File Name: symbiotic and nonsymbiotic nitrogen fixation .zip
Access to fixed or available forms of nitrogen limits the productivity of crop plants and thus food production. Nitrogenous fertilizer production currently represents a significant expense for the efficient growth of various crops in the developed world.
There are significant potential gains to be had from reducing dependence on nitrogenous fertilizers in agriculture in the developed world and in developing countries, and there is significant interest in research on biological nitrogen fixation and prospects for increasing its importance in an agricultural setting.
Biological nitrogen fixation is the conversion of atmospheric N 2 to NH 3 , a form that can be used by plants. However, the process is restricted to bacteria and archaea and does not occur in eukaryotes. Symbiotic nitrogen fixation is part of a mutualistic relationship in which plants provide a niche and fixed carbon to bacteria in exchange for fixed nitrogen.
This process is restricted mainly to legumes in agricultural systems, and there is considerable interest in exploring whether similar symbioses can be developed in nonlegumes, which produce the bulk of human food. We are at a juncture at which the fundamental understanding of biological nitrogen fixation has matured to a level that we can think about engineering symbiotic relationships using synthetic biology approaches.
This minireview highlights the fundamental advances in our understanding of biological nitrogen fixation in the context of a blueprint for expanding symbiotic nitrogen fixation to a greater diversity of crop plants through synthetic biology. There is growing interest in increasing the contribution of biological nitrogen fixation to the growth of crop plants in agriculture.
Symbiotic nitrogen fixation is largely limited to legumes in agricultural systems, but there are a number of microorganisms, including some diazotrophs, that inhabit the rhizosphere of other crop plants, some of which have been shown to enhance plant growth.
Here, we present an overview of the diversity and specificities of associations between diazotrophs and their host plants and the biology and biochemistry of these nitrogen-fixing symbiotic associations. Understanding plant and microbe mechanisms involved in the formation and functions of these symbioses to solve the nitrogen fixation problem will position us to engineer these processes into nonfixing food crops, such as cereals and agriculturally important eudicots.
Initial challenges include identifying a suitable microbial partner, initiating intracellular accommodation, controlling the plant microbiome, and keeping cheaters under control. We discuss perspectives and limitations to engineering a nitrogen-fixing ability in plants based on knowledge of symbiotic nitrogen fixation in legumes and nonlegumes.
Nitrogen-fixing bacteria are found in several phyla 1 , and representatives from most if not all of these phyla are known to engage in nitrogen-fixing symbiosis with plants 2. Reciprocally, plants have developed multiple solutions to associate with and accommodate diazotrophs in order to acquire atmospheric nitrogen. Proximity between a bacterial symbiont and plant host is a key element for nutrient exchanges between them and falls into three broad categories, based on the degree of intimacy and interdependency of the plant and microbe: loose associations with free-living nitrogen fixers, intercellular endophytic associations, and endosymbioses.
Interactions between plants and associative nitrogen-fixing bacteria, which are considered a subset of plant growth-promoting rhizobacteria PGPR Fig. These associative bacteria respond to root exudates via chemotaxis to, and colonization of, the rhizosphere of many plants but typically do not invade plant tissues 3 , 4.
Nitrogen-fixing PGPR have been identified among the bacilli and especially among the proteobacteria 5. Their proximity to the root enables them to impact plant resource acquisition nitrogen, phosphorus, and essential minerals , yield, and growth 6.
Some of the best-studied species of associative PGPR belong to the genus Azospirillum , which are able to improve the fitness of several crops, including wheat, maize, and rice 7.
Azolla ferns, which have been used as companion plants in rice agriculture for centuries, accommodate the heterocystous cyanobacterium Nostoc azollae formerly Anabaena azollae within specialized leaf cavities 8. Schematic representation of the different associations between diazotrophs and plant hosts. Diazotrophs are divided in two main groups: root-nodule bacteria and plant growth-promoting rhizobacteria PGPR. Root-nodule bacteria include rhizobia and Frankia.
