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Wel-comeMr. Sandesh Pawar
Dept. of Plant PathologyDr. PDKV., Akola
Rhizobium is a genus of Gram negative soil bacteria that fix nitrogen.Rhizobium forms an endosymbiotic nitrogen fixing association with roots of legumes and Parasponia.
The bacteria colonize plant cells within root nodules where they convert atmospheric nitrogen into ammonia and then provide organic nitrogenous compounds such as glutamine or ureides to the plant.
The plant in turn provides the bacteria with organic compounds made by photosynthesis.
This mutually beneficial relationship is true of all of the rhizobia, of which the Rhizobium genus is a typical example.
Scientific classification
Kingdom: Bacteria
Phylum: Proteobacteria
Class: Alphaproteobacteria
Order: Rhizobiales
Family: Rhizobiaceae
Genus: RhizobiumFrank 1889
Beijerinck in the Netherlands was the first to isolate and cultivate a microorganism from the nodules of legumes in 1888. He named itBacillus radicicola, which is now placed in Bergey's Manual of Determinative Bacteriology under the genus Rhizobium.
Culture of Rhizobium on agar plate
The current taxonomy of rhizobiaRhizobia are nitrogen-fixing bacteria that form root nodules on legume plants.
Most of these bacterial species are in the Rhizobiacae family in the alpha-proteobacteria and are in either the Rhizobium, Mesorhizobium, Ensifer, or Bradyrhizobium genera.
However recent research has shown that there are many other rhizobial species in addition to these.
In some cases these new species have arisen through lateral gene transfer of symbiotic genes. The list below is not 'official' by any means, and it is merely my compilation and interpretation of the literature.
An alternative list is maintained by the "ICSP Subcommittee on the taxonomy of Rhizobium and Agrobacterium".
RhizobiumClass: AlphaproteobacteriaOrder: RhizobialesFamily: Rhizobiaceae
Rhizobium alamii Rhizobium freirei
Rhizobium alkalisoli Rhizobium galegae
Rhizobium azibense Rhizobium gallicum
Rhizobium calliandrae Rhizobium giardinii
Rhizobium cauense Rhizobium grahamii
Rhizobium cellulosilyticum Rhizobium hainanense
Rhizobium daejeonense Rhizobium halophytocola
Rhizobium endophyticum Rhizobium herbae
Rhizobium etli Rhizobium huautlense
Rhizobium fabae Rhizobium indigoferae
The genus Rhizobium (Frank 1889) was the first named (from Latin meaning 'root living'), and for many years this was a 'catch all' genus for all rhizobia. Some species were later moved in to new genera based on phylogenetic analyses. It currently consists of 49 rhizobial species (and 11 non rhizobial species).
Rhizobium jaguaris Rhizobium phaseoliRhizobium laguerreae Rhizobium pisiRhizobium leguminosarum Rhizobium tibeticumRhizobium leucaenae Rhizobium sophoraeRhizobium loessense Rhizobium sophoriradicisRhizobium lusitanum Rhizobium sphaerophysaeRhizobium mayense Rhizobium sullaeRhizobium mesoamericanum Rhizobium taibaishanenseRhizobium mesosinicum Rhizobium tropiciRhizobium miluonense Rhizobium tubonenseRhizobium mongolense Rhizobium undicolaRhizobium multihospitium Rhizobium vallisRhizobium oryzae Rhizobium vignaeRhizobium paranaense Rhizobium yanglingenseRhizobium petrolearium
MesorhizobiumClass: AlphaproteobacteriaOrder: RhizobialesFamily: Phyllobacteriaceae
The genus Mesorhizobium was described by Jarvis et al. in 1997. Several Rhizobium species were transferred to this genus. It currently consists of 21 rhizobia species (and 1 non-rhizobial species).
