Shoot control of root development and nodulation is mediated by a receptor-like kinase

In legumes, root nodule organogenesis is activated in response to morphogenic lipochitin oligosaccharides that are synthesized by bacteria, commonly known as rhizobia. Successful symbiotic interaction results in the formation of highly specialized organs called root nodules, which provide a unique environment for symbiotic nitrogen fixation. In wild-type plants the number of nodules is regulated by a signalling mechanism integrating environmental and developmental cues to arrest most rhizobial infections within the susceptible zone of the root. Furthermore, a feedback mechanism controls the temporal and spatial susceptibility to infection of the root system. This mechanism is referred to as autoregulation of nodulation, as earlier nodulation events inhibit nodulation of younger root tissues. Lotus japonicus plants homozygous for a mutation in the hypernodulation aberrant root (har1) locus escape this regulation and form an excessive number of nodules. Here we report the molecular cloning and expression analysis of the HAR1 gene and the pea orthologue, Pisum sativum, SYM29. HAR1 encodes a putative serine/threonine receptor kinase, which is required for shoot-controlled regulation of root growth, nodule number, and for nitrate sensitivity of symbiotic development.     

四棱豆根瘤和类菌体的光学及超微结构

通过光学和电子显微镜观察根瘤切片,作者发现四棱豆(Psophocarpus tetragonolobus(L)DC)的根瘤具有多个感染区,类菌体力椭圆形和不规则的长形菌.包膜(membraneenvelope)包围住一个或多个类菌体,感染细胞边缘的线粒体比未感染的细胞多.根瘤感染区细胞内的根瘤菌体分裂整个过程是在包膜内进行.首先是菌体的核区拉长,随后菌体细胞中央部分的细胞壁凹陷,并逐渐加深,最后一个根瘤菌分裂成两个菌体.     

Sinorhizobium meliloti lsrB is involved in alfalfa root nodule development and nitrogen-fixing bacteroid differentiation

Rhizobia interact with host legumes to induce the formation of nitrogen-fixing nodules, which is very important in agriculture and ecology. The development of nitrogen-fixing nodules is stringently regulated by host plants and rhizobial symbionts. In our previous work, a new Sinorhizobium meliloti LysR regulator gene (lsrB) was identified to be essential for alfalfa nodulation. However, how this gene is involved in alfalfa nodulation was not yet understood. Here, we found that this gene was associated with prevention of premature nodule senescence and abortive bacteroid formation. Heterogeneous deficient alfalfa root nodules were induced by the in-frame deletion mutant of lsrB (lsrB1-2), which was similar to the plasmid-insertion mutant, lsrB1. Irregular senescence zones earlier appeared in these nodules where bacteroid differentiation was blocked at different stages from microscopy observations. Interestingly, oxidative bursts were observed in these nodules by DAB staining. The decreased expression of lipopolysaccharide core genes (lpsCDE) was correspondingly determined in these nodules. S. meliloti lipopolysaccharide is required for suppression of oxidative bursts or host cell defense. These findings demonstrate that the S. meliloti lsrB gene is involved in alfalfa root nodule development and bacteroid differentiation by suppressing oxidative bursts or defense responses in host cells.     

Reactive oxygen species produced by NADPH oxidase regulate plant cell growth

Cell expansion is a central process in plant morphogenesis, and the elongation of roots and root hairs is essential for uptake of minerals and water from the soil. Ca2+ influx from the extracellular store is required for (and sets the rates of) cell elongation in roots. Arabidopsis thaliana rhd2 mutants are defective in Ca2+ uptake and consequently cell expansion is compromised--rhd2 mutants have short root hairs and stunted roots. To determine the regulation of Ca2+ acquisition in growing root cells we show here that RHD2 is an NADPH oxidase, a protein that transfers electrons from NADPH to an electron acceptor leading to the formation of reactive oxygen species (ROS). We show that ROS accumulate in growing wild-type (WT) root hairs but their levels are markedly decreased in rhd2 mutants. Blocking the activity of the NADPH oxidase with diphenylene iodonium (DPI) inhibits ROS formation and phenocopies Rhd2-. Treatment of rhd2 roots with ROS partly suppresses the mutant phenotype and stimulates the activity of plasma membrane hyperpolarization-activated Ca2+ channels, the predominant root Ca2+ acquisition system. This indicates that NADPH oxidases control development by making ROS that regulate plant cell expansion through the activation of Ca2+ channels.     

Sinorhizobium meliloti nifA gene exerts a pleiotropic effect on nodulation through the enhanced plant defense response

Sinorhizobium meliloti nifA gene is required for the expression of a bunch of nif and fix genes. Here, we report its pleiotropic effects on the nodule formation. Compared with wild type strain, nifA mutant sig- nificantly reduced nodule suppression rate in split-root system. The plants inoculated with mutant strain produced lower amount of daidzein and less necrotic cells on their roots. In addition, the defense genes failed to be evoked by nifA mutant at the early nodulation stage. These findings indicated that host defense response was one of the mechanisms mediated by nifA gene to regulate nodule formation during symbiosis. Even though nifA mutant could increase the number of nodules in host plant, it synthesized lower Nod factors than wild type. This suggested that nifA gene mediated multiple and diverse instances in nodulation formation.     

