RluD突变能够抑制根瘤的产生,显著降低大豆植株根瘤数和根瘤干重,根瘤数由每株17.0下降到12.0,根瘤干重由每株24.0 mg下降到18.0 mg。表达量分析表明,在大豆共生结瘤过程中RluD所调控的基因能够参与到大豆的免疫信号途径。转录组结果表明,转录因子RluD主要参与根瘤菌ABC转运,鞭毛组装,胞外多糖生物合成,细菌分泌系统,碳代谢等通路。[结论]根瘤菌转录因子RluD能够正调控大豆共生结瘤,其突变情况下能够降低29.4%的根瘤数和25.0%的根瘤干重。     

结瘤因子的研究进展

结瘤因子是由根瘤菌产生的一类信号分子,它们在结瘤的起始阶段发挥着十分重要的作用。新近的研究结果证明结瘤因子大分子骨架上的不同侧链基团是决定细菌与宿主植物间相互识别的关键因素,根瘤菌细胞中一系列结瘤基因编码能够合成Lipo-chio-oligosaccharides(LCOs)的各种酶类,进而确定结瘤信号分子的特定结构。目前,一系列令人兴奋的实验结果表明：LCOs不仅可促进豆科作物的生物固氮作用,对一些非豆科作物的细胞分裂作用等同样具有刺激作用。对根瘤菌结瘤因子的研究显然有助于进一步了解细菌与植物的相互作用机理,并进而为农业生产为直接利益。本文在综述这方面的研究进展同时,还就瘤菌,豆科作用和结瘤信号分子之间的相互作用机理,以及根际促生细菌,水杨酸和结瘤信号分子之间的可能关系进行了理论分析。     

紫云英根瘤菌结瘤因子的初步研究

最近的研究结果表明,豆科植物与根瘤菌的共生识别是一种双向的信号物质交换过程.首先是豆科植物的根或种子分泌类黄酮物质,诱导根瘤菌的结瘤基因(nod genes)产生结瘤因子(nod factors),分泌到胞外,为植物所接受,从而引发植物某些基因表达,细胞分化,细胞壁形成,最终导致根毛变形等一系列变化.已经测定了几种苜蓿根瘤菌(Rhizobium meliloti)和豌豆根瘤菌(R.leguminosarum bv.viciae)结瘤因子的分子结构式,它们均属于寡糖胺类物质,在没有根瘤菌存在的条件下,结瘤因子能独立地促使根毛发生变形,这是检测结瘤因子是否存在的重要手段,即根毛变形试验(Root hairdeformation assay,简称Had试验).高浓度的结瘤因子甚至能诱导植物产生空瘤,其组织结构与典型的根瘤相同.     

菜豆根瘤菌sym和mel基因在农杆菌中的转移和表达

豆根瘤菌(Rhizobium phaseoli)共生质粒pSYM3622具有诱导宿主植物(phaseolusvulgaris cv."Jamapa")结瘤固氮的有效基因,以及菜豆根瘤菌特征性的产黑素基因(Melanin production gene)。在诱动因子RP4的存在下,共生质粒能够有效地向三叶草根瘤菌和农杆菌转移。种间和属间杂交子都能诱导菜豆植物形成无效根瘤,并且具备在特定培养基上产生黑素的能力。pSYM3622在三叶草根瘤菌菌株RCR226中具有明显的不亲和性,但是在农杆菌杂交子中,这些结癌和产生黑素的特征在植物根瘤分离菌中能够稳定地保持下去。试验同时研究了pSYM3622向假单胞菌和大肠杆菌的诱动转移。     

根瘤菌脂壳寡糖结瘤因子研究概况

共生固氮是根瘤菌与豆科植物相互作用的结果,它在农业上有重要意义。结瘤与固氮包括一系列复杂的生物学过程,它涉及微生物与植物间专一性识别、信息交换和基因协同表达等方面。近些年研究已经揭示出根瘤菌与豆科植物相互作用分子基础的基本框架。在根瘤的形成过程中,植物与根瘤菌之间首先进行信息交换,促使根瘤菌产生脂壳寡糖类物质。这类脂壳寡糖类物质能引起植物形成根瘤,因此被称为脂壳寡糖结瘤因子（Ｌｉｐｏｃｈｉｔｉｎｏｌｉｇｏｓａｃｃｈａｒｉｄｅｓ）或结瘤因子（ｎｏｄｆａｃｔｏｒｓ）［１］。脂壳寡糖结瘤因子的发现、结…     

