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1.
We compared the uptake of nitrogen, potassium and phosphorus (as well as 14C-labelled mannitol, 3H-labelled glutamate, and 32P-labelled phosphate) in three fast- and three slow-growing rhizobia. The fast-growing strains used were Rhizobium meliloti (isolated from Medicago sativa), R. trifolii (from Trifolium subterraneum), and Rhizobium spp from Leucaena leucocephala, while the slow-growing strains were R. japonicum (Glycine max), and two Rhizobium spp (from Centrosema pubescens and Crotolaria anagyroides). Slow-growing organisms preferentially utilized glutamate in the medium. Both fast- and slow-growing strains took up more NH+4-N than NO?3-N on a per cell basis. In the presence of mannitol, fast-growing strains can cause either acid or alkaline reactions, an effect that is dependent only on the N-source (NH+4 or NO?3). Uptake preferences of the fast-growing Leucaena isolate (UMKL 19) resembled those of the slow-growing rhizobia, further strengthening the argument that this organism (and others like it) may be intermediate between the normal fast- and slow-growing groups. Generally, the efficiency of uptake of N (either as NH+4 or NO?3), P, and therefore K, was greater in the fast-growing organisms.  相似文献   

2.
The average number of survivors of fast-growing medic rhizobia (3 strains), fast-growing Rhizobium leguminosarum types (6 strains) and slow-growing species (9 strains) following desiccation of sandy soil inoculated with 106 bacteria·g?1 soil was 727, 795 and 15,682 bacteria·g?1 soil, respectively. Survival in desiccated sandy soil was not influenced by the degree of extracellular polysaccharide production in strains of R. trifolii, nor was it influenced by growth of R. meliloti and slow-growing species in media of low water activity before desiccation in sandy soil.A progressive increase in numbers of fast-growing bacteria surviving desiccation was observed in sandy soil amended with increasing concentrations of powdered montmorillonite, but not with mont-morillonite added as a suspension to the soil. The clay had either a detrimental effect or no effect on the survival of the slow-growing rhizobia. Maltose, sucrose and polyvinylpyrrolidonc provided a greater degree of protection to both fast- and slow-growing rhizobia than was obtained with montmorillonite. The effect of polyethylene glycol 6000 was similar to the effect of montmorillonite, as the polymer only protected the fast-growing rhizobia and not the slow-growing species.  相似文献   

3.
Field pea (Pisum sativum L.) is widely grown in South Australia (SA), often without inoculation with commercial rhizobia. To establish if symbiotic factors are limiting the growth of field pea we examined the size, symbiotic effectiveness and diversity of populations of field pea rhizobia (Rhizobium leguminosarum bv. viciae) that have become naturalised in South Australian soils and nodulate many pea crops. Most probable number plant infection tests on 33 soils showed that R. l. bv. viciae populations ranged from undetectable (six soils) to 32×103 rhizobia g−1 of dry soil. Twenty-four of the 33 soils contained more than 100 rhizobia g−1 soil. Three of the six soils in which no R. l. bv. viciae were detected had not grown a host legume (field pea, faba bean, vetch or lentil). For soils that had grown a host legume, there was no correlation between the size of R. l. bv. viciae populations and either the time since a host legume had been grown or any measured soil factor (pH, inorganic N and organic C). In glasshouse experiments, inoculation of the field pea cultivar Parafield with the commercial Rhizobium strain SU303 resulted in a highly effective symbiosis. The SU303 treatment produced as much shoot dry weight as the mineral N treatment and more than 2.9 times the shoot dry weight of the uninoculated treatment. Twenty-two of the 33 naturalised populations of rhizobia (applied to pea plants as soil suspensions) produced prompt and abundant nodulation. These symbioses were generally effective at N2 fixation, with shoot dry weight ranging from 98% (soil 21) down to 61% (soil 30) of the SU303 treatment, the least effective population of rhizobia still producing nearly double the growth of the uninoculated treatment. Low shoot dry weights resulting from most of the remaining soil treatments were associated with delayed or erratic nodulation caused by low numbers of rhizobia. Random amplified polymorphic DNA (RAPD) polymerase chain reaction (PCR) fingerprinting of 70 rhizobial isolates recovered from five of the 33 soils (14 isolates from each soil) showed that naturalised populations were composed of multiple (5-9) strain types. There was little evidence of strain dominance, with a single strain type occupying more than 30% of trap host nodules in only two of the five populations. Cluster analysis of RAPD PCR banding patterns showed that strain types in naturalised populations were not closely related to the current commercial inoculant strain for field pea (SU303, ≥75% dissimilarity), six previous field pea inoculant strains (≥55% dissimilarity) or a former commercial inoculant strain for faba bean (WSM1274, ≥66% dissimilarity). Two of the most closely related strain types (≤15% dissimilarity) were found at widely separate locations in SA and may have potential as commercial inoculant strains. Given the size and diversity of the naturalised pea rhizobia populations in SA soils and their relative effectiveness, it is unlikely that inoculation with a commercial strain of rhizobia will improve N2 fixation in field pea crops, unless the number of rhizobia in the soil is very low or absent (e.g. where a legume host has not been previously grown and for three soils from western Eyre Peninsula). The general effectiveness of the pea rhizobia populations also indicates that reduced N2 fixation is unlikely to be the major cause of the declining field pea yields observed in recent times.  相似文献   