Mesorhizobium abyssinicae Mesorhizobium chacoenseMesorhizobium albiziae Mesorhizobium ciceriMesorhizobium alhagi Mesorhizobium erdmaniiMesorhizobium amorphae Mesorhizobium gobienseMesorhizobium australicum Mesorhizobium hawassenseMesorhizobium camelthorni Mesorhizobium huakuiiMesorhizobium caraganae Mesorhizobium jarvisii
Mesorhizobium loti Mesorhizobium shangrilenseMesorhizobium mediterraneum Mesorhizobium shonenseMesorhizobium metallidurans Mesorhizobium silamurunenseMesorhizobium muleiense Mesorhizobium septentrionaleMesorhizobium opportunistum Mesorhizobium tamadayenseMesorhizobium plurifarium Mesorhizobium tarimenseMesorhizobium qingshengii Mesorhizobium temperatumMesorhizobium robiniae Mesorhizobium tianshanenseMesorhizobium sangaii
Ensifer (formerly Sinorhizobium)class: Alphaproteobacteriaorder: Rhizobialesfamily: Rhizobiaceae
The Sinorhizobium genus was described by Chen et al. in 1988. However some recent studies show that Sinorhizobium and the genus Ensifer (Casida, 1982) belong to a single taxon. Ensiferis the earlier heterotypic synonym (it was named first) and thus takes priority (Young, 2003). This means that all Sinorhizobium spp. must be renamed as Ensifer spp. according to the Bacteriological code. There has been some discussion in the literature if Ensifer or Sinorhizobium is correct Young (2010), this website follows the opinion of the Judicial Commission, and will use Ensifer. The genus currently consists of 17 species.
Ensifer abri Ensifer melilotiSinorhizobium americanum Ensifer mexicanusEnsifer arboris 'Sinorhizobium morelense'Ensifer fredii Ensifer adhaerensEnsifer garamanticus Ensifer numidicusEnsifer indiaense Ensifer saheliEnsifer kostiensis Ensifer sojaeEnsifer kummerowiae Ensifer terangaeEnsifer medicae
Bradyrhizobiumclass: Alphaproteobacteriaorder: Rhizobialesfamily: Bradyrhizobiaceae
The Bradyrhizobium genus was described by Jordan in 1982. It currently consists of 9 rhizobia species (and 1 non rhizobial species).
Bradyrhizobium canariense Bradyrhizobium japonicum
Bradyrhizobium cytisi Bradyrhizobium jicamae
Bradyrhizobium denitrificans Bradyrhizobium liaoningense
Bradyrhizobium elkanii Bradyrhizobium pachyrhizi
Bradyrhizobium iriomotense Bradyrhizobium yuanmingense
Burkholderiaclass: Betaproteobacteriaorder: Burkholderialesfamily: Burkholderiaceae
The Burkholderia genus currently contains seven named rhizobial members and others as Burkholderia sp.
Burkholderia caribensisBurkholderia cepaciaBurkholderia mimosarumBurkholderia nodosaBurkholderia phymatumBurkholderia sabiaeBurkholderia tuberum
Phyllobacteriumclass: Alphaproteobacteria
order: Rhizobiales family: Phyllobacteriaceae
The Phyllobacterium genus currently contains three rhizobial species.
Phyllobacterium trifoliiPhyllobacterium ifriqiyensePhyllobacterium leguminum
Microvirgaclass: Alphaproteobacteria
order: Rhizobialesfamily: Methylobacteriaceae
The Azorhizobium genus was described by Dreyfus et al. in 1988. It currently consists of 2 species.
Azorhizobium caulinodansAzorhizobium doebereinerae
Ochrobactrumclass: Alphaproteobacteria
order: Rhizobialesfamily: Brucellaceae
The Ochrobactrum genus currently contains two rhizobial species.
Ochrobactrum cytisiOchrobactrum lupini
Methylobacteriumclass: Alphaproteobacteria
order: Rhizobialesfamily: Methylobacteriaceae
Methylobacterium nodulans
Cupriavidusclass: Betaproteobacteria
order: Burkholderialesfamily: Burkholderiaceae
Cupriavidus taiwanensis
Devosiaclass: Alphaproteobacteria
order: Rhizobialesfamily: Hyphomicrobiaceae
Devosia neptuniae
Shinellaclass: Alphaproteobacteria
order: Rhizobialesfamily: Rhizobiacea
Shinella kummerowiae
Root NoduleRoot nodules occur on the roots of plants (primarily Fabaceae) that associate with symbiotic nitrogen-fixing bacteria.
Undernitrogen-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known as rhizobia.
This process has evolved multiple times within the Fabaceae, as well as in other species found within the Rosidclade.