紫云英根瘤菌胞外多糖缺失变种及其结瘤性状的变化

根据紫云英根瘤菌在寄主豆科植物紫云英上的结瘤能力,经转座子Tn5诱变获得的18株Exo~-变种可分为4种结瘤类群(A-D):A类变种诱导植物产生小的瘤状突起,不具固氮能力;B类变种形成无效根瘤;C类变种产生固氮效率降低的根瘤;D类变种丧失了结瘤能力。电镜分析显示:无效瘤和瘤状突起中不存在类菌体区,根瘤细胞均为不含细菌的空细胞,侵染线不能穿透到根瘤细胞中。说明紫云英根瘤菌胞外多糖很可能参与有效根瘤的形成。     

Perception and Signal Transduction of Rhizobial NOD Factors

Referee: Dr. Gary Stacey, Director, Center for Legume Research, Department of Microbiology, M409 Walters Life Science Bldg., University of Tennessee, Knoxville, TN 37966-0845 Soil bacteria belonging to genera Rhizobium, Bradyrhizobium, Allorhizobium, Azorhizobium, Mesorhizobium, and Sinorhizobium are able to induce nodule formation on the roots of leguminous plants. In the differentiated root nodules bacteria fix as bacteroids atmospheric nitrogen and deliver it to the host plant. The interaction between bacteria and host plant starts with a complex signal exchange. After induction by plant flavonoids, rhizobia synthesize and secrete lipo-chitooligosaccharides (LCOs), known as Nod factors, which induce morphological changes and expression of early nodulin genes in the roots of host plants. Specific recognition of Nod factors by host plants and early stages of signal transduction are discussed.     

Early interactions between legumes and rhizobia: disclosing complexity in a molecular dialogue

The exchange of chemical signals between soil bacteria (rhizobia) and legumes has been termed a molecular dialogue. As initially conceived in the early 1990s, it involved two main groups of molecules: nod gene-inducing flavonoids from plants and the mitogenic lipochito-oligosaccharide Nod factors of rhizobia. This review considers how subsequent research revealed the existence of a more complex set of interactions, featuring expanded roles for the original participants and contributions from additional plant and bacterial metabolites. Rhizobia respond to chemoattractants and growth-enhancing compounds in root exudates, and several plant nonflavonoids possess nod gene-inducing properties. Expression of non-nod genes is induced by flavonoids; these include encoders of a type I secreted protein and the entire type III, and possibly also type IV, secretion systems. Many other genes and proteins in rhizobia are flavonoid-inducible but their functions are largely unknown. Rhizobia produce far more Nod factor variants than was previously envisaged and their structures can be influenced by the pH of the environment. Other symbiotically active compounds or systems of rhizobia, some of them universally present, are: the surface polysaccharides, quorum-sensing N-acyl homoserine lactones, plant growth-promoting lumichrome and two-component regulatory systems.     

Nod factor-treated Medicago truncatula roots and seeds show an increased number of nodules when inoculated with a limiting population of Sinorhizobium meliloti

Medicago truncatula is a model legume plant that interacts symbiotically with Sinorhizobium meliloti, the alfalfa symbiont. This process involves a molecular dialogue between the bacterium and the plant. Legume roots exude flavonoids that induce the expression of a set of rhizobial genes, the nod genes, which are essential for nodulation and determination of the host range. In turn, nod genes control the synthesis of lipo-chito-oligosaccharides (LCOs), Nod factors, which are bacteria-to-plant signal molecules mediating recognition and nodule organogenesis. M. truncatula roots or seeds have been treated with Nod factors and hydroponically growing seedlings have been inoculated with a limiting population of S. meliloti. It has been shown that submicromolar concentrations of Nod factors increase the number of nodules per plant on M. truncatula. Compared with roots, this increase is more noticeable when seeds are treated. M. truncatula seeds are receptive to submicromolar concentrations of Nod factors, suggesting the possibility of a high affinity LCO perception system in seeds or embryos as well.     

