Symbiosis
Symbiosis has played a key role in the evolution of life on Earth. Symbiotic mergers of once independent species drove the origin of eukaryotes. Moreover, symbiosis has enabled many species to gain novel functions and occupy new ecological niches, thus underpinning the functioning of diverse ecosystems. As endosymbionts, microbes provide their eukaryotic hosts with an array of ecological and physiological innovations, including new metabolic capabilities, such as autotrophy or nitrogen fixation, and protection against infections or environmental stressors. Microbial eukaryotes also commonly host their own endosymbionts, including bacteria and algae. Understanding the stability and resilience of symbioses is key to predicting the response of important ecosystems, such as coral reefs, to global change. Manipulating symbiotic associations also has far-reaching economic, environmental and medical implications, through the potential to improve crop productivity, reduce reliance on fertilisers, and control the insect vectors of infectious diseases.
This collection, guest edited by Professor Michael Brockhurst (University of Manchester) and Dr. Rebecca J Hall (University of Birmingham), will feature microbe-focused studies of symbiosis, ranging from the molecular mechanisms of host-symbiont interactions, their genetic and genomic diversity, to understanding the impacts of symbioses in natural and manmade ecosystems.
Collection Contents
51 - 68 of 68 results
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Phage Induction of Lysogenic Rhizobium leguminosarum biovar trifolii in both the Free-living and the Symbiotic Form
More LessA lysogenic strain, Rhizobium leguminosarum biovar trifolii UK-1: φU, was isolated from a wild white clover nodule. It was symbiotically effective on white clover. A lysogenic phage (U-mole) was induced from this strain by treatment with either UV irradiation or mitomycin C. Phage U-mole had an icosahedral head (40 nm wide), a short tail (9 nm long) and tail fibres (15 nm long). Phage U-mole was induced with mitomycin C both from bacterial cells in infection threads and from bacteroids in nodules. Southern hybridization using EcoRI fragments of phage U-mole as probe indicated that phage U-mole DNA was integrated into the chromosome of strain UK-1: øU at a site involving the 60 kb EcoRI fragment of phage U-mole. Phage U-mole also lysogenized the wild-type strain R. leguminosarum biovar trifolii 4S. The resulting lysogenized strain, 4S:øU, had lost its 315 kb Sym plasmid and its nodulation ability.
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Symbiotic Plasmid Rearrangement in a Hyper-recombinant Mutant of Rhizobium leguminosarum biovar phaseoli
More LessThe symbiotic plasmid of Rhizobium leguminosarum biovar phaseoli strain CFN23 has been previously shown to undergo a series of genetic rearrangements that modify the bacterial symbiotic phenotype. A hyper-recombinant derivative of strain CFN23 was isolated. This derivative (strain CFN2300) has a similar phenotype to that reported for Escherichia coli mutants defective in DNA metabolism. The frequency of CFN23 symbiotic plasmid deletion is increased in the CFN2300 background, suggesting that homologous genetic recombination is involved in the generation of this genetic rearrangement.
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Chemotaxis of Rhizobium leguminosarum biovar phaseoli towards Flavonoid Inducers of the Symbiotic Nodulation Genes
Chemotaxis of Rhizobium leguminosarum biovar phaseoli RP8002 towards a range of carbohydrates, phenolic compounds and flavonoids was assayed. Xylose (peak response 10−4 M), sucrose (peak response 10−6 M) and raffinose (peak response 10−5 M) were strong chemoattractants amongst the carbohydrates, whilst glucose, fructose, galactose and maltose produced little or no detectable response. Of the monocyclic phenolic compounds, vanillyl alcohol, p-hydroxybenzoic acid (both peak responses 10−6 M) and 3,4-dihydroxybenzoic acid (peak response 10−4 M) all evoked strong chemotactic responses. Amongst the nod-inducing flavonoids, apigenin and luteolin were both strong chemoattractants (peaks at 10−5 M) while naringenin produced a very low response. Competition experiments suggest that apigenin and luteolin are recognized by a common receptor, but that there exists a separate receptor for luteolin alone. The inhibitors of nod-induction, umbelliferone and acetosyringone, both produced strong chemotactic responses, with peaks at 10−3 M and 10−2 M respectively. This evidence is indicative of a role for chemotaxis towards nod-inducing flavonoids in the initiation of root nodule formation by rhizobia, and also suggests that chemotaxis may influence the host range of the interaction.
