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1.
The Medicago sativa-Sinorhizobium symbiosis is challenged by acidity, resulting in generally poor nodulation and production. Medicago murex, however, can nodulate and grow at low pH. The effect of low pH on signal exchange in the Sinorhizobium-Medicago symbiosis was studied to gain a greater understanding of the basis for poor nodulation of M. sativa compared to M. murex. Root exudates from M. sativa and M. murex grown in buffered nutrient solution at pH 4.5, 5.8 and 7.0, were collected to measure the expression of nodB induction in Sinorhizobium. A nodB-gusA fusion was constructed and inserted into Sinorhizobium medicae strains WSM419 (acid tolerant) and CC169 (acid sensitive). We identified greater induction by root exudates from both Medicago spp. collected at pH 4.5 than at pH 5.8 and 7.0, less induction by M. murex than M. sativa and less induction of WSM419 than CC169. The same major inducing compounds, 4′,7-dihydroxyflavanone (liquiritigenin), 4′,7-dihydroxyflavone, and 2′,4′,4-trihydroxychalcone (isoliquiritigenin), were identified in exudates of M. murex and M. sativa at all pH values, although in increasing amounts at lower pH. Poor nodulation of M. sativa relative to M. murex under acid conditions is not the consequence of decreased induction of Sinorhizobium nodB by chemical inducers present in the root exudates of both species at low pH.  相似文献   

2.
In acid soil, low pH, reduced availability of nutrients, and toxicity of Al and Mn limit plant growth and the survival and effectiveness of rhizobia. The symbiosis between legumes and rhizobia is particularly sensitive to acid soil stress. A pot experiment evaluated whether Bradyrhizobium japonicum strain growth on acidic agar media would predict ability to colonize the rhizosphere and form effective nodules in acidic soils. Three Indonesian strains of B. japonicum with similar effectiveness at neutral pH in sand culture but with different tolerance of acid soil stress factors in agar media, and an acid-tolerant commercial strain (CB1809) of comparable effectiveness, were tested in three acid soils using the Al tolerant soybean (Glycine max cv PI 416937). At 7 days after inoculation all strains had achieved large rhizosphere populations, but by day 14 the rhizosphere population of the acid-sensitive strain had decreased, while the more acid-tolerant strains increased. The acid-tolerant strains had significantly greater nodulation and symbiotic effectiveness than plants inoculated with the acid-sensitive strain. Laboratory prescreening of B. japonicum for acid, Al and Mn tolerance in acid media successfully identified strains which were symbiotically competent in low pH soils.  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.
Biserrula pelecinus is a pasture legume species new to Australian agriculture. The potential N benefit from B. pelecinus pastures in agricultural systems may not be realised if its symbiotic interactions with Mesorhizobium spp. are not well understood. This study evaluated the symbiotic interactions of four strains of Biserrula root-nodule bacteria (WSM1271, WSM1283, WSM1284, WSM1497) with four genotypes of B. pelecinus (cv. Casbah, 93GRC4, 93ITA33, IFBI1) and with a range of related legumes, including species known to be nodulated by strains of Mesorhizobium loti and other Mesorhizobium spp. Structures of root nodules were studied using light and electron microscopy enabling the ultrastructure of effective and ineffective nodules to be compared. B. pelecinus always formed typical indeterminate, finger-like nodules. The number of bacteroids inside symbiosomes varied between host×strain combinations, however, nodules formed by ineffective associations had well developed peribacteroid membranes and abundant bacteroids. Considerable variation was found in N2-fixing effectiveness of strains isolated from B. pelecinus on the four B. pelecinus genotypes. Strains WSM1271, WSM1284 and WSM1497 nodulated Astragalus membranaceus, only strains WSM1284 and WSM1497 nodulated Astragalus adsurgens. Strain WSM1284 also nodulated Dorycnium rectum, Dorycnium hirsutum, Glycyrrhiza uralensis, Leucaena leucocephala, Lotus edulis, Lotus glaber, Lotus maroccanus, Lotus ornithopodioides, Lotus pedunculatus, Lotus peregrinus, Lotus subbiflorus and Ornithopus sativus. The four strains from B. pelecinus did not nodulate Amorpha fruticosa, Astragalus sinicus, Cicer arietinum, Hedysarum spinosissimum, Lotus parviflorus, Macroptilium atropurpureum or Trifolium lupinaster. M. loti strain SU343 nodulated all four genotypes of B. pelecinus. However, M. loti strain CC829 only nodulated B. pelecinus genotypes 93ITA33 and IFBI1 and the nodules were ineffective. The root nodule isolates from H. spinosissimum (E13 and H4) nodulated B. pelecinus cv. Casbah whereas the commercial inoculant strain for Cicer (CC1192) could not nodulate any genotype of B. pelecinus. These results indicate that strains WSM1271, WSM1283 and WSM1497 isolated originally from B. pelecinus have a specific host range while strain WSM1284 is promiscuous in its capacity to nodulate with a broad range of related species. As B. pelecinus can be nodulated by Mesorhizobium spp. from other agricultural legumes, particularly Lotus, there is an opportunity to utilise this trait in cultivar development.  相似文献   