4.
We have evaluated the genetic diversity and phylogeny of alfalfa rhizobia, originating from different types of soils in Serbia and their ability to establish an effective symbiosis with alfalfa (Medicago sativa L.). A collection of 65 strains isolated from root nodules of alfalfa were characterized by rep-PCR analysis, partial and complete 16S rDNA gene and recA gene sequencing, as well as atpD gene sequencing and DNA–DNA hybridizations. The results of the sequence analyses revealed that Sinorhizobium meliloti is the dominant species in alfalfa nodules. Only one strain was identified as Sinorhizobium medicae, two strains as Rhizobium tibeticum and one strain as Rhizobium sp. Despite the fact that the majority of strains were identified as S. meliloti, a high genetic diversity at strain level was detected. Almost all isolates shared the ability to nodulate and fix nitrogen with M. sativa, except 11 of them, which were incapable of fixing nitrogen with this species. About 50% of the isolates showed values of symbiotic effectiveness (SE) above 50%, while 10% of the strains were highly effective with SE values above 70%. Some of the strains which were highly effective in nitrogen fixation at the same time could intensively solubilize phosphates, offering a possibility for multipurpose inoculum development. This was the first genetic study of rhizobia isolated from this region and also the first report of natural presence of R. tibeticum in root nodules of M. sativa.  相似文献   

5.
Biological nitrogen fixation plays a key role in agriculture sustainability, and assessment of rhizobial diversity contributes to worldwide knowledge of biodiversity of soil microorganisms, to the usefulness of rhizobial collections and to the establishment of long-term strategies aimed at increasing contributions of legume-fixed N to agriculture. Although in recent decades the use of molecular techniques has contributed greatly to enhancing knowledge of rhizobial diversity, concerns remain over simple issues such as the effects of sampling on estimates of diversity. In this study, rhizobia were isolated from nodules of plants grown under field conditions, in pots containing soil, or in Leonard jars receiving a 10−2 or a 10−4 serially-diluted soil inoculum, using one exotic (soybean, Glycine max) and one indigenous (common bean, Phaseolus vulgaris) legume species. The experiments were performed using an oxisol with a high population (105 cells g−1 soil) of both soybean rhizobia, composed of naturalized strains introduced in inoculants and of indigenous common-bean rhizobia. BOX-PCR was used to evaluate strain diversity, while RFLP-PCR of the ITS (internally transcribed spacer) region with five restriction enzymes aimed at discriminating rhizobial species. In both analyses the genetic diversity of common-bean rhizobia was greater than that of soybean. For the common bean, diversity was greatly enhanced at the 10−4 dilution, while for the soybean dilution decreased diversity. Qualitative differences were also observed, as the DNA profiles differed for each treatment in both host plants. Differences obtained can be attributed to dissimilarity in the history of the introduction of both the host plant and the rhizobia (exotic vs. indigenous), to host-plant specificity, rhizobial competitiveness, and population structure, including ease with which some types are released from microcolonies in soil. Therefore, sampling method should be considered both in the interpretation and comparison of the results obtained in different studies, and in the setting of the goals of any study, e.g. selection of competitive strains, or collection of a larger spectrum of rhizobia. Furthermore, effects of sampling should be investigated for each symbiosis.  相似文献   