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Symbiotic Phenotypes of Auxotrophic Mutants of Rhizobium meliloti104A14
More LessAuxotrophic mutants of Rhizobium meliloti 104A14 were isolated using nitrous acid mutagenesis followed by penicillin enrichment. Mutants in ornithine transcarbamylase, argininosuccinate synthetase or serine-glycine biosynthesis formed nitrogen-fixing (Fix+) nodules on the roots of alfalfa (Medicago sativa). Mutants with defects in ornithine, pyrimidine, purine, asparagine, leucine, methionine or tyrosine biosynthesis, in one-carbon metabolism or in carbamoylphosphate synthetase formed nodules but these nodules were unable to fix nitrogen. Prototrophic revertants were always Fix+. Plasmids that would complement many of these auxotrophs were isolated by transduction with a P2 cosmid gene bank of R. meliloti 104A14. These plasmids were then introduced into mutants of the same and different classes and the growth and symbiotic phenotypes of the new strains were determined. In all cases, complementation of the nutritional defect restored symbiotic nitrogen fixation.
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Plasmids and Symbiotic Effectiveness of Representative Phage Types from Two Indigenous Populations of Rhizobium meliloti
More LessSUMMARY: Phage types representative of the population of indigenous Rhizobium meliloti at each of two sites were evaluated for plasmid content by agarose gel electrophoresis and for symbiotic effectiveness with Medicago sativa cv. Saranac. Relative to four strains used commercially, 55 and 65 phage types representing these populations showed a high average level of symbiotic effectiveness; only a single type from one site was relatively ineffective in symbiosis. On the basis of plasmid number and molecular mass, 160 isolates comprising 45 and 48 types from both sites were placed in 22 different groups with 17 and 13 groups from the respective sites. The number of plasmids varied between one and five per isolate with molecular masses ranging from 5 MDa to considerably greater than 267 MDa. Only five isolates lacked a plasmid with mobility in agarose gels corresponding to that of a reference megaplasmid but instead showed a band of lesser mobility and therefore greater molecular mass. Phage types, which were divided into plasmid groups solely on the basis of differences between isolates from each site, may reflect adaptation of R. meliloti to their respective sites. Differences between isolates within certain phage types due to the presence or absence of a single plasmid may have resulted from genetic interchange between indigenous R. meliloti. There was no significant correlation between plasmid number or mass and symbiotic effectiveness or phage sensitivity of the phage types from either site.
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Ammonia Transport in Free-living and Symbiotic Rhizobium sp. ANU289
More LessSummary: [14C]Methylamine uptake by free-living Rhizobium sp. ANU289 had Michaelis–Menten kinetics (apparent K m 6.6 μm). Uptake was competitively inhibited by ammonia (K i 0.4 μm) and was dependent on an energized membrane. Uptake by bacteria had an optimum at pH 7.0. Methylamine uptake by bacteroids from siratro root nodules was much slower than that by free-living bacteria at pH 7.0 but increased exponentially with the pH of the medium. Uptake by bacteroids did not show saturation kinetics and was insensitive to the presence of ammonia or uncouplers. These results suggest that free-living bacteria (grown under conditions where ammonia is limiting) have an active transport mechanism for the uptake of ammonium ions; this carrier is not operative in the symbiotic state, where passive diffusion of ammonia occurs. In the free-living state, the ammonium carrier is under genetic control, being repressed by growth on high concentrations of ammonia. Derepression occurs under conditions of nitrogen starvation.
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Effect of Transfer of Symbiotic Plasmids and of Hydrogenase Genes (hup) on Symbiotic Efficiency of Rhizobium leguminosarum Strains
More LessSummary: Plasmid-encoded symbiotic determinants from the Rhizobium leguminosarum strain MA1 (817) with uptake hydrogenase activity (Hup+) and from the Hup− strain MC1 (18a) were mobilized by recombination with the self-transmissible plasmid pVW5JI. The symbiotic determinants were transferred by conjugation from strain MA1 to strain MC1 and to a derivative of MC1 without the symbiotic plasmid, and vice versa, thus constructing four types of transconjugants. The determinants for a total recycling hydrogenase in strain MA1 were found to be encoded on the symbiotic plasmid.
Strain MC1 fixed 60% more N2 in pea root nodules, determined as mg nitrogen per plant, than strain MA1. This difference was not increased in the MC1 derivative that obtained hydrogenase activity. Plants inoculated with a derivative of strain MA1, however, where the symbiotic plasmid was replaced by that of strain MC1 had a high percentage nitrogen content. It was concluded that the symbiotic plasmid and the genetic background were more important for plant nitrogen accumulation than uptake hydrogenase.