5.
For optimum production, the use of commercial rhizobial inoculant on pea (Pisum sativum L.) at seeding is necessary in the absence of compatible rhizobial strains or when rhizobial soil populations are low or symbiotically ineffective. Multiple site experiments were conducted to characterize the abundance and effectiveness of resident populations of Rhizobium leguminosarum bv. viciae (Rlv) in eastern Canadian prairie soils. A survey of 20 sites across a broad geographical range of southern Manitoba was carried out in 1998 and was followed by more intensive study of five of the sites in 1999 and 2000. Appreciable nodulation of uninoculated pea was observed at all sites which had previously grown inoculated pea. However, uninoculated pea grown at two sites, which had not previously grown pea, had negligible nodulation. Likewise, wild Lathyrus sp. and Vicia sp. plants collected from uncultivated areas adjacent to agricultural sites were poorly nodulated. In the more intensively studied sites, there was a tendency towards higher nodulation in pea plants receiving commercial inoculant containing Rlv strain PBC108 across all site-years (e.g., 4.7% in nodulation and 22% in nodule mass), but the effect was significant at only 2 of 10 site-years. Despite a relatively high range of soil pH (6-8), regression analysis indicated that decreasing soil pH resulted in lower nodulation rates. Likewise, electrical conductivity (EC) was correlated to nodulation levels, however the effect of EC was likely more indicative of the influence of soil texture and organic matter than salinity. As with nodulation, commercial inoculation tended to increase above-ground dry matter (DM) and fixed-N (estimated by the difference method) at the early pod-filling stage, but again the effects were significant at only 2 of 10 site-years. Specifically, above-ground DM and fixed-N levels were up to 29 and 51% greater, respectively, in inoculated compared to non-inoculated treatments at these sites. Addition of N-fertilizer at a rate of 100 kg N ha−1 decreased nodulation at almost all site-years (by as much as 70% at one site), but rarely resulted in increases in above-ground DM compared to inoculated plots. The study indicates for the first time that populations of infective, and generally effective strains of Rlv occur broadly in agricultural soils across the eastern Canadian prairie, but that there is a tendency for increased symbiotic efficiency with the use of commercial inoculant.  相似文献   