A receptor kinase gene of the LysM type is involved in legume perception of rhizobial signals

Plants belonging to the legume family develop nitrogen-fixing root nodules in symbiosis with bacteria commonly known as rhizobia. The legume host encodes all of the functions necessary to build the specialized symbiotic organ, the nodule, but the process is elicited by the bacteria. Molecular communication initiates the interaction, and signals, usually flavones, secreted by the legume root induce the bacteria to produce a lipochitin-oligosaccharide signal molecule (Nod-factor), which in turn triggers the plant organogenic process. An important determinant of bacterial host specificity is the structure of the Nod-factor, suggesting that a plant receptor is involved in signal perception and signal transduction initiating the plant developmental response. Here we describe the cloning of a putative Nod-factor receptor kinase gene (NFR5) from Lotus japonicus. NFR5 is essential for Nod-factor perception and encodes an unusual transmembrane serine/threonine receptor-like kinase required for the earliest detectable plant responses to bacteria and Nod-factor. The extracellular domain of the putative receptor has three modules with similarity to LysM domains known from peptidoglycan-binding proteins and chitinases. Together with an atypical kinase domain structure this characterizes an unusual receptor-like kinase.     

Host sanctions and the legume-rhizobium mutualism

Explaining mutualistic cooperation between species remains one of the greatest problems for evolutionary biology. Why do symbionts provide costly services to a host, indirectly benefiting competitors sharing the same individual host? Host monitoring of symbiont performance and the imposition of sanctions on 'cheats' could stabilize mutualism. Here we show that soybeans penalize rhizobia that fail to fix N(2) inside their root nodules. We prevented a normally mutualistic rhizobium strain from cooperating (fixing N(2)) by replacing air with an N(2)-free atmosphere (Ar:O(2)). A series of experiments at three spatial scales (whole plants, half root systems and individual nodules) demonstrated that forcing non-cooperation (analogous to cheating) decreased the reproductive success of rhizobia by about 50%. Non-invasive monitoring implicated decreased O(2) supply as a possible mechanism for sanctions against cheating rhizobia. More generally, such sanctions by one or both partners may be important in stabilizing a wide range of mutualistic symbioses.     

Amino-acid cycling drives nitrogen fixation in the legume-Rhizobium symbiosis

The biological reduction of atmospheric N2 to ammonium (nitrogen fixation) provides about 65% of the biosphere's available nitrogen. Most of this ammonium is contributed by legume-rhizobia symbioses, which are initiated by the infection of legume hosts by bacteria (rhizobia), resulting in formation of root nodules. Within the nodules, rhizobia are found as bacteroids, which perform the nitrogen fixation: to do this, they obtain sources of carbon and energy from the plant, in the form of dicarboxylic acids. It has been thought that, in return, bacteroids simply provide the plant with ammonium. But here we show that a more complex amino-acid cycle is essential for symbiotic nitrogen fixation by Rhizobium in pea nodules. The plant provides amino acids to the bacteroids, enabling them to shut down their ammonium assimilation. In return, bacteroids act like plant organelles to cycle amino acids back to the plant for asparagine synthesis. The mutual dependence of this exchange prevents the symbiosis being dominated by the plant, and provides a selective pressure for the evolution of mutualism.     

诱导不结瘤豆科树种结瘤固氮的研究

根据国内外有关诱导非豆科作物结瘤固氮的现状,结合作者几年来的工作,对诱导不结瘤豆科树种结瘤固氮的可行性进行了探讨,并报道了作者近年来人工诱导不结瘤的豆科树种—山皂荚(Gledtsia melanacantha)的试验结果。     

河北省豆科植物根瘤菌资源的初步调查研究

通过对河北省全境豆科植物的结瘤和固氮状况进行调查 ,共采集到 2 4属 5 3种豆科植物的根瘤样品 2 3 5份 ,98.1的被调查豆科植物可自然结瘤 ,96.2的自然瘤可测到固氮酶活性 .其中海边香豌豆 (L athyrus maritimus)、野百合 (Crotalaria sessilif lora)、阴山胡枝子 (L athyrusinschanica)、山岩黄耆 (H edysarum alpinum)、辽西扁苜蓿 (Melilotus ruthenicus)的结瘤固氮情况为首次报道 .     

Characteristics of the Distribution and Control Factors of Polymetallic Nodules in the West Region of CCFZ （China Pioneer Area）

The tectonic setting of the west region of the Reserved Area of the International Seabed Authority is the same as that of China Pioneer Area. They have similar characteristics of topography and nodule distribution. To research and analyse the characteristics and control factors of nodule distribution can help the establishment of a geological model and the assessment of the polymetallic nodule resource. From the year of 1991 till 2000 COMRA (China Ocean Mineral Resource Research and Development Association) carried out phase 1 and 2 of the exploration and resource assessment project of polymetallic nodules in China Pioneer Area. The features of topography and geology, and the distribution of polymetallic nodules and control factors have been clearly understood. The results show that the formation and types and spatial features of abundance and grade of nodules resulted from several factors, the tectonic features (volcanism and faulting)and seafloor topography play an important role in the geological process of the formation of polymetallic nodules.     