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Sugar Metabolism and the Symbiotic Properties of Carbohydrate Mutants of Rhizobium leguminosarum
More LessRhizobium leguminosarum metabolizes sugars via the Entner-Doudoroff and pentose phosphate pathways but does not have a functional Embden-Meyerhof pathway. Although some sugar catabolizing enzymes are constitutive, activities of the ‘Entner-Doudoroff’ enzymes vary with the carbon source. Bacteroids have complete pathways for sugar catabolism even though the specific activities of some enzymes, e.g., glucokinase, are lower than in free-living cells. Tn5-induced mutants lacking glucokinase, fructokinase and pyruvate dehydrogenase have been isolated. Although these mutants are unable to utilize sugars, they all nodulate peas and fix N2. The capacity to utilize particular C6 and C12 sugars is apparently not essential for bacteroid development or the establishment of effective N2 fixation.
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Contribution of the Symbiotic Plasmid to the Competitiveness of Rhizobium leguminosarum
More LessFour different symbiotic plasmids from Rhizobium leguminosarum were introduced into three different recipient strains that lacked plasmid-linked symbiotic determinants. The twelve synthetic strains so constructed were each tested for competitiveness against a standard reference strain. The recipient strain and the introduced symbiotic plasmid contributed about equally to competitiveness in forming root nodules on pea plants: there was also significant interaction between strain and plasmid, although this was much less important than the main effects. Competitiveness for growth on the legume root surface (the rhizosphere) was attributable entirely to the recipient strain; the introduced plasmid had no significant effect.
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Antibiotic Activity of Xenorhabdus spp., Bacteria Symbiotically Associated with Insect Pathogenic Nematodes of the Families Heterorhabditidae and Steinernematidae
More LessA wide range of micro-organisms, including yeasts, was found to be inhibited by the primary form of Xenorhabdus spp., but not by the secondary form. Only one Xenorhabdus strain, the symbiont of Neoaplectana glaseri, did not inhibit any of the micro-organisms tested; it is suggested that this strain may not have been isolated in the primary form. Gram-positive bacteria were sensitive to all active isolates of Xenorhabdus; each of the yeasts and almost all of the Gram-negative bacteria were sensitive to some but not all Xenorhabdus isolates. Each Xenorhabdus isolate was sensitive to some other Xenorhabdus isolates. The antibiotic activity of X. nematophilus was unaffected by autoclaving but was lost after dialysis. Anaerobically incubated Xenorhabdus spp. did not exhibit antibiotic activity.
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Improvement of Symbiotic Properties in Rhizobium leguminosarum by Plasmid Transfer
More LessTransfer of plasmid pIJ1008, a recombinant of two indigenous Rhizobium leguminosarum plasmids into R. leguminosarum strain 300 produced strain 3960, which reduced significantly more N2 in pea root nodules than did strain 300 itself. Strain 3960 was superior to the field isolate 128C53, from which the symbiotic determinants of pIJ1008 were derived, and transfer of plasmid pIJ1008 into two other genetic backgrounds also improved symbiotic performance compared to the introduction of other nodulation plasmids.
As plasmid pIJ1008 carries genetic determinants for an uptake hydrogenase activity (Hup+) as well as nodulation capability (Nod+) and other determinants for symbiotic nitrogen fixation (Fix+), the increased effectiveness of strains carrying pIJ1008 may result from their capacity to conserve energy by recovering H2 evolved by nitrogenase.
Intact plant studies with 15N showed that the superior N2 fixation capability associated with pIJ1008 was enhanced by 2 mm-NO3 −, a common concentration of soil N. It was also shown that, as plants grew older, the hydrogenase determined by pIJ1008 was not able to recycle all the hydrogen evolved by pea root nodules.
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Only One of the Large Plasmids in Rhizobium leguminosarum Strain PRE is Strongly Expressed in the Endosymbiotic State
More LessPlasmid DNA from six strains of Rhizobium leguminosarum was separated by agarose gel electrophoresis and showed a large variation in the number and size of the plasmids. Blotting on to nitrocellulose and hybridization with a probe containing nif (nitrogen fixation) genes from Rhizobium meliloti demonstrated that all strains contain only one plasmid carrying nif genes. The molecular weights of the nif plasmids ranged between approximately 130 × 106 and 550 × 106. Using DNA-DNA hybridization no homologous sequences were detected between the nif plasmid of strain PRE and the other plasmid present in this strain. Hybridizations with RNA from bacteroids showed that only the nif plasmid is strongly expressed in the endosymbiotic state. No selective amplification of plasmid DNA occurs during the transition of free-living bacteria into bacteroids.
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Glutamate Synthase Activity in Symbiotic Cyanobacteria
More LessGlutamate synthase (GOGAT) activities of the symbiotic cyanobacteria of the lichens Peltigera canina and Peltigera aphthosa and of the water fern Azolla caroliniana have been determined and, like glutamine synthetase (GS) activity, found to be substantially reduced compared with the activities found in the free-living cyanobacteria. A similar reduction in GOGAT activity was not noted in the symbiotic green alga (Coccomyxa sp.) in P. aphthosa. The selective reduction of GS-GOGAT activity in symbiosis may be related to the production of extracellular nitrogen by the symbiotic cyanobacterium.