6.
The ability of 4 strains of Rhizobium trifolii to compete with naturalized strains in nodulating Trifolium subterruneum cv. Mt Barker and cv. Woogenellup was assessed at 5 sites in New South Wales. The populations of naturalized rhizobia at these sites ranged from 4 × 106 rhizobia/g to one where no rhizobia were detected. The introduced strains were inoculated singly or as mixed strain inocula onto seed of the host at 2 × 106 rhizobia/seed. There were marked differences in competitive ability between the strains but these differences were modified by the host cultivar and the site.At the R. trifolii-free site the inoculum strain formed 100% of the nodules in the 1st yr; by the second year serologically unrelated strains had invaded the plots and these formed almost all of the nodules in the 3rd yr. At the site where competition was greatest (4 × 106 naturalized rhizobia/g), there were no differences in the competitive abilities of the strains in the first year but at all other sites WU95 was superior whether used as a single strain or in a mixed strain inoculum. In these sites also the proportion of nodules formed by the inoculum strains declined markedly by the 2nd yr.  相似文献   

7.
Although rhizobia for common bean (Phaseolus vulgaris L.) are established in most Brazilian soils, understanding of their genetic diversity is very poor. This study characterized bean strains from two contrasting ecosystems in Brazil, the Northeast Region, with a semi-arid climate and neutral soils and the South Region, with a humid subtropical climate and acid soils. Seedlings of the cultivars Negro Argel and Aporé were used to trap 243 rhizobial isolates from 12 out of 14 sites. An analysis of ERIC-PCR products revealed enormous variability, with 81% of the isolates representing unique strains considering a level of 70% of similarity. In general, there was no effect of either the bean cultivar, or the ecosystem on rhizobial diversity. One-hundred and one strains showing genetic relatedness (ERIC-PCR) less than 70% were further analyzed using restriction fragment length polymorphism (RFLP) of the 16 S rDNA cleaved with five restriction enzymes. Twenty-five different profile combinations were obtained. Rhizobium etli was the predominant species, with 73 strains showing similar RFLP profiles, while 12 other strains differed only by the profile with one restriction enzyme. Fifty strains were submitted to sequencing of a 16 S rDNA fragment, and 34 clustered with R. etli, including strains with RFLP-PCR profiles similar to those species or differing by one restriction enzyme. However, other strains differing by one or two enzymes were genetically distant from R. etli and two strains with identical profiles showed higher similarity to Sinorhizobium fredii. Other strains showed higher similarity of bases with R. tropici, R. leguminosarum and Mesorhizobium plurifarium, but some strains were quite dissimilar and may represent new species. Great variability was also verified among the sequenced strains in relation to the ability to grow in YMA at 40 °C, in LB, to synthesize melanin in vitro, as well as in symbiotic performance, including differences in relation to the described species, e.g. many R. etli strains were able to grow in LB and in YMA at 40 °C, and not all R. tropici were able to nodulate Leucaena.  相似文献   

8.
In buffered media Trichoderma grew best with l-alanine, l-aspartic acid, l-glutamic acid and casamino acids as sources of N. Growth on NH+4-N was consistently superior to growth on NO?3-N. Some isolates of T. koningii Oud. and T. hamatum (Bon.) Bain, were incapable of utilizing NO3?. The best carbon sources were dextrose, fructose, mannose, galactose, xylose, ribose, trehalose and cellobiose. The ability to use melezitose, raffinose, sucrose and inulin as sources of carbon were related to taxonomic units. It is proposed that the ability to use these carbon sources as well as tests for extracellular oxidase, NO?3 utilization and temperature maxima may be useful to delineate and identify species. All isolates tested readily decomposed pure cellulose but the time required to adapt to the cellulose medium varied considerably. The relative vigor of cellulose decomposition was not related to the loss in weight of pine needles or dogwood leaves induced by Trichoderma.  相似文献   

