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53-405: Rhizobia are a "group of soil bacteria that infect the roots of legumes to form root nodules ". Rhizobia are found in the soil and, after infection, produce nodules in the legume where they fix nitrogen gas (N 2 ) from the atmosphere, turning it into a more readily useful form of nitrogen. From here, the nitrogen is exported from the nodules and used for growth in the legume. Once the legume dies,

106-414: A classic tragedy of the commons scenario presents itself. Cheater strains may hoard plant resources such as polyhydroxybutyrate for the benefit of their own reproduction without fixing an appreciable amount of nitrogen . Given the costs involved in nodulation and the opportunity for rhizobia to cheat, it may be surprising that this symbiosis exists. The formation of the symbiotic relationship involves

159-425: A mutual relationship with host sanctions pushes the relationship toward mutualism rather than parasitism and is likely a contributing factor to why the symbiosis exists. However, other studies have found no evidence of plant sanctions. The partner choice hypothesis proposes that the plant uses prenodulation signals from the rhizobia to decide whether to allow nodulation, and chooses only noncheating rhizobia. There

212-447: A nodule, some of the bacteria differentiate into nitrogen fixing bacteroids, which have been found to be unable to reproduce. Therefore, with the development of a symbiotic relationship, if the host sanctions hypothesis is correct, the host sanctions must act toward whole nodules rather than individual bacteria because individual targeting sanctions would prevent any reproducing rhizobia from proliferating over time. This ability to reinforce

265-413: A signal exchange between both partners that leads to mutual recognition and the development of symbiotic structures. The most well understood mechanism for the establishment of this symbiosis is through intracellular infection. Rhizobia are free living in the soil until they are able to sense flavonoids , derivatives of 2-phenyl-1.4-benzopyrone, which are secreted by the roots of their host plant, triggering

318-452: A significant amount of nitrogen and allow the pines to colonize nutrient-poor sites. Polyhydroxybutyrate Polyhydroxybutyrate ( PHB ) is a polyhydroxyalkanoate (PHA), a polymer belonging to the polyesters class that are of interest as bio-derived and biodegradable plastics . The poly-3-hydroxybutyrate (P3HB) form of PHB is probably the most common type of polyhydroxyalkanoate, but other polymers of this class are produced by

371-399: A single homeotic mutation. Legumes release organic compounds as secondary metabolites called flavonoids from their roots, which attract the rhizobia to them and which also activate nod genes in the bacteria to produce nod factors and initiate nodule formation. These nod factors initiate root hair curling . The curling begins with the very tip of the root hair curling around

424-566: A symbiotic relationship with a host-specific strain of bacteria known as rhizobia . This process has evolved multiple times within the legumes, as well as in other species found within the Rosid clade. Legume crops include beans , peas , and soybeans . Within legume root nodules, nitrogen gas (N 2 ) from the atmosphere is converted into ammonia (NH 3 ), which is then assimilated into amino acids (the building blocks of proteins), nucleotides (the building blocks of DNA and RNA as well as

477-466: A tendency to develop lateral roots in response to abscisic acid may enable the later evolution of root nodules. Some fungi produce nodular structures known as tuberculate ectomycorrhizae on the roots of their plant hosts. Suillus tomentosus , for example, produces these structures with its plant host lodgepole pine ( Pinus contorta var. latifolia ). These structures have, in turn, been shown to host nitrogen fixing bacteria , which contribute

530-547: A variety of non- symbiotic bacteria. For instance, the plant pathogen Agrobacterium is a closer relative of Rhizobium than the Bradyrhizobium that nodulate soybean. Although much of the nitrogen is removed when protein -rich grain or hay is harvested , significant amounts can remain in the soil for future crops. This is especially important when nitrogen fertilizer is not used, as in organic rotation schemes or in some less- industrialized countries. Nitrogen

583-432: A variety of organisms: these include poly-4-hydroxybutyrate (P4HB), polyhydroxyvalerate (PHV), polyhydroxyhexanoate (PHH), polyhydroxyoctanoate (PHO) and their copolymers . PHB is produced by microorganisms (such as Cupriavidus necator , Methylobacterium rhodesianum or Bacillus megaterium ) apparently in response to conditions of physiological stress; mainly conditions in which nutrients are limited. The polymer