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Transfer of Symbiotic Genes with Bacteriocinogenic Plasmids in Rhizobium leguminosarum
More LessTransfer of derivatives of the bacteriocinogenic plasmid pRL1JI into eight symbiotically defective strains of R. leguminosarum resulted in suppression of the mutant phenotype in four cases. These included non-infective, ineffective and temperature-sensitive ineffective phenotypes. However, in none of these strains was the defect suppressible after high frequency transfer of derivatives of two other bacteriocinogenic plasmids, pRL3JI or pRL4JI. Nodulation ability was co-transferred at high frequency (> 95 %) with bacteriocin production by the plasmid pRL1JI. The other two plasmids, pRL3JI and pRL4JI, also mediated the transfer of nodulation ability but at much lower frequency (10−3 to 10−4 per plasmid transfer). Sometimes, transconjugants that had acquired nodulation ability after the transfer of derivatives of plasmids pRL3JI or pRL4JI acted subsequently as high frequency donors for nodulation ability in a manner that was apparently similar to strains containing pRL1JI.
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Carbohydrate Metabolism in Rhizobium trifolii: Identification and Symbiotic Properties of Mutants
More LessCrude extracts of Rhizobium trifolii strain 7000 contained enzymes of the Entner–Doudoroff and pentose phosphate pathways. No phosphofructokinase (EC 2.7.1.11) activity and only a low activity of fructose-1,6-bisphosphate aldolase (EC 4.1.2.13) were found, suggesting that the Embden–Meyerhof–Parnas pathway was not physiologically important in this strain.
Independent carbohydrate-negative mutants of R. trifolii were isolated and characterized as deficient in glucokinase (glk; EC 2.7.1.2), fructose uptake (fup), the Entner–Doudoroff pathway (edp) and pyruvate carboxylase (pyc; EC 6.4.1.1). Glucokinase was essential for glucose phosphorylation in R. trifolii and was also required for growth on sucrose. The edp mutant was impaired in growth on all hexoses tested except galactose, suggesting that the ED pathway was the major pathway used by R. trifolii for the catabolism of these sugars. Galactose may be catabolized via a different pathway, possibly involving an NADP+-linked galactose dehydrogenase. Pyruvate carboxylase was an important anaplerotic enzyme in R. trifolii required for growth on all carbon sources tested, except succinate.
All the mutants, including a glk fup double mutant, formed an effective symbiosis on red clover, suggesting that neither glucose, fructose nor sucrose are used by the bacteroids to provide ATP and reductant for nitrogen fixation. The bacteroids probably receive a supply of tricarboxylic acid cycle intermediates from the plant cytosol, and these may be their major source of energy.
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Genetic Relationships among Bacterial Endosymbionts of Paramecium aurelia
More LessGenetic relationships among bacterial endosymbionts of Paramecium assigned to the genus Caedobacter Preer et al. 1974 were determined using DNA-DNA hybridization techniques. It was shown that mu, nu and pi endosymbionts are closely related to each other, but not to any of the strains of kappa endosymbionts tested, and that kappa consists of at least three distinct and genetically diverse groups. Therefore, it is proposed that Caedobacter be split into two genera, Caedobacter and Pseudocaedobacter, depending upon the ability or lack of ability, respectively, to produce cells containing R bodies. Strains were screened for covalently closed, circular DNA by ethidium bromide/CsCl centrifugation, but only pi and hump-killer kappa particles were found to contain this form of DNA.
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The Fine Structure of a Nuclear Envelope Associated Endosymbiont of Paramecium
More LessSummaryParticles of a newly described endosymbiont of Paramecium, here referred to as epsilon, have a fine structure that is essentially identical to that of Gram-negative bacteria. The symbionts occur only within loculi formed by extension of the outer membrane of the nuclear envelopes.
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Mutations in Symbiotic Effectiveness in Rhizobium trifolii Caused by Transforming DNA and Other Agents
More LessSUMMARYA strain of Bhizobium trifolii able to fix nitrogen in root nodules of clover plants lost its effectiveness when treated with deoxyribonucleic acid (DNA) from an ineffective strain. Attempts to transform two ineffective strains to effectiveness failed, even when the donor of DNA and the recipient strains were genetically related and apparently differed only in symbiotic property. The efficiency of transformation by DNA to ineffectiveness was compared with mutagenic and selective treatments. The results support the idea that symbiotic effectiveness involves compatability between several plant and bacterial factors, changes in any one of which makes the bacterium ineffective.
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