6.
Environmentally friendly control measures are needed for the soil-borne pathogen, Pythium ultimum. This pathogen can cause severe losses to field- and greenhouse-grown cucumber and other cucurbits. Live cells and ethanol extracts of cultures of the bacterium Serratia marcescens N4-5 provided significant suppression of damping-off of cucumber caused by P. ultimum when applied as a seed treatment. Live cells of this bacterium also suppressed damping-off caused by P. ultimum on cantaloupe, muskmelon, and pumpkin. Culture filtrates from strain N4-5 contained chitinase and protease activities while ethanol extracts contained the antibiotic prodigiosin, the surfactant serrawettin W1, and possibly other unidentified surfactants. Production of prodigiosin and serrawettin W1 was temperature-dependent, both compounds being detected in extracts from N4-5 grown at 28 °C but not in extracts from N4-5 grown at 37 °C. Ethanol extracts from strain N4-5 grown at 28 °C inhibited germination of sporangia and mycelial growth by P. ultimum in in vitro experiments. There was no in vitro inhibition of P. ultimum associated with ethanol extracts of strain N4-5 grown at 37 °C. Prodigiosin, purified from two consecutive thin-layer chromatography runs using different solvent systems, inhibited germination of sporangia and mycelial growth of P. ultimum. Another unidentified compound(s) also inhibited germination of sporangia but did not inhibit mycelial growth. There was no in vitro inhibition associated with serrawettin W1. These results demonstrate that live cells and cell-free extracts of S. marcescens N4-5 are effective for suppression of damping-off of cucumber caused by P. ultimum possibly due in part to the production of the antibiotic prodigiosin.  相似文献   

7.
In previous work, Variovorax paradoxus strain HB44, next to Burkholderia terrae BS001 and Dyella japonica BS003, were found to be selected in the mycosphere of the tricholomataceous fungi Laccaria proxima and Lyophyllum sp. strain Karsten in an acid soil denoted G. V. paradoxus HB44 showed poor survival in G bulk soil, irrespective of prior soil sterilization, and this poor survival also occurred for B. terrae BS001 and D. japonica BS003. In contrast, the survival rate of strain HB44 in two other soils, with pH values > 5.5, was significantly raised. Also, significantly enhanced strain HB44 survival in G soil was found if the pH was raised to 5.5 or 6.5, and it was even shown to grow (in the presence of the exogenous carbon source glycerol) at such pH values in the sterile G soil. This behaviour was similar to that of the V. paradoxus type strain. Strikingly, Lyophyllum sp. strain Karsten, when colonizing the sterilized G soil, significantly raised the soil pH from about 4.6 to ≥5.0. The pH raise was dependent on time, hyphal development, as well as on initial soil pH, but was consistent throughout. The modulated soil pH conditions were shown to be permissive for the survival and growth of strain HB44, and this was extended to strains BS001 and BS003. These findings corroborate the hypothesis that L. sp. strain Karsten provides a suitable habitat for acid-sensitive strains like HB44, BS001 and BS003 in its mycosphere in acid soil, which is strongly defined by the establishment of a growth-permissive pH.  相似文献   

8.
Lime pelleting of the inoculated seed is recommended for most pasture legume species to improve survival of the rhizobia on the seed and to counter deleterious effects of soil or fertiliser acidity on rhizobial numbers. Except for New South Wales, lime pelleting is specifically not recommended for serradella (Ornithopus spp.). Our objectives were to evaluate effects of lime pelleting on bradyrhizobial numbers on seed, and nodulation and growth of the serradella plants. Three experiments are reported at two acid-soil sites in northern New South Wales involving four cultivars of yellow serradella (Ornithopus compressus) and Bradyrhizobium sp. (Lupinus) strains WSM471 (current inoculant strain) and WU425 and WSM480. Lime pelleting increased bradyrhizobial numbers on seed, 24 h after inoculation, by an average of 90%. Similarly, lime pelleting increased nodulation and shoot dry matter of the inoculated plants by an average of 57 and 28%, respectively. The three strains were similar in effects on plant growth. Relative values for shoot dry weight, averaged over sites, were 100 for WSM471 and 98 for both WU425 and WSM480. Our results confirmed previous research that lime pelleting inoculated serradella seed was not deleterious to survival of the bradyrhizobial inoculum, and showed that it could result in enhanced symbiotic activity of the inoculum in some instances. We recommend lime pelleting of serradella and that WSM471 remain the inoculant strain.  相似文献   