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636-461: Is being done on North American legumes. Rhizobia are a paraphyletic group that fall into two classes of Pseudomonadota —the alphaproteobacteria and betaproteobacteria . As shown below, most belong to the order Hyphomicrobiales , but several rhizobia occur in distinct bacterial orders of the Pseudomonadota. Alphaproteobacteria Betaproteobacteria These groups include

689-427: Is currently used in the medical industry for internal suture . It is nontoxic and biodegradable, so it does not have to be removed after recovery. TephaFLEX is a bacterially derived poly-4-hydroxybutyrate, manufactured using a recombinant fermentation process by Tepha Medical Devices, intended for a variety of medical applications that require biodegradable materials such as absorbable sutures . Polyhydroxybutyrate

742-416: Is evidence for sanctions in soybean plants, which reduce rhizobium reproduction (perhaps by limiting oxygen supply) in nodules that fix less nitrogen. Likewise, wild lupine plants allocate fewer resources to nodules containing less-beneficial rhizobia, limiting rhizobial reproduction inside. This is consistent with the definition of sanctions, although called "partner choice" by the authors. Some studies support

795-511: Is helpful to compare the rhizobia-legume symbiosis to a more ancient symbiotic relationship, such as that between endomycorrhizae fungi and land plants, which dates back to almost 460 million years ago. Endomycorrhizal symbiosis can provide many insights into rhizobia symbiosis because recent genetic studies have suggested that rhizobia co-opted the signaling pathways from the more ancient endomycorrhizal symbiosis. Bacteria secrete Nod factors and endomycorrhizae secrete Myc-LCOs. Upon recognition of

848-414: Is observed. Instead, the bacteria penetrate between cells through cracks produced by lateral root emergence. Inside the nodule, the bacteria differentiate morphologically into bacteroids and fix atmospheric nitrogen into ammonium using the enzyme nitrogenase . Ammonium is then converted into amino acids like glutamine and asparagine before it is exported to the plant. In return, the plant supplies

901-413: Is primarily a product of carbon assimilation (from glucose or starch ) and is employed by microorganisms as a form of energy storage molecule to be metabolized when other common energy sources are not available. Microbial biosynthesis of PHB starts with the condensation of two molecules of acetyl-CoA to give acetoacetyl-CoA which is subsequently reduced to hydroxybutyryl-CoA. This latter compound

954-822: Is the most commonly deficient nutrient in many soils around the world and it is the most commonly supplied plant nutrient. The supply of nitrogen through fertilizers has severe environmental concerns . Specific strains of rhizobia are required to make functional nodules on the roots able to fix the N 2 . Having this specific rhizobia present is beneficial to the legume, as the N 2 fixation can increase crop yield. Inoculation with rhizobia tends to increase yield. Rhizobia has been found to increase legume resistance to insect herbivores, particularly when several species of rhizobia are present. Legume inoculation has been an agricultural practice for many years and has continuously improved over time. 12–20 million hectares of soybeans are inoculated annually. An ideal inoculant includes some of

1007-424: Is the most widely studied type of nodule, but the details are quite different in nodules of peanut and relatives and some other important crops such as lupins where the nodule is formed following direct infection of rhizobia through the epidermis and where infection threads are never formed. Nodules grow around the root, forming a collar-like structure. In these nodules and in the peanut type the central infected tissue

1060-405: Is then used as a monomer to polymerize PHB. PHAs granules are then recovered by disrupting the cells. Most commercial plastics are synthetic polymers derived from petrochemicals . They tend to resist biodegradation . PHB-derived plastics are attractive because they are compostable and derived from renewables and are bio-degradable. ICI had developed the material to pilot plant stage in

1113-429: Is uniform, lacking the uninfected ells seen in nodules of soybean and many indeterminate types such as peas and clovers. Actinorhizal-type nodules are markedly different structures found in non-legumes. In this type, cells derived from the root cortex form the infected tissue, and the prenodule becomes part of the mature nodule. Despite this seemingly major difference, it is possible to produce such nodules in legumes by

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1166-451: The Rhizobium . Within the root tip, a small tube called the infection thread forms, which provides a pathway for the Rhizobium to travel into the root epidermal cells as the root hair continues to curl. Partial curling can even be achieved by nod factor alone. This was demonstrated by the isolation of nod factors and their application to parts of the root hair. The root hairs curled in

