Ensifer meliloti | |
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Sinorhizobium meliloti strain Rm1021 on an agar plate. | |
Scientific classification ![]() | |
Domain: | Bacteria |
Phylum: | Pseudomonadota |
Class: | Alphaproteobacteria |
Order: | Hyphomicrobiales |
Family: | Rhizobiaceae |
Genus: | Ensifer |
Species: | E. meliloti |
Binomial name | |
Ensifer meliloti (Dangeard, 1926) Young, 2003 | |
Type strain | |
ATCC 9930 CCUG 27879 Contents | |
Biovars | |
Synonyms [9] | |
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Ensifer meliloti (formerly Rhizobium meliloti and Sinorhizobium meliloti) [10] are an aerobic, Gram-negative, and diazotrophic species of bacteria. S. meliloti are motile and possess a cluster of peritrichous flagella. [11] S. meliloti fix atmospheric nitrogen into ammonia for their legume hosts, such as alfalfa. S. meliloti forms a symbiotic relationship with legumes from the genera Medicago , Melilotus and Trigonella , including the model legume Medicago truncatula . This symbiosis promotes the development of a plant organ, termed a root nodule. Because soil often contains a limited amount of nitrogen for plant use, the symbiotic relationship between S. meliloti and their legume hosts has agricultural applications. [12] These techniques reduce the need for inorganic nitrogenous fertilizers. [13]
Symbiosis between S. meliloti and its legume hosts begins when the plant secretes an array of betaines and flavonoids into the rhizosphere: 4,4′-dihydroxy-2′-methoxychalcone, [14] chrysoeriol, [15] cynaroside, [15] 4′,7-dihydroxyflavone, [14] 6′′-O-malonylononin, [16] liquiritigenin, [14] luteolin, [17] 3′,5-dimethoxyluteolin, [15] 5-methoxyluteolin, [15] medicarpin, [16] stachydrine, [18] and trigonelline. [18] These compounds attract S. meliloti to the surface of the root hairs of the plant where the bacteria begin secreting nod factors. This initiates root hair curling. The rhizobia then penetrate the root hairs and proliferate to form an infection thread. Through the infection thread, the bacteria move toward the main root. The bacteria develop into bacteroids within newly formed root nodules and perform nitrogen fixation for the plant. A S. meliloti bacterium does not perform nitrogen fixation until it differentiates into a endosymbiotic bacteroid. A bacteroid depends on the plant for survival. [19]
Leghemoglobin, produced by leguminous plants after colonization of S. meliloti, interacts with the free oxygen in the root nodule where the rhizobia reside. Rhizobia are contained within symbiosomes in the root nodules of leguminous plants. The leghemoglobin reduces the amount of free oxygen present. Oxygen disrupts the function of the nitrogenase enzyme in the rhizobia, which is responsible for nitrogen fixation. [20]
The S. meliloti genome contains four genes coding for flagellin. These include fliC1C2–fliC3C4. [11] The genome contains three replicons: a chromosome (~3.7 megabases), a chromid (pSymB; ~1.7 megabases), and a plasmid (pSymA; ~1.4 megabases). Individual strains may possess additional, accessory plasmids. Five S. meliloti genomes have been sequenced to date: Rm1021, [21] AK83, [22] BL225C, [22] Rm41, [23] and SM11 [24] with 1021 considered to be the wild type. Indeterminate nodule symbiosis by S. meliloti is conferred by genes residing on pSymA. [25]
The proteins encoded by E. meliloti genes uvrA, uvrB and uvrC are employed in the repair of DNA damages by the process of nucleotide excision repair. E. meliloti is a desiccation tolerant bacterium. However, E. meliloti mutants defective in either genes uvrA, uvrB or uvrC are sensitive to desiccation, as well as to UV light. [26] This finding indicates that the desiccation tolerance of wild-type E. meliloti depends on the repair of DNA damages that can be caused by desiccation.