9.
We previously reported that commercial Rhizobium leguminosarum bv. trifolii inoculants failed to outcompete naturalized strains for nodule occupation of clover sown into an alkaline soil [Aust. J. Agric. Res. 53 (2002) 1019]. Two field isolates that dominated nodule occupancy at the field site were labeled with a PnifH-gusA marker. Marked strains were chosen on the basis that they were equally competitive and fixed similar amounts of nitrogen in comparison to their parental strain. The minitransposon insertions were cloned and sequence analysis revealed that neither lesion disrupted the integrity of any known gene. The marked strains were then used to follow nodule occupancy of Trifolium alexandrinum in competition against the commercial inoculant TA1 under a range of experimental conditions. In co-inoculation experiments in sand-vermiculite, TA1 outcompeted each marked field isolate for nodule occupancy. However, using TA1-inoculated seed sown into alkaline soil containing a marked field strain, it was demonstrated that by increasing the cell number of marked rhizobia in the soil and reducing the cell number of the commercial inoculant, the proportion of nodules occupied by TA1 was reduced. These studies indicate that the ability of the field isolates to dominate nodule occupancy in the alkaline field soils was most likely caused by poor commercial inoculant survival providing the advantage for naturalized soil rhizobia to initiate nodulation.  相似文献   

10.
It is not known why sweet potato (Ipomoea batatas) cultivated in tropical regions tolerates acid soil. Here, we report the involvement of mycorrhizal symbiosis in this tolerance. Plants were grown in root-boxes filled with either acidic soil (pH 4.2) or the same soil amended with lime (pH 5.2) for 30 d in a growth chamber. In the inoculated treatments, the percentage of root length colonized by Gigaspora margarita was not affected by soil pH (23±9% at pH 4.2 vs. 30±12% at pH 5.2). The root and shoot dry weights of the non-mycorrhizal plants at pH 4.2 were 27 and 35%, respectively, of those at pH 5.2. The root and shoot dry weights of the mycorrhizal plants at pH 4.2 were 70 and 51% of those at pH 5.2. Growth promotion in mycorrhizal plants was significant only at pH 4.2 (2-fold increase in whole plant dry weight), but not at pH 5.2. As a result, no significant difference was detected in whole plant dry weight between the mycorrhizal plants at pH 4.2 and non-mycorrhizal plants at pH 5.2. The mycorrhizal plants at pH 4.2 showed reduced toxic symptoms of Mn (brown specks on mature leaves) and Al (poor root growth) compared to non-mycorrhizal ones, but tissue concentrations of P, K and Ca did not increase in mycorrhizal plants. We assume that the mycorrhizal colonization can reduce toxic effects of those elements while the exact mechanisms should be further investigated.  相似文献   

11.
The effect of pure gluconic acid and of gluconic-acid-producing bacteria on the activity of three protozoan species, Colpoda steinii (a ciliate), Vahlkampfia sp. (an amoeba) and Neobodo designis (a flagellate), was determined in vitro and in soil microcosms. Pure gluconic acid was shown to mediate disappearance of active cells, due to encystment and/or death of protozoa, at 0.15 mM in saline medium. Similarly, the presence of gluconic acid inhibited excystment of the three protozoa tested. Enterobacter intermedium 60-2G (Wt), a gluconic acid-producing rhizobacterium, elicited the same effects on protozoa when co-cultured in the presence of 5 g L−1 glucose. However, the effect was not observed when glucose was omitted from the medium. Similarly, a pqqA isogenic mutant strain, unable to produce gluconic acid from glucose, exhibited a reduced effect on protozoan activity. Rhizosphere-microcosm studies performed with wheat (Triticum aestivum L.) confirmed the reduced ability of the pqqA mutant to limit protozoa reproduction compared to the Wt strain. Since the sodium salt of gluconic acid did not cause any significant stress to protozoa and considering that addition of 50 mM Tris-Cl (pH 7.2) abolished the deleterious effect of gluconic acid, acidification of the medium appeared as the key factor that induced encystment/death of protozoa. We propose that production and excretion of gluconic acid should be considered an efficient mechanism evolved by bacteria to escape, tolerate or defend themselves against protozoan grazing in rhizosphere environments.  相似文献   