9.
It is generally accepted that there are two major centers of genetic diversification of common beans (Phaseolus vulgaris L.): the Mesoamerican (Mexico, Colombia, Ecuador and north of Peru, probably the primary center), and the Andean (southern Peru to north of Argentina) centers. Wild common bean is not found in Brazil, but it has been grown in the country throughout recorded history. Common bean establishes symbiotic associations with a wide range of rhizobial strains and Rhizobium etli is the dominant microsymbiont at both centers of genetic diversification. In contrast, R. tropici, originally recovered from common bean in Colombia, has been found to be the dominant species nodulating field-grown common-bean plants in Brazil. However, a recent study using soil dilutions as inocula has shown surprisingly high counts of R. etli in two Brazilian ecosystems. In the present study, RFLP-PCR analyses of nodABC and nifH genes of 43 of those Brazilian R. etli strains revealed unexpected homogeneity in their banding patterns. The Brazilian R. etli strains were closely similar in 16S rRNA sequences and in nodABC and nifH RFLP-PCR profiles to the Mexican strain CFN 42T, and were quite distinct from R. etli and R. leguminosarum strains of European origin, supporting the hypothesis that Brazilian common bean and their rhizobia are of Mesoamerican origin, and could have arrived in Brazil in pre-colonial times. R. tropici may have been introduced to Brazilian soils later, or it may be a symbiont of other indigenous legume species and, due to its tolerance to acidic soils and high temperature conditions became the predominant microsymbiont of common bean.  相似文献   

10.
The mechanisms for maintaining the species diversity of plant communities under conditions of resource limitation is an important subject in ecology. How interspecific relationships influence the pattern of nutrient absorption by coexisting species in N-limited ecosystems is still disputed. We investigated the effect of neighbor species on the uptake of inorganic and organic N by three common plant species using 15N tracer techniques in a semi-arid alpine steppe on the northern Tibet. The results showed that the plant species varied in their capacity to absorb NO3 ?-N, NH4 +-N, and glycine-N with or without neighbor species. Carex moorcroftii and Leontopodium nanum showed much more plasticity in resource utilization than Stipa purpurea when neighbor species were present. When C. moorcroftii and S. purpurea coexisted, they all increased their 15N uptake for the NO3 ?-N (C. moorcroftii 2.2-fold increase and S. purpurea 2.2-fold increase) and glycine-N treatments (C. moorcroftii 2.9-fold increase and S. purpurea 3.4-fold increase), which indicated that neighborhood had a positive effect for N absorption between the two species. However, L. nanum was a less effective competitor for N utilization than the neighbor species across almost all treatments. The dominant species appeared to have an inhibitory effect on N absorption by the accompanying species in this alpine steppe environment. Thus, interspecific neighbor pairs may result in both a mutually beneficial cooperative relationship and a competitive relationship among neighbors in resource use patterns in extreme environments. Resource use plasticity in altered neighbor species may be due to phenotypic plasticity based on the conditions of the realized niche, offering a valuable insight into niche complementarity and providing a general and important mechanism for resource partitioning in an alpine area.  相似文献   

11.
The diversity of 110 rhizobial strains isolated from Acacia abyssinica, A. seyal, A. tortilis, Faidherbia albida, Sesbania sesban, Phaseolus vulgaris, and Vigna unguiculata grown in soils across diverse agro-ecological zones in southern Ethiopia was assessed using the Biolog™ system and amplified fragment length polymorphism (AFLP) fingerprinting technique. By cluster analysis of the metabolic and genomic fingerprints, the test strains were grouped into 13 Biolog and 11 AFLP clusters. Twenty-two strains in the Biolog method and 15 strains in the AFLP analysis were linked to eight and four reference species, respectively, out of the 28 included in the study. Most of the test strains (more than 80% of 110) were not related to any of the reference species by both methods. Forty-six test strains (42% of 110) were grouped into seven corresponding Biolog and AFLP clusters, suggesting that these groups represented the same strains, or in some cases clonal descendants of the same organisms. In contrast to the strains from S. sesban, isolates from Acacia spp. were represented in several Biolog and AFLP clusters indicating the promiscuous nature of the latter and widespread occurrence of compatible rhizobia in most of the soil sampling locations. The results showed that indigenous rhizobia nodulating native woody species in Ethiopian soils constituted metabolically and genomically diverse groups that are not linked to reference species.  相似文献   

