Differences between strains of rhizobium in sensitivity to canavanine

Summary Four strains of rhizobia that nodulate canavanine-synthesizing legumes and four strains that nodulate noncanavanine-synthesizing legumes were tested for sensitivity to L-canavanine. The effect of canavanine on growth depends upon the strain of Rhizobium tested rather than the canavanine synthesizing capability of the host legume. In both groups of rhizobia, some strains were inhibited in growth by canavanine. Canavanine enhancement of growth was observed in rhizobia that nodulate noncanavanine-synthesizing legumes.Canavanine was found to enhance incorporation of uridine-H³ and L-leucine-H³ into trichloroacetic acid insoluble fractions of starved cells of two strains of rhizobia tested. This demonstrated that under certain conditions some rhizobia can detoxify canavanine and utilize it in synthetic processes. re]19760729     

Mercury-resistant rhizobial bacteria isolated from nodules of leguminous plants growing in high Hg-contaminated soils

A survey of symbiotic bacteria from legumes grown in high mercury-contaminated soils (Almadén, Spain) was performed to produce a collection of rhizobia which could be well adapted to the environmental conditions of this region and be used for restoration practices. Nineteen Hg-tolerant rhizobia were isolated from nodules of 11 legume species (of the genera Medicago, Trifolium, Vicia, Lupinus, Phaseolus, and Retama) and characterized. Based on their growth on Hg-supplemented media, the isolates were classified into three susceptibility groups. The minimum inhibitory concentrations (MICs) and the effective concentrations that produce 50% mortality identified the patterns of mercury tolerance and showed that 15 isolates were tolerant. The dynamics of cell growth during incubation with mercury showed that five isolates were unaffected by exposure to Hg concentrations under the MICs. Genetic analyses of the 16S rRNA gene assigned ten strains to Rhizobium leguminosarum, six to Ensifer medicae, two to Bradyrhizobium canariense, and one to Rhizobium radiobacter. Inoculation of host plants and analysis of the nodC genes revealed that most of them were symbiotically effective. Finally, three isolates were selected for bioremediation processes with restoration purposes on the basis of their levels of Hg tolerance, their response to high concentrations of this heavy metal, and their genetic affiliation and nodulation capacity.     

Nodulation and nitrogen fixation in extreme environments

Biological nitrogen fixation is a phenomenon occurring in all known ecosystems. Symbiotic nitrogen fixation is dependent on the host plant genotype, theRhizobium strain, and the interaction of these symbionts with the pedoclimatic factors and the environmental conditions. Extremes of pH affect nodulation by reducing the colonization of soil and the legume rhizosphere by rhizobia. Highly acidic soils (pH<4.0) frequently have low levels of phosphorus, calcium, and molybdenum and high concentrations of aluminium and manganese which are often toxic for both partners; nodulation is more affected than host-plant growth and nitrogen fixation. Highly alkaline soils (pH>8.0) tend to be high in sodium chloride, bicarbonate, and borate, and are often associated with high salinity which reduce nitrogen fixation. Nodulation and N-fixation are observed under a wide range of temperatures with optima between 20–30°C. Elevated temperatures may delay nodule initiation and development, and interfere with nodule structure and functioning in temperate Iegumes, whereas in tropical legumes nitrogen fixation efficiency is mainly affected. Furthermore, temperature changes affect the competitive ability ofRhizobium strains. Low temperatures reduce nodule formation and nitrogen fixation in temperate legumes; however, in the extreme environment of the high arctic, native legumes can nodulate and fix nitrogen at rates comparable to those observed with legumes in temperate climates, indicating that both the plants and their rhizobia have successfully adapted to arctic conditions. In addition to low temperatures, arctic legumes are exposed to a short growing season, a long photoperiod, low precipitation and low soil nitrogen levels. In this review, we present results on a number of structural and physiological characteristics which allow arctic legumes to function in extreme environments.     