Several bacteriophages that infect Sinorhizobium meliloti have been described: [27] Φ1, [28] Φ1A, [29] Φ2A, [29] Φ3A, [30] Φ4 (=ΦNM8), [31] Φ5t (=ΦNM3), [31] Φ6 (=ΦNM4), [31] Φ7 (=ΦNM9), [31] Φ7a, [28] Φ9 (=ΦCM2), [31] Φ11 (=ΦCM9), [31] Φ12 (=ΦCM6), [31] Φ13, [32] Φ16, [32] Φ16-3, [33] Φ16a, [32] Φ16B, [30] Φ27, [28] Φ32, [33] Φ36, [33] Φ38, [33] Φ43, [28] Φ70, [28] Φ72, [33] Φ111, [33] Φ143, [33] Φ145, [33] Φ147, [33] Φ151, [33] Φ152, [33] Φ160, [33] Φ161, [33] Φ166, [33] Φ2011, [34] ΦA3, [28] ΦA8, [28] ΦA161, [34] ΦAL1, [35] ΦCM1, [34] ΦCM3, [34] ΦCM4, [34] ΦCM5, [34] ΦCM7, [34] ΦCM8, [34] ΦCM20, [34] ΦCM21, [34] ΦDF2, [35] Φf2D, [35] ΦF4, [36] ΦFAR, [35] ΦFM1, [34] ΦK1, [37] ΦL1, [32] ΦL3, [32] ΦL5, [32] ΦL7, [32] ΦL10, [32] ΦL20, [32] ΦL21, [32] ΦL29, [32] ΦL31, [32] ΦL32, [32] ΦL53, [32] ΦL54, [32] ΦL55, [32] ΦL56, [32] ΦL57, [32] ΦL60, [32] ΦL61, [32] ΦL62, [32] ΦLO0, [35] ΦLS5B, [34] ΦM1, [27] [38] ΦM1, [27] [39] ΦM1-5, [34] ΦM2, [40] ΦM3, [28] ΦM4, [28] ΦM5, [27] [28] [41] ΦM5 (=ΦF20), [27] [38] ΦM5N1, [34] ΦM6, [38] ΦM7, [38] ΦM8, [40] ΦM9, [38] ΦM10, [38] ΦM11, [38] ΦM11S, [34] ΦM12, [38] [42] ΦM14, [38] ΦM14S, [34] ΦM19, [43] ΦM20S, [34] [44] ΦM23S, [34] ΦM26S, [34] ΦM27S, [34] ΦMl, [45] ΦMM1C, [34] ΦMM1H, [34] ΦMP1, [46] ΦMP2, [46] ΦMP3, [46] ΦMP4, [46] ΦN2, [28] ΦN3, [28] ΦN4, [28] ΦN9, [28] ΦNM1, [34] [44] ΦNM2, [34] [44] ΦNM6, [34] [44] ΦNM7, [34] [44] ΦP6, [36] ΦP10, [36] ΦP33, [36] ΦP45, [36] ΦPBC5, [47] ΦRm108, [48] ΦRmp26, [49] ΦRmp36, [49] ΦRmp38, [49] ΦRmp46, [49] ΦRmp50, [49] ΦRmp52, [49] ΦRmp61, [49] ΦRmp64, [49] ΦRmp67, [49] ΦRmp79, [49] ΦRmp80, [49] ΦRmp85, [49] ΦRmp86, [49] ΦRmp88, [49] ΦRmp90, [49] ΦRmp145, [49] ΦSP, [28] ΦSSSS304, [50] ΦSSSS305, [50] ΦSSSS307, [50] ΦSSSS308, [50] and ΦT1. [28] Of these, ΦM5, [41] ΦM12, [42] Φ16-3 [51] and ΦPBC5 [47] have been sequenced.
As of March 2020 the International Committee on Taxonomy of Viruses (ICTV) has accepted the following species in its Master Species List 2019.v1 (#35):
Rhizobia are diazotrophic bacteria that fix nitrogen after becoming established inside the root nodules of legumes (Fabaceae). To express genes for nitrogen fixation, rhizobia require a plant host; they cannot independently fix nitrogen. In general, they are gram negative, motile, non-sporulating rods.
Rhizobium is a genus of Gram-negative soil bacteria that fix nitrogen. Rhizobium species form an endosymbiotic nitrogen-fixing association with roots of (primarily) legumes and other flowering plants.
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 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.
Nod factors, are signaling molecules produced by soil bacteria known as rhizobia in response to flavonoid exudation from plants under nitrogen limited conditions. Nod factors initiate the establishment of a symbiotic relationship between legumes and rhizobia by inducing nodulation. Nod factors produce the differentiation of plant tissue in root hairs into nodules where the bacteria reside and are able to fix nitrogen from the atmosphere for the plant in exchange for photosynthates and the appropriate environment for nitrogen fixation. One of the most important features provided by the plant in this symbiosis is the production of leghemoglobin, which maintains the oxygen concentration low and prevents the inhibition of nitrogenase activity.
Sharon Rugel Long is an American plant biologist. She is the Steere-Pfizer Professor of Biological Science in the Department of Biology at Stanford University, and the Principal Investigator of the Long Laboratory at Stanford.
suhB, also known as mmgR, is a non-coding RNA found multiple times in the Agrobacterium tumefaciens genome and related alpha-proteobacteria. Other non-coding RNAs uncovered in the same analysis include speF, ybhL, metA, and serC.
Bradyrhizobium is a genus of Gram-negative soil bacteria, many of which fix nitrogen. Nitrogen fixation is an important part of the nitrogen cycle. Plants cannot use atmospheric nitrogen (N2); they must use nitrogen compounds such as nitrates.
Ensifer is a genus of nitrogen-fixing bacteria (rhizobia), three of which have been sequenced.