12.
Naturally growing Sesbania species with tolerance to unfavourable habitats are widely distributed in non-cultivated seasonally wetland areas in Uruguay. We investigated the relative abundance, diversity and symbiotic efficiency of Sesbania punicea and S. virgata rhizobia in three ecologically different undisturbed and water-logged sites in Uruguay. Numbers of native-soil rhizobia infective on S. punicea or S. virgata were low, with higher numbers associated with the presence of S. virgata. Plants of S. virgata inoculated with soil suspension showed aerial and nodule biomass greater than that obtained with S. punicea. The rhizobia nodulating Sesbania species in water-logged lands in different regions of Uruguay were diverse differing in growth rates, acid production, growth at 39°C and in LB medium, host range and symbiotic efficiency. Seventeen representative strains clustered into four groups on the basis of phenotypic characteristics, ARDRA and DNA fingerprinting (GTG5-PCR). Partial sequence of 16S rRNA from eight of these strains classified them into at least two genera with four species: Azorhizobium doebereinerae, Rhizobium sp. related to R. etli and two different Rhizobium sp.-Agrobacterium. Our results confirm the presence of the specie Azorhizobium doebereinerae as microsymbionts of S. virgata in South America. No strain of Rhizobium etli has previously been reported as a microsymbiont of Sesbania, though R. etli like organisms have also been recovered from Dalea purpurea and Desmanthus illinoensis. Significant increases in dry matter production were obtained with S. virgata plants inoculated with selected rhizobial strains under growth chamber conditions.  相似文献   

13.
Free-living rhizobia are sensitive to soils and artificial media that are acidic. Both excessive H+ and Al released from acid-soluble minerals appear to be toxic. The complex, heterotrophic nutrient requirements of rhizobia and the joint occurrence of Al3+ and hydroxo-, sulphato-, phosphato-, fluoro-, and other Al species have prevented a precise attribution of toxicity to the Al species. In the present study, a medium composed of 0.3 mM MgSO4, 2 mM CaCl2, and 10 mM sucrose (the basal medium) enabled a 1000-fold cell increase at pH 4.6 or above. Additions of 1 μM AlCl3 to the basal medium were highly intoxicating, especially at higher pH: below pH 5.0 cell numbers increased slightly; at pH 5.0 cell numbers did not change from the inoculum; at higher pH values the cell numbers declined. Similar trends were observed for La3+ and Cu2+ intoxication. Uptake of methylene blue, a positively charged dye useful as a probe of cell-surface electrical potential, was inhibited by pH reductions between pH 3.5 and 6.0. Factors that decrease cell-surface negativity (such as lower pH) reduce the intoxication by cations in plant roots, but the pH responsiveness of the rhizobia in our system was much greater than the pH responsiveness of plants. Although plant-root intoxication by mononuclear hydroxo-Al species has been discounted, rhizobia may be sensitive to those species. These results have implications for the management of rhizobia in acidic soils and for the development of resistant strains.  相似文献   

14.
The shoot dry weight of alfalfa inoculated with an effective strain of Rhizobium meliloti mixed with an ineffective strain in different ratios was found to be directly proportional to the log of the number of effective nodules. Consequently the comparison of the shoot dry weight of plants inoculated with a mixture of effective and ineffective strains with the shoot dry weight of plants inoculated with the effective strain should allow the estimation of the relative competitiveness of the effective strains. To check this. the competitiveness of 14 antibiotic-resistant strains of R. leguminosarum was evaluated in this way and compared with the ability of the strains to form nodules when inoculated to seeds of Vicia faba planted in a soil containing indigenous R. leguminosarum. The percentage of recovery of the inoculum strains in the nodules of field-grown fababeans was positively correlated with the competitiveness of the strains as estimated by the greenhouse test. This simple way of evaluating the nodulating competitiveness of strains of rhizobia being indicative of their competitive behaviour with indigenous rhizobia in the field could therefore be useful for screening a large number of strains for competitiveness.  相似文献   