Ecological genetics of Rhizobium meliloti: symbiotic plasmid transfer in the Medicago sativa rhizosphere

A potentially important aspect of the ecological genetics of fast-growing rhizobia is lateral transfer of symbiosis-related genes between strains in the host-legume rhizosphere. We have therefore characterized transfer of the Rhizobium leguminosarum symbiotic (Sym) plasmids pJB5JI and pSymR1897 to strain WL113, a non-attaching, non-nodulating Rhizobium meliloti recipient, in the rhizosphere of its legume host Medicago sativa. Interspecific transfer of pJB5JI generated symbiotically proficient transconjugants of WL113, demonstrating that a mechanism exists for genetic flux in indigenous R. meliloti populations in response to selection pressure provided by the host legume. This could generate fluid groupings of R. meliloti rather than discrete and stable strains.     

Mineral constraints to nitrogen fixation

Mineral nturient defiencies are a major constraint limiting legume nitrogen fixation and yield. In this review general techniques for assessing nutrient involvement in symbiotic nitrogen fixation are described and specific methods are outlined for determining which developmental phase of the symbiosis is most sensitive to nutrient deficiency. The mineral nutrition of the Rhizobium component of the symbiosis is considered both as the free living organism in the soil and as bacteroids in root nodules. Rhizobial growth and survival in soils is not usually limited by nutrient availability. Multiplication of rhizobia in the legume rhizosphere is limited by low Ca availability. Nodule initiation is affected by severe Co deficiency through effects on rhizobia. Nodule development is limited by severe B deficiency via an effect on plant cell growth. Fe deficiency limits nodule development by affecting rhizobia and strains of rhizobia differ widely in their ability to acquire sufficient Fe for their symbiotic development. Nodule function requires more Mo than does the host plant, and in some symbioses nitrogen fixation may be specifically limited by low availability of Ca, Co, Cu and Fe. The importance of the peribacteriod membrane in determining nutrient availability to bacteroids is considered. It is concluded that the whole legume-Rhizobium symbiosis should be considered when improving legume growth and yield under nutrient stress conditions. Differences among rhizobial strains in their ability to obtain mineral nutrients from their environment may be agronomically important.     

Molecular Diversity of Rhizobia Occurring on Native Shrubby Legumes in Southeastern Australia

The structure of rhizobial communities nodulating native shrubby legumes in open eucalypt forest of southeastern Australia was investigated by a molecular approach. Twenty-one genomic species were characterized by small-subunit ribosomal DNA PCR-restriction fragment length polymorphism and phylogenetic analyses, among 745 rhizobial strains isolated from nodules sampled on 32 different legume host species at 12 sites. Among these rhizobial genomic species, 16 belonged to the Bradyrhizobium subgroup, 2 to the Rhizobium leguminosarum subgroup, and 3 to the Mesorhizobium subgroup. Only one genomic species corresponded to a known species (Rhizobium tropici). The distribution of the various genomic species was highly unbalanced among the 745 isolates, legume hosts, and sites. Bradyrhizobium species were by far the most abundant, and Rhizobium tropici dominated among the Rhizobium and Mesorhizobium isolates in the generally acid soils where nodules were collected. Although a statistically significant association occurred between the eight most common genomic species and the 32 hosts, there was sufficient overlap in distributions that no clear specificity between rhizobial genomic species and legume taxa was observed. However, for three legume species, some preference for particular genomic species was suggested. Similarly, no geographical partitioning was found.     

The effectiveness and competitiveness of some Indonesian Rhizobium strains on tropical legumes grown in four soil types of Java

Several Indonesian and some imported Rhizobium strains were assessed for their effectiveness in nodulating four legume species in four soil types of Java. Naturally occurring Rhizobia formed effective symbioses onVigna unguiculata, Macroptilium atropurpureum andDesmodium heterocarpon in all four soils and the applied strains, with some exceptions, did not infect a majority of nodules of these legumes.Centrosema pubescens was more specific in its Rhizobia requirements and applied strains formed effective symbioses in two clay soils, but not in two sandy loam soils.     