Bradyrhizobium japonicum is a species of legume-root nodulating, microsymbiotic nitrogen-fixing bacteria. The species is one of many Gram-negative, rod-shaped bacteria commonly referred to as rhizobia. Within that broad classification, which has three groups, taxonomy studies using DNA sequencing indicate that B. japonicum belongs within homology group II.
enod40, also known as early nodulin 40, is a gene found in flowering plants. The gene has characteristics of both protein and Non-coding RNA genes. There is some evidence that the non-coding characteristics of this gene are more widely conserved than the protein coding sequences. In soyabeans enod40 was found to be expressed during early stages of formation of nitrogen-fixing root nodules that are associated with symbiotic soil rhizobial bacteria. The gene is also active in roots containing fungi forming phosphate-acquiring arbuscular mycorrhiza. An interaction with a novel RNA-binding protein MtRBP1 investigated in the development of Root nodule suggests ENOD40 has a function of cytoplasmic relocalization of nuclear proteins. In the study of non-legume plants, the over-expression of ENOD40 in transgenic Arabidopsis lines was observed a reduction of cell expansion.
Medicarpin is a pterocarpan, a derivative of isoflavonoids.
Within genetics, post-genomic research has rendered bacterial small non-coding RNAs (sRNAs) as major players in post-transcriptional regulation of gene expression in response to environmental stimuli. The Alphaproteobacteria includes Gram-negative microorganisms with diverse life styles; frequently involving long-term interactions with higher eukaryotes.
αr14 is a family of bacterial small non-coding RNAs with representatives in a broad group of α-proteobacteria. The first member of this family (Smr14C2) was found in a Sinorhizobium meliloti 1021 locus located in the chromosome (C). It was later renamed NfeR1 and shown to be highly expressed in salt stress and during the symbiotic interaction on legume roots. Further homology and structure conservation analysis identified 2 other chromosomal copies and 3 plasmidic ones. Moreover, full-length Smr14C homologs have been identified in several nitrogen-fixing symbiotic rhizobia, in the plant pathogens belonging to Agrobacterium species as well as in a broad spectrum of Brucella species. αr14C RNA species are 115-125 nt long and share a well defined common secondary structure. Most of the αr14 transcripts can be catalogued as trans-acting sRNAs expressed from well-defined promoter regions of independent transcription units within intergenic regions (IGRs) of the α-proteobacterial genomes.
αr35 is a family of bacterial small non-coding RNAs with representatives in a reduced group of Alphaproteobacteria from the order Hyphomicrobiales. The first member of this family (Smr35B) was found in a Sinorhizobium meliloti 1021 locus located in the symbiotic plasmid B (pSymB). Further homology and structure conservation analysis have identified full-length SmrB35 homologs in other legume symbionts, as well as in the human and plant pathogens Brucella anthropi and Agrobacterium tumefaciens, respectively. αr35 RNA species are 139-142 nt long and share a common secondary structure consisting of two stem loops and a well conserved rho independent terminator. Most of the αr35 transcripts can be catalogued as trans-acting sRNAs expressed from well-defined promoter regions of independent transcription units within intergenic regions of the Alphaproteobacterial genomes.
αr45 is a family of bacterial small non-coding RNAs with representatives in a broad group of α-proteobacteria from the order Hyphomicrobiales. The first member of this family (Smr45C) was found in a Sinorhizobium meliloti 1021 locus located in the chromosome (C). Further homology and structure conservation analysis identified homologs in several nitrogen-fixing symbiotic rhizobia, in the plant pathogens belonging to Agrobacterium species as well as in a broad spectrum of Brucella species, in Bartonella species, in several members of the Xanthobactereacea family, and in some representatives of the Beijerinckiaceae family. αr45C RNA species are 147-153 nt long and share a well defined common secondary structure. All of the αr45 transcripts can be catalogued as trans-acting sRNAs expressed from well-defined promoter regions of independent transcription units within intergenic regions (IGRs) of the α-proteobacterial genomes.
Trigonella suavissima is a herbaceous plant that is endemic to Australia. It is a member of the genus Trigonella and the family Fabaceae. Common names include Cooper clover, Menindee clover, calomba, Darling trigonella, sweet fenugreek, channel clover, sweet-scented clover and Australian shamrock.
Ensifer fredii is a nitrogen fixing bacterium. It is a fast-growing root nodule bacterium. Ensifer fredii exhibits a broad host-range and is able to nodulate both determinant hosts, such as soy, as well as indeterminate hosts including the pigeon pea. Because of their ease of host infection there is interest in their genetics and the symbiotic role in host infection and nodule formation.
Mesorhizobium mediterraneum is a bacterium from the genus Mesorhizobium, which was isolated from root nodule of the Chickpea in Spain. The species Rhizobium mediterraneum was subsequently transferred to Mesorhizobium mediterraneum. This species, along with many other closely related taxa, have been found to promote production of chickpea and other crops worldwide by forming symbiotic relationships.
Ensifer medicae is a species of gram-negative, nitrogen-fixing, rod-shaped bacteria. They can be free-living or symbionts of leguminous plants in root nodules. E.medicae was first isolated from root nodules on plants in the genus Medicago. Some strains of E.medicae, like WSM419, are aerobic. They are chemoorganotrophic mesophiles that prefer temperatures around 28 °C. In addition to their primary genome, these organisms also have three known plasmids, sized 1,570,951 bp, 1,245,408 bp and 219,313 bp.
A symbiosome is a specialised compartment in a host cell that houses an endosymbiont in a symbiotic relationship.