Rhizobia alpha- and betaproteobacteria enter into a symbiotic association with legumes and Frankia with actinorhizal plants. Alphaproteobacteria can also nodulate Parasponia species. Some plants develop endosymbiotic interactions with nitrogen-fixing cyanobacteria Nostoc. PGPRs include proteobacteria alpha-, beta-, and gammaproteobacteria , actinobacteria, bacilli, and cyanobacteria. Many PGPRs develop associative or endophytic associations with cereals.
Some cyanobacteria found within plant tissues are classified as endophytes. Many species of diazotrophic bacteria have evolved beyond surface colonization to spread and multiply within plant tissues without causing damage and eliciting significant defense reactions.
These bacteria, such as Azoarcus , Herbaspirillum , and Gluconacetobacter Fig. Bacterial endophytes are ubiquitous and have been isolated from surface-sterilized tissue from almost all plants examined to date Their association can be obligate or facultative, and they exhibit complex interactions with their hosts that range from mutualism to parasitism.
They typically enter plant tissues through natural openings stomata or through cracks at the site of lateral root emergence, for instance Research on bacterial endophytes has mainly focused on quantifying the amount of nitrogen fixed and on identification of the diazotrophs; consequently, very little is known about the molecular mechanisms involved in forming and maintaining the cooperation. Cyanobacteria are also frequently found within plant tissues.
Nostoc is endophytic with two genera of liverworts Blasia and Cavicularia and all hornworts. Colonization can take place in dome-shaped auricles on the thallus of liverworts or in slime cavities of the thallus or mucilage-filled canals that run parallel to the thallus of hornworts Nostoc is also able to endophytically colonize coralloid roots of cycads.
The mechanism of recruitment is unknown, but the cyanobacteria are found embedded in mucilage in a specific cortical layer of the coralloid root between elongated specialized cells The most elaborate form of nitrogen-fixing plant microbe association is endosymbiosis.
Bacterial endosymbionts are generally acquired from the environment and are accommodated inside plant cells within plant-derived membranes. Some plants interact with nitrogen-fixing cyanobacteria. In the symbiosis between plants of the genus Gunnera and cyanobacteria of the genus Nostoc , seedlings recruit the endosymbiont by secretion of carbohydrate-rich mucilage.
Nostoc subsequently enters through specialized glands and then is accommodated within cells of the inner cortex. Filaments of Nostoc are surrounded by the host's plasma membrane, which acts as the interface for nutrient exchange The most well-studied plant endosymbioses are those between actinorhizal plants and Frankia bacteria and between legumes and rhizobia, which we will discuss in more depth below.
The establishment and functioning of an effective symbiosis is dependent on genetic determinants in both plant and bacteria. The fully compatible symbiosis proceeds from recognition, penetration, stimulation of host cell division, and differentiation of the endosymbiont.
Legume- Rhizobium symbiosis starts with a molecular dialogue between the two partners. The legume secrete a cocktail of phenolic molecules, predominantly flavonoids and isoflavonoids, into the rhizosphere. These signals are taken up by rhizobia, bind the transcriptional regulator NodD, and activate a suite of bacterial nodulation genes These nodulation genes are responsible for the production of lipochitooligosaccharides LCOs called Nod factors.
Nod factors are key symbiotic signals and are indispensable in the specific host- Rhizobium interaction and at later stages in the infection process and nodule organogenesis Nod factors are active at very low concentrations nanomolar to picomolar range. Nod factors from different rhizobia share the same chitin-like N -acetylglucosamine oligosaccharide backbone with a fatty acyl chain at the nonreducing end, but they differ in the length of their backbone, size and saturation of the fatty acyl chain, and have additional modifications at either end, such as glycosylation and sulfation.
Such decorations on the ends of LCOs play a crucial role in determining whether the Nod factors can be perceived by a specific host It has been demonstrated by genetic and molecular analyses in pea, soybean, and Lotus japonicus that NFRs are host determinants of symbiosis specificity 17 , — Nod factors trigger plant cell division and meristem formation, and the rhizobia infect legume roots through crack entry, intercellular colonization of epidermal cells, or the well-studied formation of infection threads Rhizobia eventually enter root cortical cells via endocytosis, where they differentiate into nitrogen-fixing bacteroids within a unique plant organelle called the symbiosome.