1219-506: The actinomycete , Frankia , which form symbiotic root nodules in actinorhizal plants , although these bacteria have a much broader host range, implying the association is less specific than in legumes. Additionally, several cyanobacteria like Nostoc are associated with aquatic ferns , Cycas , and Gunneras , although they do not form nodules. Additionally, loosely associated plant bacteria, termed endophytes , have been reported to fix nitrogen in planta . These bacteria colonize

1272-432: The legume family – Fabaceae  – with taxa such as kudzu , clovers , soybeans , alfalfa , lupines , peanuts , and rooibos . They contain symbiotic bacteria called rhizobia within the nodules, producing nitrogen compounds that help the plant to grow and compete with other plants. When the plant dies, the fixed nitrogen is released, making it available to other plants, and this helps to fertilize

1325-500: The root cortex ; they are then surrounded by a plant-derived symbiosome membrane and differentiate into bacteroids that fix nitrogen . Effective nodulation takes place approximately four weeks after crop planting , with the size, and shape of the nodules dependent on the crop. Crops such as soybeans, or peanuts will have larger nodules than forage legumes such as red clover, or alfalfa, since their nitrogen needs are higher. The number of nodules, and their internal color, will indicate

1378-427: The soil . The great majority of legumes have this association, but a few genera (e.g., Styphnolobium ) do not. In many traditional farming practices, fields are rotated through various types of crops, which usually includes one consisting mainly or entirely of a leguminous crop such as clover, in order to take advantage of this. Although by far the majority of plants able to form nitrogen-fixing root nodules are in

1431-491: The 1980s, but interest faded when it became clear that the cost of material was too high, and its properties could not match those of polypropylene . Some bottles were made for Wella's "Sanara" range of shampoo; an example using the tradename "Biopol" is in the collection of the Science Museum , London . In 1996 Monsanto (who sold PHB as a copolymer with PHV) bought all patents for making the polymer from ICI/Zeneca including

1484-458: The Nod factor/Myc-LCO, the plant proceeds to induce a variety of intracellular responses to prepare for the symbiosis. It is likely that rhizobia co-opted the features already in place for endomycorrhizal symbiosis because there are many shared or similar genes involved in the two processes. For example, the plant recognition gene SYMRK (symbiosis receptor-like kinase) is involved in the perception of both

1537-538: The accumulation of a large population of cells and eventually attachment to root hairs . These flavonoids then promote the DNA binding activity of NodD, which belongs to the LysR family of transcriptional regulators and triggers the secretion of nod factors after the bacteria have entered the root hair. Nod factors trigger a series of complex developmental changes inside the root hair, beginning with root hair curling and followed by

1590-605: The axils of lateral or adventitious roots and are formed following infection via cracks where these roots emerge and not using root hairs . Their internal structure is quite different from those of the soybean type of nodule. Indeterminate nodules are found in the majority of legumes from all three sub-families, whether in temperate regions or in the tropics. They can be seen in Faboideae legumes such as Pisum (pea), Medicago (alfalfa), Trifolium (clover), and Vicia (vetch) and all mimosoid legumes such as acacia s,

1643-509: The bacteria with carbohydrates in the form of organic acids. The plant also provides the bacteroid oxygen for cellular respiration , tightly bound by leghaemoglobins , plant proteins similar to human hemoglobins . This process keeps the nodule oxygen poor in order to prevent the inhibition of nitrogenase activity. Recently, a Bradyrhizobium strain was discovered to form nodules in Aeschynomene without producing nod factors, suggesting

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1696-414: The direction of the application, demonstrating the action of a root hair attempting to curl around a bacterium. Even application on lateral roots caused curling. This demonstrated that it is the nod factor itself, not the bacterium that causes the stimulation of the curling. When the nod factor is sensed by the root, a number of biochemical and morphological changes happen: cell division is triggered in

1749-483: The existence of alternative communication signals other than nod factors, possibly involving the secretion of the plant hormone cytokinin. It has been observed that root nodules can be formed spontaneously in Medicago without the presence of rhizobia. This implies that the development of the nodule is controlled entirely by the plant and simply triggered by the secretion of nod factors . There are two main hypotheses for

1802-461: The few nodulated caesalpinioid legumes such as partridge pea . They earned the name "indeterminate" because they maintain an active apical meristem that produces new cells for growth over the life of the nodule. This results in the nodule having a generally cylindrical shape, which may be extensively branched. Because they are actively growing, indeterminate nodules manifest zones which demarcate different stages of development/symbiosis: This