12.
Two indole-producing Paenibacillus species, known to be associated with propagules of arbuscular mycorrhizal (AM) fungi, were examined for their mycorrhization helper bacteria activity at pre-symbiotic and symbiotic stages of the AM association. The effects were tested under in vitro and in vivo conditions using an axenically propagated strain of the AM fungus Glomus intraradices and Glycine max (soybean) as the plant host. The rates of spore germination and re-growth of intraradical mycelium were not affected by inoculation with Paenibacillus strains in spite of the variation of indole production measured in the bacterial supernatants. However, a significant promotion in pre-symbiotic mycelium development occurred after inoculation of both bacteria under in vitro conditions. The Paenibacillus rhizosphaerae strain TGX5E significantly increased the extraradical mycelium network, the rates of sporulation, and root colonization in the in vitro symbiotic association. These results were also observed in the rhizosphere of soybean plants grown under greenhouse conditions, when P. rhizosphaerae was co-inoculated with G. intraradices. However, soybean dry biomass production was not associated with the increased development and infectivity values of G. intraradices. Paenibacillus favisporus strain TG1R2 caused suppression of the parameters evaluated for G. intraradices during in vitro symbiotic stages, but not under in vivo conditions. The extraradical mycelium network produced and the colonization of soybean roots by G. intraradices were promoted compared to the control treatments. In addition, dual inoculation had a promoting effect on soybean biomass production. In summary, species of Paenibacillus associated with AM fungus structures in the soil, may have a promoting effect on short term pre-symbiotic mycelium development, and little impact on AM propagule germination. These findings could explain the associations found between some bacterial strains and AM fungus propagules.  相似文献   

13.
Phytoextraction of soil Cd and Zn may require reduction in soil pH in order to achieve high metal uptake. Reducing the pH of high metal soil, however, could negatively affect soil ecosystem function and health. The objectives of this study were to characterize the quantitative causal relationship between pH and soil biological activities in two Zn and Cd contaminated soils and to investigate the relationship between metals and soil biological activities under low pH. Soils were adjusted to five or six different pH levels by sulfur addition, followed by salt leaching. Thlaspi caerulescens was grown for 6 months, and both the rhizosphere and non-rhizosphere soil biological activities were tested after harvest. Reducing pH significantly lowered soil alkaline phosphatase activity, arylsulphatase activity, nitrification potential, and respiration. However, acid phosphatase activity was increased with decreasing pH. The relationship between soil biological activities and pH was well characterized by linear or quadratic regression models with R2 values ranging from 0.57 to 0.99. In general, the three enzyme activities, nitrification potential, and the ratio of alkaline phosphatase to acid phosphatase activity were very sensitive indicators of soil pH status while soil respiration was not sensitive to pH change. The rhizosphere soil had higher biological activities than non-rhizosphere soil. The negative effects observed in the non-rhizosphere soil were alleviated by the rhizosphere influence. However, rhizosphere soil after 6 months phytoextraction showed lower nitrification potential than non-rhizosphere soil, probably due to substrate limitation in our study.  相似文献   

14.
We investigated the taxonomic position and symbiotic capabilities of two root-nodule bacterial strains isolated from the South African herbaceous, papilionoid legume Rhynchosia ferulifolia. The 16S rRNA gene sequence of the two strains was determined along with intragenic sequences of nodA and nifH, together with their symbiotic capabilities when inoculated onto the papilionoid legumes R. ferulifolia, Rhynchosia caribaea, Rhynchosia minima and Macroptilium atropurpureum (Siratro). Burkholderia phymatum STM815T, Cupriavidus taiwanensis LMG 19424T and root-nodule bacteria isolated from R. minima and Rhynchosia totta were included in the study. Root-nodule bacteria isolated from R. ferulifolia, WSM3937 and WSM3930, belong to the genus Burkholderia and are most closely related to Burkholderia terricola (98.8% similarity). The phylogenetic analysis of nodA and nifH revealed substantial similarity of the novel strains with Burkholderia tuberum STM678T, a β-rhizobium also originated from South Africa, and only a distant relationship with South American Mimosa-nodulating β-rhizobia. R. ferulifolia was effectively nodulated only by Burkholderia sp. WSM3937 and WSM3930 and not by bradyrhizobia isolated from Rhynchosia minima and Rhynchosia totta or STM815 and LMG 19924. Nodules induced by the novel strains were determinate and hosted well organized symbiosomes within infected cells. In this study we describe a new symbiotic N-fixing relationship between Burkholderia sp. and the South African legume R. ferulifolia. This is the first report of N-fixation between β-rhizobia and an herbaceous, papilionoid legume from which the strains were originally isolated. The level of N-fixation in this symbiosis approached that achieved by effectively nodulated Medicago sativa and suggests that the β-rhizobia may have a role in N-fixation in agricultural systems.  相似文献   