铁对刺槐根瘤形成中根部传递细胞诱发的影响

对接种根瘤菌后的刺槐根部进行了光学显微镜和透射电镜观察。结果表明:培养于缺铁培养液中的接菌刺槐苗发生形态变化的根部,在不同时期(根毛密集形变、根部膨大、初期小瘤一直到成瘤)根外层细胞的外切向壁上可形成具壁内突的传递细胞;培养于含铁培养液中的未接菌刺槐苗根部的外层细胞,以及接菌但无形态变化的刺槐菌根部均未观察到传递细胞。电镜观察结果表明,在根瘤形成过程中,根瘤菌是刺槐根外层传递细胞的主要诱因,而缺铁仅是次要因子。     

大豆根毛对大豆根瘤菌的生化吸附及其对结瘤影响的研究

本文研究证明大豆根毛与大豆根瘤菌具有专化性生化吸附作用。吸附主要发生在根毛端部。同一棵根上不是所有的根毛都能发生生化吸附,吸附与根毛的生长令期有关。用13种大豆根瘤菌株系与7个大豆品种根毛生化吸附实验表明,吸附率明显地存在有差异性,但这种差异与结瘤量无相关性。使用本研究发现的生化吸附率高而不能结瘤的1.174大豆根瘤菌株系与吸附率较低而结瘤力强的113—2或61A67大豆根瘤菌株系双接菌,证明能明显降低113—2或地61A76的结瘤量,表现出大豆根毛生化吸附具有竞争性抑制大豆根瘤菌的侵入、结瘤作用,对理论研究和实际应用具有一定重要意义。     

旋扭山绿豆根瘤菌的分离与回接

用纯培养方法,从旋扭山绿豆(Desmodium intortum)根瘤中分离获得快生型和慢生型菌株,经回接试验表明这两种菌株都能使旋扭山绿豆结瘤,但结瘤数,固氮活性及植株生长都有差异.     

Identification of genes induced during Medicago sativa nodule development by using the cDNA-AFLP technique

Under limited nitrogen conditions, rhizobia are ableto induce the formation of nitrogen-fixing root nodules on their leguminous plant host. This organogenetic process is triggered by a complex exchange of molecu- lar signals between the host plant and bac…     

A receptor kinase gene regulating symbiotic nodule development

Leguminous plants are able to establish a nitrogen-fixing symbiosis with soil bacteria generally known as rhizobia. Metabolites exuded by the plant root activate the production of a rhizobial signal molecule, the Nod factor, which is essential for symbiotic nodule development. This lipo-chitooligosaccharide signal is active at femtomolar concentrations, and its structure is correlated with host specificity of symbiosis, suggesting the involvement of a cognate perception system in the plant host. Here we describe the cloning of a gene from Medicago sativa that is essential for Nod-factor perception in alfalfa, and by genetic analogy, in the related legumes Medicago truncatula and Pisum sativum. The identified 'nodulation receptor kinase', NORK, is predicted to function in the Nod-factor perception/transduction system (the NORK system) that initiates a signal cascade leading to nodulation. The family of 'NORK extracellular-sequence-like' (NSL) genes is broadly distributed in the plant kingdom, although their biological function has not been previously ascribed. We suggest that during the evolution of symbiosis an ancestral NSL system was co-opted for transduction of an external ligand, the rhizobial Nod factor, leading to development of the symbiotic root nodule.     

八种豆科树种根瘤的形态与结构研究

对8种豆科树种根瘤的形态和结构进行观察,结果显示:根瘤形态多样,分有限根瘤和无限根瘤两类,根瘤形状、大小与根瘤类型之间有一定的相关性。豆科树种根瘤具有豆科植物根瘤的基本结构,即从外向内依次为皮层、维管束和含菌细胞区,但在皮层中发现有特殊的根瘤形成层细胞,可能与根瘤增粗及根瘤维管束的分化形成有关。豆科树种根瘤中维管束粗而发达,并具有木本植物根的中柱结构,包括有典型的环纹导管、筛管、纤维细胞等。     

中国沙棘营养器官结构特征及其与生境关系的研究

对中国沙棘初生结构的分化和次生生长过程进行了观察。发现沙棘根系角质层发达，外被粘质物，外皮层细胞的细胞壁大量栓质化。根瘤的形成与木质部和皮层有密切关系，弗兰克氏菌的侵入刺激皮层形成大量的薄壁细胞，向外突出形成根瘤。茎的皮层外部细胞较小，排列整齐，细胞壁厚，表皮外有很厚的角质层和一定量的蜡质。叶的结构更加体现了抗旱、耐寒植物的特点，栅栏组织发达，排列紧密，海绵组织不发达，细胞间隙较大，气孔深陷，且具有角质层和密而叠置的星状毛及鳞片等。这些均为生活在旱、寒环境里表现出的适应性。