1855-825: The first lineage to branch off; thus, the ability to fix nitrogen may be plesiomorphic and subsequently lost in most descendants of the original nitrogen-fixing plant; however, it may be that the basic genetic and physiological requirements were present in an incipient state in the last common ancestors of all these plants, but only evolved to full function in some of them: Betulaceae : Alnus (alders) Cannabaceae : Trema Casuarinaceae : ...... Coriariaceae : Coriaria Datiscaceae : Datisca Elaeagnaceae : ...... Myricaceae : ...... Rhamnaceae : ...... Rosaceae : Two main types of nodule have been described in legumes: determinate and indeterminate. Determinate nodules are found on certain tribes of tropical legume such as those of

1908-429: The following aspects; maximum efficacy, ease of use, compatibility, high rhizobial concentration, long shelf-life, usefulness under varying field conditions, and survivability. These inoculants may foster success in legume cultivation. As a result of the nodulation process, after the harvest of the crop, there are higher levels of soil nitrate, which can then be used by the next crop. Rhizobia are unique in that they are

1961-438: The formation of the infection thread, a cellulose lined tube that the bacteria use to travel down through the root hair into the root cells. The bacteria then infect several other adjacent root cells. This is followed by continuous cell proliferation, resulting in the formation of the root nodule . A second mechanism, used especially by rhizobia that infect aquatic hosts, is called crack entry. In this case, no root hair deformation

2014-453: The genera Glycine (soybean), Phaseolus (common bean), and Vigna . and on some temperate legumes such as Lotus . These determinate nodules lose meristematic activity shortly after initiation, thus growth is due to cell expansion resulting in mature nodules which are spherical in shape. Another type of determinate nodule is found in a wide range of herbs, shrubs and trees, such as Arachis ( peanut ). These are always associated with

2067-404: The important energy molecule ATP ), and other cellular constituents such as vitamins , flavones , and hormones . Their ability to fix gaseous nitrogen makes legumes an ideal agricultural organism as their requirement for nitrogen fertilizer is reduced. Indeed, high nitrogen content blocks nodule development as there is no benefit for the plant of forming the symbiosis. The energy for splitting

2120-466: The intercellular spaces of leaves, stems, and roots in plants but do not form specialized structures like rhizobia and Frankia . Diazotrophic bacterial endophytes have very broad host ranges, in some cases colonizing both monocots and dicots . Root nodules Root nodules are found on the roots of plants , primarily legumes , that form a symbiosis with nitrogen-fixing bacteria . Under nitrogen -limiting conditions, capable plants form

2173-530: The legume family Fabaceae , there are a few exceptions: The ability to fix nitrogen is far from universally present in these families. For instance, of 122 genera in the Rosaceae , only 4 genera are capable of fixing nitrogen. All these families belong to the orders Cucurbitales , Fagales , and Rosales , which together with the Fabales form a nitrogen-fixing clade (NFC) of eurosids . In this clade, Fabales were

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2226-443: The legume family has emerged and evolved over the past 66 million years. Although evolution tends to swing toward one species taking advantage of another in the form of noncooperation in the selfish-gene model, management of such symbiosis allows for the continuation of cooperation. When the relative fitness of both species is increased, natural selection will favor symbiosis. To understand the evolutionary history of this symbiosis, it

2279-608: The loss of AON receptor kinase activity, suggesting that nodule growth and root development are functionally linked. Investigations into the mechanisms of nodule formation showed that the ENOD40 gene, coding for a 12–13 amino acid protein [41], is up-regulated during nodule formation [3]. Root nodules apparently have evolved three times within the Fabaceae but are rare outside that family. The propensity of these plants to develop root nodules seems to relate to their root structure. In particular,

2332-605: The mechanism that maintains legume-rhizobium symbiosis (though both may occur in nature). The sanctions hypothesis theorizes that legumes cannot recognize the more parasitic or less nitrogen fixing rhizobia and must counter the parasitism by post-infection legume sanctions. In response to underperforming rhizobia, legume hosts can respond by imposing sanctions of varying severity to their nodules. These sanctions include, but are not limited to, reduction of nodule growth, early nodule death, decreased carbon supply to nodules, or reduced oxygen supply to nodules that fix less nitrogen. Within