The Fabaceae include legume crops such as beans and peas.
Within legume nodules, nitrogen gas from the atmosphere is converted into ammonia, which is then assimilated into amino acids (the building blocks of proteins), nucleotides (the building blocks of DNA and RNA as well as the important energy molecule ATP), and other cellular constituents such as vitamins, flavones, and hormones.
Cross section though a soybean (Glycine max'Essex') root nodule. The bacterium, Bradyrhizobium japonicum, colonizes the roots and establishes a nitrogen fixing symbiosis. This high magnification image shows part of a cell with single bacteroids within their symbiosomes. In this image, endoplasmic reticulum, dictysome and cell wall can be seen.
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 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 conversion of nitrogen gas to ammonia.
Root nodule symbiosisLegume family
Plants that contribute to nitrogen fixation include the legume family – Fabaceae – with taxa such as kudzu, clovers, soybeans, alfalfa, lupines, peanuts, and rooibos.
They containsymbiotic 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 the soil.
The great majority of legumes have this association, but a few genera (e.g.,Styphnolobium) do not.
In many traditional and organic farming practices, fields are rotated through various types of crops, which usually includes one consisting mainly or entirely of clover or buckwheat (non-legume family Polygonaceae), which are often referred to as "green manure".Inga alley farming relies on the leguminous genus Inga, a small tropical, tough-leaved, nitrogen-fixing tree.[4]
Non-leguminous
Although by far the majority of plants able to form nitrogen-fixing root nodules are in the legume family Fabaceae, there are a few exceptions:
•Parasponia, a tropical genus in the Cannabaceae also able to interact with rhizobia and form nitrogen-fixing nodules
•Actinorhizal plants such as alder and bayberry can also form nitrogen-fixing nodules, thanks to a symbiotic association with Frankiabacteria. These plants belong to 25 genera[6] distributed among 8 plant families.
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 Fabalesform a clade of eurosids.
A sectioned alder treeroot nodule.
Types of NodulesTwo main types of nodule have been described: determinate and indeterminate.
Determinate nodules are found on certain tribes of tropical legume such as those of 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 types of determinate nodule is found in a wide range of herbs, shrubs and trees, such as Arachis (peanut).
These are always associated with 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 papilioinoid legumes such as Pisum (pea), Medicago (alfalfa), Trifolium (clover), and Vicia (vetch) and all mimosoid legumes such as acacias (mimosas), 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.
Indeterminate nodules growing on the roots of Medicago italica
Diagram illustrating the different zones of an indeterminate root nodule
Zone I—the active meristem. This is where new nodule tissue is formed which will later differentiate into the other zones of the nodule.
Zone II—the infection zone. This zone is permeated with infection threads full of bacteria. The plant cells are larger than in the previous zone and cell division is halted.
Interzone II–III—Here the bacteria have entered the plant cells, which contain amyloplasts. They elongate and begin terminally differentiating into symbiotic, nitrogen-fixing bacteroids.
Zone III—the nitrogen fixation zone. Each cell in this zone contains a large, central vacuole and the cytoplasm is filled with fully differentiated bacteroids which are actively fixing nitrogen. The plant provides these cells with leghemoglobin, resulting in a distinct pink color.
Zone IV—the senescent zone. Here plant cells and their bacteroid contents are being degraded.
The breakdown of the heme component of leghemoglobin results in a visible greening at the base of the nodule.
This 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 is uniform, lacking the uninfected ells seen in nodules of soybean and many indeterminate types such as peas and clovers.
NodulationLegumes release compounds called flavonoids from their roots, which trigger the production of nod factors by the bacteria.
When the nod factor is sensed by the root, a number of biochemical and morphological changes happen: cell division is triggered in the root to create the nodule, and the root hair growth is redirected to wind around the bacteria multiple times until it fully encapsulates 1 or more bacteria.
The bacteria encapsulated divide multiple times, forming a microcolony. From this microcolony, the bacteria enter the developing nodule through a structure called an infection thread, which grows through the root hair into the basal part of the epidermis cell, and onwards into the root cortex; they are then surrounded by a plant-derived membrane and differentiate into bacteroids that fix nitrogen.
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 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 theENOD40 gene, coding for a 12–13 amino acid protein [41], is up-regulated during nodule formation [3].
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