The symbiosis between oriental goat's rue and the root nodule bacteria Rhizobium galegae: specificity and competitiveness

Competitiveness and genetic variation of the Rhizobium galegae strains from the collection of the All-Russia Institute of Agricultural Microbiology, Russian Academy of Agricultural Sciences, causing nodulation of oriental goat's rue under conditions of Bashkortostan soils (lacking this rhizobial species) were studied. It was demonstrated that of all the tested strains, the strains CIAM 0702 and CIAM 0704, each carrying two megaplasmids of 1500 and 2000 MDa, were the most competitive. RAPD (random amplified polymorphic DNA) analysis showed that R. galegae strains were able to intensively exchange the genetic material in the host plant rhizosphere. We did not succeed in detecting the local root nodule bacteria that were either initially able to infect oriental goat's rue or had adapted to infecting this species due to various genetic rearrangements.     

Survival and symbiotic characteristics of rhizobium in saline-alkali soils

Summary Survival, growth and symbiotic performance of rhizobia isolated from normal, saline-sodic and mildly acidic soils were studied in original and amended saline-alkali soils. Rhizobia of 4 out of 9 legumes studied for nodulation were found to be present in a highly saline-sodic soil. Majority of the strains of these bacteria did not survive in the original saline-sodic soil of pH 10.5 but as the pH was amended to lower than 10.0, all the strains survived in the soil. Virtually no differences were noticed in the survival and symbiotic characteristics of native and exotic strains ofRhizobium leguminosarum andRhizobium trifolii in the saline-sodic soil, though wide variations were observed among individual strains irrespective of their ecological origin. Rhizobia were found to possess greater tolerance for alkalinity than their host legumes. However, delayed nodulation in lentil (Lens esculenta) and berseem (Trifolium alexandrinum) resulting in decreased yield of the plants at pH values higher than 9.0 was observed.     

Phosphorus pools and other soil properties in the rhizosphere of wheat and legumes growing in three soils in monoculture or as a mixture of wheat and legume

Background and aims

Phosphorus and nitrogen availability and forms are affected by soil properties as well as by plant species and further modulated by soil microbes. Additionally, close contact of the roots of two plant species may affect concentrations and forms of N and P. The aim of this study was to assess properties related to N and P cycling in the rhizosphere of wheat and legumes grown in monoculture or in wheat/legume mixtures in three soils differing in pH.

Methods

Faba bean, white lupin and wheat were grown in three soils differing in pH (4.8, 7.5 and 8.8) in monoculture or in mixed culture of wheat and legumes. Rhizosphere soil was collected at flowering and analyzed for P pools by sequential fractionation, available N as well as community structure of bacteria, fungi, ammonia oxidizers, N₂-fixers and P mobilizers by polymerase chain reaction (PCR)—denaturing gradient gel electrophoresis (DGGE).

Results

Soil type was the major factor determining plant growth, rhizosphere nutrient dynamics and microbial community structure. Among the crop species, only faba bean had a significant effect on nitrification potential activity (PNA) in all three soils with lower activity compared to the unplanted soil. Soil type and plant spieces affected the community composition of ammonia-oxidizing archaea (AOB), ammonia-oxidizing archaea (AOA), N₂-fixers (nifH), P mobilizers (ALP gene) and fungi, but not that of bacteria. Among the microbial groups, the AOA and nifH community composition were most strongly affected by crop species, cropping system and soil type, suggesting that these groups are quite sensitive to environmental conditions. All plants depleted some labile as well as non-labile P pools whereas the less labile organic P pools (NaOH extractable P pools, acid extractable P pools) accumulated in the rhizosphere of legumes. The pattern of depletion and accumulation of some P pools differed between monoculture and mixed culture as well as among soils.

Conclusions

Plant growth and rhizosphere properties were mainly affected by soil type, but also by crop species whereas cropping system had the least effect. Wheat and the legumes depleted less labile inorganic P pools in some soils whereas less labile organic P pools (NaOH extractable P, acid extractable P) accumulated in the rhizosphere of legumes.     