15.
Kura clover (Trifolium ambiguum M.B.) is a perennial rhizomatous forage legume whose use is currently limited by difficulties in its establishment in part attributable to nodulation problems and very specific rhizobial requirements. A limited number of Kura clover-nodulating rhizobial strains are currently available and many have a limited effectiveness. In this study, 128 rhizobia were isolated from four sites in the center of origin of Kura clover (i.e., two in Azerbaijan, one in Armenia, and one in Northwest Iran) using the three ploidy levels of Kura clover (diploid, tetraploid, and hexaploid), red clover (Trifolium pratense L.), and white clover (Trifolium repens L.) plants as trap hosts. Rhizobia were fingerprinted using repetitive extragenic palindromic polymerase chain reaction (BOXA1R primer) and their genetic diversity was measured using the Shannon-Weaver diversity index. The nodulation specificity and phenotypic diversity of a subset of 13 isolates was determined. Genetic diversity among the 128 isolates was large and similar for rhizobia grouped according to their geographic origin or original host plant. Phenotypic diversity was significant; percentage of similarity among 13 isolates ranging between 38 and 92%. Nodulation specificity of the Kura clover-nodulating rhizobial isolates studied was less complex and not as clearly delineated as previously reported. Some strains originally isolated from Kura clover could effectively nodulate more than one ploidy level of Kura clover and even one or both of two other Trifolium species (i.e., red clover and white clover). Three strains formed effective nodules on both Kura clover and white clover; however, none promoted plant growth of both species to levels currently obtained with commercial inoculants when evaluated in a growth chamber. Rhizobial isolates that are highly effective with both species have yet to be identified.  相似文献   

16.
Rhizobial symbionts were isolated from the surface (0-0.5 M) and phreatic (3.9-5.0 M) root environments of a mature mesquite woodland in the Sonoran Desert of Southern California, and from variable depths (0-12 m) of non-phreatic mesquite ecosystems in the Chihuahuan Desert of New Mexico. They were tested for their ability to tolerate high salinity, and respire NO3− as mechanisms of free-living survival. Sixteen of 25 isolates were grown in yeast-extract mannitol (YEM) broth at NaCl concentrations of 2 (basal concentration), 100, 300, 500 and 600 mM, and their specific growth rates, cell dry weight and lag times were determined. Twenty of the 25 isolates were also grown in YEM broth under anaerobic conditions with or without 10 mM KNO3. Three categories of NaCl salinity responses were observed: (1) eight isolates showed decreased specific growth rates at NaCl concentrations of 100, 300 and 500 mM, but they nevertheless remained viable at 500 mM NaCl concentration; (2) the specific growth rate of six isolates increased significantly at 100 and 300 mM NaCl; and (3) specific growth rates of two isolates were significantly greater than the base-rate at all concentrations of NaCl. Five of 11 of the Bradyrhizobium isolates tested respired NO3−, but showed no growth. Seven Rhizobium isolates, three from the deep (3.9-5 m) phreatic rhizobial community, and four from the surface community denitrified NO3− but only the isolates from the phreatic community displayed anaerobic growth. Long-term interactions between rhizobial and bradyrhizobial communities and the surface and phreatic root environments of the mature Sonoran Desert mesquite woodland appear to have selected for strains of NO3− respiring rhizobia, general salt tolerance of both rhizobial and bradyrhizobial symbionts, and strains of weak facultative halophilic bradyrhizobia. These survival characteristics of mesquite rhizobia may be important regarding mesquite's establishment and long-term productivity in marginal desert soils, and may provide novel types of rhizobia for food crops growing in harsh environments.  相似文献   