Nod factor structures,responses, and perception during initiation of nodule development

The onset of nodule development, the result of rhizobia-legume symbioses, is determined by the exchange of chemical compounds between microsymbiont and leguminous host plant. Lipo-chitooligosaccharidic nodulation (Nod) factors, secreted by rhizobia, belong to these signal molecules. Nod factors consist of an acylated chitin oligomeric backbone with various substitutions at the (non)reducing-terminal and/or nonterminal residues. They induce the formation and deformation of root hairs, intra- and extracellular alkalinization, membrane potential depolarization, changes in ion fluxes, early nodulin gene expression, and formation of nodule primordia. Nod factors play a key role during nodule initiation and act at nano- to picomolar concentrations. A correct chemical structure is required for induction of a particular plant response, suggesting that Nod factor-receptor interaction(s) precede(s) a Nod factor-induced signal transduction cascade. Current data on Nod factor structures and Nod factor-induced responses are highlighted as well as recent advances in the characterization of proteins, possibly involved in recognition of Nod factors by the host plant.     

LysM domains of Medicago truncatula NFP protein involved in Nod factor perception. Glycosylation state, molecular modeling and docking of chitooligosaccharides and Nod factors

The establishment of the symbiosis between legume plants and rhizobial bacteria depends on the production of rhizobial lipo-chitooligosaccharidic signals (the Nod factors) that are specifically recognized by roots of the host plant. In Medicago truncatula, specific recognition of Sinorhizobium meliloti and its Nod factors requires the NFP (Nod factor perception) gene, which encodes a putative serine/threonine receptor-like kinase (RLK). The extracellular region of this protein contains three tandem lysin motifs (LysMs), a short peptide domain that is implicated in peptidoglycan or chitin binding in various bacterial or eukaryotic proteins, respectively. We report here the homology modeling of the three LysM domains of M. truncatula NFP based on the structure of a LysM domain of the Escherichia coli membrane-bound lytic murein transglycosidase D (MltD). Expression of NFP in a homologous system (M. truncatula roots) revealed that the protein is highly N-glycosylated, probably with both high-mannose and complex glycans. Surface analysis and docking calculations performed on the models of the three domains were used to predict the most favored binding modes for chitooligosaccharides and Nod factors. A convergent model can be proposed where the sulfated, O-acetylated lipo-chitooligosaccharidic Nod factor of S. meliloti binds in similar orientation to the three LysM domains of M. truncatula NFP. N-glycosylation is not expected to interfere with Nod factor binding in this orientation.     

Nod factors stimulate seed germination and promote growth and nodulation of pea and vetch under competitive conditions

Nod factors are lipochitooligosaccharide (LCO) produced by soil bacteria commonly known as rhizobia acting as signals for the legume plants to initiate symbiosis. Nod factors trigger early symbiotic responses in plant roots and initiate the development of specialized plant organs called nodules, where biological nitrogen fixation takes place. Here, the effect of specific LCO originating from flavonoid induced Rhizobium leguminosarum bv. viciae GR09 culture was studied on germination, plant growth and nodulation of pea and vetch. A crude preparation of GR09 LCO significantly enhanced symbiotic performance of pea and vetch grown under laboratory conditions and in the soil. Moreover, the effect of GR09 LCOs seed treatments on the genetic diversity of rhizobia recovered from vetch and pea nodules was presented.     

Recent advances in the study of nod factor perception and signal transduction.

Rhizobial lipochitooligosaccharidic Nod factors mediate the specific recognition between leguminous plants and their prokaryotic symbionts. This review summarizes recent findings on the way plants could perceive and transduce these bacterial signals. It starts by summarizing knowledge about Nod factor binding sites, before moving to the potential implications in Nod factor signal transduction of G proteins, root-hair plasma membrane depolarisation, cytoplasmic and extracellular alkalinisation and finally variations in cytoplasmic calcium concentration.