A new root-nodulating symbiont of the tropical legume Sesbania, Rhizobium sp SIN-1, is closely related to R. galegae, a species that nodulates temperate legumes

Abstract Rhizobium sp. SIN-1, isolated in India from root nodules on the tropical legume Sesbania aculeata , also induces nitrogen-fixing nodules on roots of S. macrocarpa, S. speciosa, S. procumbens, S. punicea, S. rostrata , and Vigna unguiculata . Unlike Azorhizobium caulinodans , SIN-1 does not induce stem nodules on S. rostrata . The nodules induced by SIN-1 develop exclusively at the bases of secondary roots. Electron microscopic studies of mature nodule sections revealed rhizobia within intercellular spaces, indicating a 'crack entry' mechanism of root infection. SIN-1 is a fast-growing, acid-producing, salt-tolerant Rhizobium that utilizes a wide variety of carbon sources. The nodulation ( nod ) genes of this strain are located on a 300-MDa symbiosis ( sym ) plasmid. Fatty acid profile and sequence comparison of a 260-bp conserved region of the 16S rRNA gene demonstrated that SIN-1 is phylogenetically closely related to R. galegae , a species that nodulates temperate legumes.     

土壤氮磷添加下豆科草本植物生物固氮与磷获取策略的权衡机制

豆科草本植物固氮是陆地生态系统重要的自然氮输入方式, 影响着草地生产的经济性和可持续性。为探讨氮磷交互作用影响豆科草本植物生物固氮率的潜在生理生态机制, 该研究选取8种豆科草本植物分别种植在对照、氮肥添加、磷肥添加和氮磷耦合添加处理的土壤中, 进行野外盆栽实验。测定了初花期植物生物量和营养含量、根部碳水化合物含量、根际pH、根际柠檬酸含量、根际有效磷含量、植物根瘤生物量、磷含量及其生物固氮率。主要结果: 依赖于豆科物种, 氮添加显著促进了豆科草本植物根际磷的活化, 降低了根生物量分配以及根系非结构性碳水化合物含量。在两种磷添加处理下, 氮添加导致8种豆科草本植物根瘤生物量平均下降27%-36%, 生物固氮率平均下降20%-33%。磷添加降低了根际的磷活化, 但促进了豆科草本植物根系发育和非结构性碳水化合物的积累。在施氮和不施氮条件下, 磷添加分别使8种豆科草本植物的生物固氮率提高了45%-69%和0-47%。氮添加降低豆科草本植物生物固氮率, 其原因是氮添加提高了植物磷需求, 为活化更多磷, 豆科草本植物降低根系生物量和根系非结构性碳水化合物的含量, 导致根瘤发育受到限制。在氮添加的同时进行磷添加, 能够改善土壤氮磷平衡, 促进根系生长和非结构性碳水化合物积累, 缓解了增氮对生物固氮的抑制作用。     

Biogeography of symbiotic and other endophytic bacteria isolated from medicinal Glycyrrhiza species in China

A total of 159 endophytic bacteria were isolated from surface-sterilized root nodules of wild perennial Glycyrrhiza legumes growing on 40 sites in central and northwestern China. Amplified fragment length polymorphism (AFLP) genomic fingerprinting and sequencing of partial 16S rRNA genes revealed that the collection mainly consisted of Mesorhizobium, Rhizobium, Sinorhizobium, Agrobacterium and Paenibacillus species. Based on symbiotic properties with the legume hosts Glycyrrhiza uralensis and Glycyrrhiza glabra, we divided the nodulating species into true and sporadic symbionts. Five distinct Mesorhizobium groups represented true symbionts of the host plants, the majority of strains inducing N2-fixing nodules. Sporadic symbionts consisted of either species with infrequent occurrence (Rhizobium galegae, Rhizobium leguminosarum) or species with weak (Sinorhizobium meliloti, Rhizobium gallicum) or no N2 fixation ability (Rhizobium giardinii, Rhizobium cellulosilyticum, Phyllobacterium sp.). Multivariate analyses revealed that the host plant species and geographic location explained only a small part (14.4%) of the total variation in bacterial AFLP patterns, with the host plant explaining slightly more (9.9%) than geography (6.9%). However, strains isolated from G. glabra were clearly separated from those from G. uralensis, and strains obtained from central China were well separated from those originating from Xinjiang in the northwest, indicating both host preference and regional endemism.     