The symbiosome is delimited by a plant-derived membrane that controls nutrient exchange between the symbionts. Two main types of nodules are formed on the various legume species, indeterminate or determinate, depending on whether or not the meristem remains active for the life of the nodule, respectively. Both of types of legume nodules have a peripheral vasculature, in contrast to roots The strategies used by Frankia spp.
Depending on both the host species and Frankia clade, root hair, crack entry, or an intercellular infection mode is employed Actinorhizal nodules are indeterminate, have a central vasculature like roots , and fix nitrogen in amounts comparable to legumes.
In addition to legumes and actinorhizal plants, Parasponia andersonii family Cannabaceae displays a unique nitrogen-fixing symbiosis, as it is the only nonlegume known to be nodulated by rhizobia 23 Fig. Rhizobia invade Parasponia spp. The rhizobia are never released into cells in symbiosomes, nor do they terminally differentiate. Because Parasponia evolved this ability relatively recently, it has been suggested that it represents a fairly primitive form of nodulation The degree of specificity between legumes and rhizobia varies.
For example, although the Nod factors produced by Rhizobium etli and Rhizobium loti are identical, the two species have distinct host ranges Phaseolus spp. Furthermore, two rhizobia that nodulate the same plant may secrete different Nod factors. Rhizobium tropici and R. Likewise, Bradyrhizobium elkanii , Bradyrhizobium japonicum , Rhizobium sp. They have been reported in numerous studies as being symbiotically important, and depending on the particular system, a defect in surface polysaccharides may cause failure of symbiosis at either an early or late stage 27 , — It was recently reported that some strains of Frankia possess the ability to produce LCOs 33 , but the majority of Frankia strains employ an unknown signal that is may be structurally unrelated to LCOs In Azolla - Nostoc symbiosis, specificity is maintained by vertical inheritance of the cyanobacterium.
During sporulation, Nostoc filaments are packaged into sporocarps by sporangial pair hairs and retained until nutrient exchange can be reestablished during embryogenesis. This specificity has been maintained over the course of evolution, with the cyanobacteria cospeciating with the fern In the Gunnera - Nostoc symbiosis, the flow of mucilage excludes most bacteria, and only compatible symbionts achieve intracellular infection.
Elements in the mucilage of all Nostoc hosts act as chemoattractants and induce differentiation into specialized motile filaments called hormogonia 12 , All plants release a significant amount of organic carbon into the soil in the form of cell lysates, intact border cells, mucilage, and root exudates The amount and type of exudates depend on plant genotype and growth stage, vary across different environmental conditions soil type, soil moisture, nutrient availability, or toxicity , and are greatly affected by the organisms living in the rhizosphere.
Exudates are complex mixtures of low-molecular-weight organic substances, like sugars, amino and organic acids, fatty acids, sterols, growth factors, and vitamins It is well known that root exudates can influence the soil microbial community structure and biogeochemical cycles of key nutrients, such as nitrogen and phosphorous The composition of exudates is highly varied between plant species and allows the recruitment of unique populations of prokaryotes and eukaryotes Plants can enrich their rhizosphere with specific microbiota by the secretion of particular carbon sources.
For example, dicarboxylates in tomato root exudates favor the growth of Pseudomonas biocontrol strains 40 , Pea plants select for their symbiont Rhizobium leguminosarum by the excretion of homoserine into the rhizosphere 42 , In fact, Rhizobium leguminosarum has been shown to contain a pea-rhizosphere-specific plasmid that is globally upregulated in the pea rhizosphere Root exudates also play an important role in plant defense through the secretion of phytochemicals that can inhibit the growth of certain microbes The ability to tolerate these chemicals can play an important role in the ability to colonize the plant.