2385-489: The nitrogen gas in the nodule comes from sugar that is translocated from the leaf (a product of photosynthesis ). Malate as a breakdown product of sucrose is the direct carbon source for the bacteroid. Nitrogen fixation in the nodule is very oxygen sensitive. Legume nodules harbor an iron containing protein called leghaemoglobin , closely related to animal myoglobin , to facilitate the diffusion of oxygen gas used in respiration. Plants that contribute to N2 fixation include

2438-487: The nodule breaks down and releases the rhizobia back into the soil, where they can live individually or reinfect a new legume host. The first known species of rhizobia, Rhizobium leguminosarum , was identified in 1889, and all further species were initially placed in the Rhizobium genus . Most research has been done on crop and forage legumes such as clover , alfalfa , beans , peas , and soybeans ; more research

2491-490: The only nitrogen-fixing bacteria living in a symbiotic relationship with legumes . Common crop and forage legumes are peas, beans, clover, and soy. The legume–rhizobium symbiosis is a classic example of mutualism —rhizobia supply ammonia or amino acids to the plant and, in return, receive organic acids (mainly malate and succinate , which are dicarboxylic acids ) as a carbon and energy source. However, because several unrelated strains infect each individual plant,

2544-413: The partner choice hypothesis. While both mechanisms no doubt contribute significantly to maintaining rhizobial cooperation, they do not in themselves fully explain the persistence of mutualism . The partner choice hypothesis is not exclusive from the host sanctions hypothesis, as it is apparent that both of them are prevalent in the symbiotic relationship. The symbiosis between nitrogen fixing rhizobia and

2597-624: The rhizobial Nod factors as well as the endomycorrhizal Myc-LCOs. The shared similar processes would have greatly facilitated the evolution of rhizobial symbiosis because not all the symbiotic mechanisms would have needed to develop. Instead, the rhizobia simply needed to evolve mechanisms to take advantage of the symbiotic signaling processes already in place from endomycorrhizal symbiosis. Many other species of bacteria are able to fix nitrogen ( diazotrophs ), but few are able to associate intimately with plants and colonize specific structures like legume nodules. Bacteria that do associate with plants include

2650-428: The root to create the nodule, and the root hair growth is redirected to curl around the bacteria multiple times until it fully encapsulates one or more bacteria. The bacteria encapsulated divide multiple times, forming a microcolony . From this microcolony, the bacteria enter the developing nodule through the infection thread, which grows through the root hair into the basal part of the epidermis cell, and onwards into

2703-888: The status of nitrogen fixation in the plant. Nodulation is controlled by a variety of processes, both external (heat, acidic soils, drought, nitrate) and internal (autoregulation of nodulation, ethylene). Autoregulation of nodulation controls nodule numbers per plant through a systemic process involving the leaf. Leaf tissue senses the early nodulation events in the root through an unknown chemical signal, then restricts further nodule development in newly developing root tissue. The Leucine rich repeat (LRR) receptor kinases (NARK in soybean ( Glycine max ); HAR1 in Lotus japonicus , SUNN in Medicago truncatula ) are essential for autoregulation of nodulation (AON). Mutation leading to loss of function in these AON receptor kinases leads to supernodulation or hypernodulation. Often root growth abnormalities accompany

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2756-454: The trademark "Biopol" [1] . However, Monsanto's rights to Biopol were sold to the American company Metabolix in 2001 and Monsanto's fermenters producing PHB from bacteria were closed down at the start of 2004. Monsanto began to focus on producing PHB from plants instead of bacteria. But now with so much media attention on GM crops, there has been little news of Monsanto's plans for PHB. Biopol

2809-692: Was first isolated and characterized in 1925 by French microbiologist Maurice Lemoigne . Firmicutes and proteobacteria can degrade PHB. Bacillus , Pseudomonas and Streptomyces species can degrade PHB. Pseudomonas lemoigne , Comamonas sp. Acidovorax faecalis , Aspergillus fumigatus and Variovorax paradoxus are soil microbes capable of degradation. Alcaligenes faecalis , Pseudomonas , and Illyobacter delafieldi , are obtained from anaerobic sludge. Comamonas testosteroni and Pseudomonas stutzeri were obtained from sea water. Few of these are capable of degrading at higher temperatures; notably excepting thermophilic Streptomyces sp. and

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