Phosphorus benefits of different legume crops to subsequent wheat grown in different soils of Western Australia

Certain legume crops, including white lupin (Lupinus albus L.), mobilise soil-bound phosphorus (P) through root exudates. The changes in the rhizosphere enhance P availability to these crops, and possibly to subsequent crops growing in the same soil. We conducted a pot experiment to compare phosphorus acquisition of three legume species with that of wheat, and to determine whether the legume crops influence growth and P uptake of a subsequent wheat crop. Field pea (Pisum sativum L.), faba bean (Vicia faba L.), white lupin (Lupinus albus L.) and wheat (Triticum aestivum L.) were grown in three different soils to which we added no or 20 mg P kg^–1 soil (P0, P20). Growth, P content and rhizosphere carboxylates varied significantly amongst crops, soils and P levels. Total P content of the plants was increased with applied phosphorus. Phosphorus content of faba bean was 3.9 and 8.8 mg/pot, at P0 and P20, respectively, which was about double that of all other species at the respective P levels. Field pea and white lupin had large amounts of rhizosphere carboxylates, whereas wheat and faba bean had negligible amounts in all three soils at both P levels. Wheat grew better after legumes than after wheat in all three soils. The effect of the previous plant species was greater when these previous species had received P fertiliser. All the legumes increased plant biomass of subsequent wheat significantly over the unplanted pots in all the soils. Faba bean was unparalleled in promoting subsequent wheat growth on all fertilised soils. This experiment clearly demonstrated a residual benefit of the legume crops on the growth of the subsequent wheat crop due to enhanced P uptake. Faba bean appeared to be a suitable P-mobilising legume crop plant for use in rotations with wheat.     

Molecular diversity and phylogeny of rhizobia associated with wild legumes native to Xinjiang, China

A total of 111 rhizobial strains were isolated from wild legumes in Xinjiang, an isolated region of northwest China. Nine genomic species belonging to four genera of Rhizobium, Mesorhizobium, Ensifer, and Bradyrhizobium were defined among these strains based on the characterization of amplified 16S ribosomal DNA restriction analysis (ARDRA), restriction fragment length polymorphism (RFLP) analysis of 16S-23S rDNA intergenic spacers (IGS), 16S rRNA gene sequencing and multilocus sequence analysis (MLSA). Twenty-five nodC types corresponding to eight phylogenetic clades were divided by RFLP and sequence analysis of the PCR-amplified nodC gene. The acid-producing Rhizobium and Mesorhizobium species were predominant, which may be related to both the local environments and the hosts sampled. The present study also showed the limitation of using nod genes to estimate the host specificity of rhizobia.     

Promotion of Peanut Growth by Co-inoculation with Selected Strains of <Emphasis Type="Italic">Bradyrhizobium</Emphasis> and <Emphasis Type="Italic">Azospirillum</Emphasis>

The ability of inoculated rhizobial strains to increase root nodulation of host legumes often depends on their competitiveness with existing native soil strains. Results of studies to date on rhizobial inoculation for improvement of peanut (Arachis hypogaea L.) production in Argentina have been inconsistent and controversial. In many cases, nodulation and yield of peanut crops have been increased by inoculation of specific rhizobial strains. Native peanut-nodulating strains are generally present in soils of agricultural areas, but their growth-promoting effect is often lower than that of inoculated strains. Many species of the genus Bradyrhizobium interact in a host-specific manner with legume species and form nitrogen-fixing root nodules. Other free-living rhizobacteria such as species of the genus Azospirillum are facultatively capable of interacting with legume roots and promoting plant growth. We evaluated and compared the effects of various single inoculation and co-inoculation treatments on peanut growth parameters in greenhouse and field experiments. In the greenhouse studies, co-inoculation with various Bradyrhizobium strains (native 15A and PC34, and recommended peanut inoculant C145), and Azospirillum brasilense strain Az39 generally resulted in increases in the measured parameters. The growth-promoting effect of 15A was similar to or higher than that of C145. In the field studies, 15A-Az39 co-inoculation had a greater promoting effect on measured growth parameters than did C145-Az39 co-inoculation. Our findings indicate that careful selection of native rhizobacterial strains adapted to peanut soils is useful in strategies for growth promotion, and that 15A in particular is a promising candidate for future inoculant formulation.     