17.
This work investigated the effect of plant species (Eucalyptus camaldulensis vs Arundo donax) on N-turnover during land application of pre-treated municipal wastewater. The study was carried out in 40-L pots under field conditions and revealed strong effects of plant species on N uptake, potential nitrification rate (PNR), and gaseous losses of N. E. camaldulensis accumulated 38% more N in its biomass than A. donax. With regard to the soil N, there was no effect of plant species on total Kjeldahl N content. The lower PNR measured in pots planted with A. donax could not be explained by differences in archaeal or bacterial amoA gene copies; suggesting that plant species affected the activity of nitrifiers.The loads of NH4+-N applied were not found to have delayed the oxidation of NH4+-N, despite the differences in PNR, as indicated by soil solution and soil NH4+-N concentrations in the rhizosphere of the two plant species throughout the period of the study. However, decreased concentrations of NO3−-N were measured in the pots planted with A. donax from the end of June onwards. This finding implies increased losses of N through denitrification and reveals a direct effect of plant species on the activity of denitrifiers since differences in copy numbers of denitrification genes (nirK, nirS, nosZ) were observed only early in the season. Our findings reveal a critical role of plant species on N cycling in terrestrial environments with important implications for the management/restoration of N-polluted areas, such as riparian zones, and for land treatment systems and constructed wetlands.  相似文献   

18.
We examined the bean rhizobia community other than the predominant species Rhizobium etli present in soils of a region that is part of the range occupied by the host in Northwest Argentina, which showed Rep and 16S rDNA RFLP polymorphism. Two populations represented by isolates T29N3L and T44N22P were found to be distinct chromosomal genotypes and closely related to species Rhizobium tropici and Agrobacterium rhizogenes. Their symbiotic genes were analyzed and found to cluster with those from R. tropici as well as with rhizobia isolated from leguminous trees. Three nodulation metabolites produced by T44N22P were detected which are tetra- and pentameric chitocompounds, N-methylated, O-carbamoylated, and N-substituted either by a C18:0 or C18:1 acyl chain at their non-reducing end, and all them sulphated at the reducing end. Isolates T29N3L and T44N22P exhibited broad host range but unlike T29N3L, only T44N22P was able to efficiently nodulate Medicago truncatula.  相似文献   

19.
The behaviour of Rhizobium strains introduced separately into soil from a contaminated site with high concentrations of heavy metals (mainly Zn and Hg), and the role of plasmids in the ecology of these rhizobia strains were studied. Six Rhizobium leguminosarum biovar trifolii strains, from different sources and with different plasmid contents, were selected. Two of them were isolated from nodules of subterranean clover plants (Trifolium subterraneum) grown in the contaminated soil and four were from an uncontaminated soil. After inoculation with approximately 107 cells g−1 soil, of each strain, survival and plasmid stability were assessed over a period of 12-18 months. Differences in survival of Rhizobium strains were only detected more than 12 months after inoculation. After 18 months it was clear that survival in contaminated soil was greatest in the two strains originally isolated from that contaminated soil, and also by two of the strains originally isolated from uncontaminated soil. The latter two strains were also the only ones that showed changes in their plasmid profiles. The remaining isolates had the lowest populations, and their plasmid profiles were unchanged and similar to the parent strains.  相似文献   

20.
Isolates of Rhizobium phaseoli resistant to spergon (2,3,5,6-tetrachloro-l,4-benzoquinone), Rhizobium meliloti resistant to thiram (tetramethylthiuram disulfide) and of a cowpea Rhizobium resistant to phygon (2,3-dichloro-l,4-naphthoquinone) were obtained by culturing the bacteria in media with increasing concentrations of these fungicides. The cultures grew in media with 200 μg thiram ml?1, 150 μg spergon ml?1 or 400 μg phygon ml?1. Spergon-tolerant R. phaseoli was sensitive to thiram, and thiram-tolerant R. meliloti was sensitive to spergon. The dry weights of beans, alfalfa and cowpeas and the amount of N2 fixed were the same for plants inoculated with the fungicide-resistant or the sensitive parent rhizobia. However, when the three parent Rhizobium strains were applied to seeds treated with the three fungicides, the plants that developed were stunted, chlorotic, grew poorly and fixed little or no N2. By contrast, beans, alfalfa or cowpea plants derived from seeds coated with spergon, thiram or phygon and inoculated with the resistant rhizobia grew as well and fixed as much N2 as legumes derived from seeds not treated with the pesticides. These findings provide the basis for a simple method for simultaneously allowing for N2 fixation and seed protection of legumes.  相似文献   

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