For example, the PGPR Pseudomonas putida is both tolerant of and attracted by the main antimicrobial benzoxazinoid produced by maize Pseudomonads also possess specialized gene sets that allow them to overcome nonhost isothiocyanate resistance in Arabidopsis
Metrics details. Nitrogen, a component of many bio-molecules, is essential for growth and development of all organisms. Most nitrogen exists in the atmosphere, and utilisation of this source is important as a means of avoiding nitrogen starvation. However, the ability to fix atmospheric nitrogen via the nitrogenase enzyme complex is restricted to some bacteria. Eukaryotic organisms are only able to obtain fixed nitrogen through their symbiotic interactions with nitrogen-fixing prokaryotes. These symbioses involve a variety of host organisms, including animals, plants, fungi and protists.
Nitrogen fixation is a chemical process by which molecular nitrogen N 2 in the air is converted into ammonia NH 3 or related nitrogenous compounds in soil. Biological nitrogen fixation or diazotrophy is an important microbially mediated process that converts dinitrogen N 2 gas to ammonia NH 3 using the nitrogenase protein complex Nif. Nitrogen fixation is essential to life because fixed inorganic nitrogen compounds are required for the biosynthesis of all nitrogen-containing organic compounds , such as amino acids and proteins , nucleoside triphosphates and nucleic acids. As part of the nitrogen cycle , it is essential for agriculture and the manufacture of fertilizer. It is also, indirectly, relevant to the manufacture of all nitrogen chemical compounds, which includes some explosives, pharmaceuticals, and dyes. Nitrogen fixation is carried out naturally in soil by microorganisms termed diazotrophs that include bacteria such as Azotobacter and archaea.
Non-symbiotic nitrogen (N2) fixation by diazotrophic bacteria is a potential source for biological N inputs in non-leguminous crops and pastures.
Part of ammonification. Nitrogen is the most important, limiting element for plant production. Nitrogen fixation is an extremely important concept in agriculture, where many crops cannot perform nitrogen fixation … See more. These lightning and radiations splits the molecular nitrogen into nitrogen atom. It then combines with hydrogen or oxygen of atmospheric water forming the ammonia or nitric oxides … Ammonification.
Nitrogen-fixing bacteria , microorganisms capable of transforming atmospheric nitrogen into fixed nitrogen inorganic compounds usable by plants. More than 90 percent of all nitrogen fixation is effected by these organisms, which thus play an important role in the nitrogen cycle. Nitrogen is a component of proteins and nucleic acids and is essential to life on Earth.
Access to fixed or available forms of nitrogen limits the productivity of crop plants and thus food production. Nitrogenous fertilizer production currently represents a significant expense for the efficient growth of various crops in the developed world. There are significant potential gains to be had from reducing dependence on nitrogenous fertilizers in agriculture in the developed world and in developing countries, and there is significant interest in research on biological nitrogen fixation and prospects for increasing its importance in an agricultural setting. Biological nitrogen fixation is the conversion of atmospheric N 2 to NH 3 , a form that can be used by plants. However, the process is restricted to bacteria and archaea and does not occur in eukaryotes. Symbiotic nitrogen fixation is part of a mutualistic relationship in which plants provide a niche and fixed carbon to bacteria in exchange for fixed nitrogen.
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Symbiotic nitrogen-fixing bacterial populations trapped from soils under agroforestry systems in the Western Amazon. Cowpea Vigna unguiculata is an important grain-producing legume that can forego nitrogen fertilization by establishing an efficient symbiosis with nitrogen-fixing bacteria. Although inoculating strains have already been selected for this species, little is known about the genotypic and symbiotic diversity of native rhizobia. Recently, Bradyrhizobium has been shown to be the genus most frequently trapped by cowpea in agricultural soils of the Amazon region. We investigated the genetic and symbiotic diversity of bacterial strains with different phenotypic and cultural properties isolated from the nodules of the trap species cowpea, which was inoculated with samples from soils under agroforestry systems from the western Amazon. Sixty non-nodulating strains indicated a high frequency of endophytic strains in the nodules. The 88 authenticated strains had varying symbiotic efficiency.
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