Two C4-dicarboxylate transport systems in Rhizobium sp. NGR234: rhizobial dicarboxylate transport is essential for nitrogen fixation in tropical legume symbioses.

To investigate the role of dicarboxylate transport in nitrogen-fixing symbioses between Rhizobium and tropical legumes, we made a molecular genetic analysis of the bacterial transport system in Rhizobium sp. NGR234. This braod host range strain fixes nitrogen in association with evolutionarily divergent legumes. Two dicarboxylate transport systems were cloned from Rhizobium NGR234. One locus was chromosomally located, whereas the other was carried on the symbiotic plasmid (pSym) and contained a dctA carrier protein gene, which was analyzed in detail. Although the DNA and derived amino acid sequences of the structural gene were substantially homologous to that of R. meliloti, its promoter sequences was quite distinct, and the upstream sequence also exhibited no homology to dctB, which is found at this position in R. meliloti. A site-directed internal deletion mutant in dctA of NGR234 exhibited a (unique) exclusively symbiotic phenotype that could grow on dicarboxylates ex planta, but could not fix nitrogen in planta. This phenotype was found for tested host plants of NGR234 with either determinate- or indeterminate-type nodules, confirming for the first time that symbiosis-specific uptake of dicarboxylates is a prerequisite for nitrogen fixation in tropical legume symbioses.     

Nod factor signaling genes and their function in the early stages of Rhizobium infection

A lipochitosaccharide-based signal molecule that is secreted by Rhizobium, named Nod factor (NF), induces root nodule formation in legumes. This molecule is also essential for the establishment of bacterial infection. Genetic analyses in the legume species Lotus japonicus and Medicago truncatula have led to the identification of many components of the NF signaling cascade. At least three of these genes do not function exclusively in the Rhizobium symbiosis but are also essential for the formation of mycorrhiza, an endosymbiosis found in many higher plant species. Recent studies have advanced our understanding of the functions of NF signaling genes in the Rhizobium infection process and the extent to which these genes are unique to legumes.     

Biodiversity of Mimosa pudica rhizobial symbionts (Cupriavidus taiwanensis, Rhizobium mesoamericanum) in New Caledonia and their adaptation to heavy metal-rich soils

Rhizobia are soil bacteria able to develop a nitrogen-fixing symbiosis with legumes. They are taxonomically spread among the alpha and beta subclasses of the Proteobacteria. Mimosa pudica, a tropical invasive weed, has been found to have an affinity for beta-rhizobia, including species within the Burkholderia and Cupriavidus genera. In this study, we describe the diversity of M.?pudica symbionts in the island of New Caledonia, which is characterized by soils with high heavy metal content, especially of Ni. By using a plant-trapping approach on four soils, we isolated 96 strains, the great majority of which belonged to the species Cupriavidus taiwanensis (16S rRNA and recA gene phylogenies). A few Rhizobium strains in the newly described species Rhizobium mesoamericanum were also isolated. The housekeeping and nod gene phylogenies supported the hypothesis of the arrival of the C.?taiwanensis and R.?mesoamericanum strains together with their host at the time of the introduction of M.?pudica in New Caledonia (NC) for its use as a fodder. The C.?taiwanensis strains exhibited various tolerances to Ni, Zn and Cr, suggesting their adaptation to the specific environments in NC. Specific metal tolerance marker genes were found in the genomes of these symbionts, and their origin was investigated by phylogenetic analyses.