15.
Achromobacter piechaudii strain TBPZ is a 2,4,6-tribromophenol (TBP) degrader that was isolated from contaminated desert soil. In the current study we have found that yeast extract has a substantial impact on the debromination activity of non-dividing cells of TBPZ harvested during the stationary phase. The results suggest that yeast extract provides a factor that is necessary for activity and that is missing in stationary cells. Nevertheless, strain TBPZ from the stationary phase did survive and degrade TBP in soils with different degrees of contamination without yeast extract supplements. Experiments with soil extracts showed that a soluble factors is responsible for this phenomenon in a similar way to yeast extract. To test whether other soil bacteria provide these factors, co-culture experiments with strain TBPZ and a bisphenol A (BPA) degrader, Sphingomonas strain WH1, that was isolated from the same contaminated desert soil, were carried out. These experiments clearly demonstrated that growth of strain WH1 on BPA enables strain TBPZ to degrade TBP rapidly, suggesting that a metabolic product from WH1 is involved. In experiments with mixed amino acid blend (casamino acid), slower growth was observed when compared to growth with yeast extract. Providing vitamins and a defined mixture of two amino acids, tryptophan and phenylalanie, caused rapid degradation of TBP without the rapid growth seen in yeast extract treatment, indicating that the amino acids are needed to sustain biodegradation activity. The results of this study highlight that degradation of TBP by strain TBPZ is possibly controlled by micronutrients. In addition, the results suggest that an auxotrophic bacterium able to degrade halo-organic pollutants can survive in a natural environment due to its interaction with natural organic matter or other microorganisms.  相似文献   

16.
Twenty-eight Rhizobium strains were isolated from the root nodules of faba bean (Vicia faba L.) collected from 11 governorates in Egypt. A majority of these strains (57%) were identified as Rhizobium leguminosarum bv. viciae (Rlv) based on analysis of a nodC gene fragment amplified using specific primers for these faba bean symbionts. The strains were characterized using a polyphasic approach, including nodulation pattern, tolerance to environmental stresses, and genetic diversity based on amplified ribosomal DNA-restriction analysis (ARDRA) of both 16S and 23S rDNA. Analysis of tolerance to environmental stresses revealed that some of these strains can survive in the presence of 1% NaCl and a majority of them survived well at 37 °C. ARDRA indicated that the strains could be divided into six 16S rDNA genotypes and five 23S rDNA genotypes. Sequence analysis of 16S rDNA indicated that 57% were Rlv, two strains were Rhizobium etli, one strain was taxonomically related to Rhizobium rubi, and a group of strains were most closely related to Sinorhizobium meliloti. Results of these studies indicate that genetically diverse rhizobial strains are capable of forming N2-fixing symbiotic associations with faba bean and PCR done using nodC primers allows for the rapid identification of V. faba symbionts.  相似文献   

17.
《Applied soil ecology》2007,35(2):441-448
The size of the background rhizobial population can often determine the success of field nodulation and persistence of inoculant rhizobia. Field experiments were conducted to determine the nodulation response of annual medics (Medicago spp.) in a pasture-wheat-pasture rotation when grown in soils of contrasting pH and rhizobial populations. Medicago truncatula Gaertn. and M. polymorpha L. were inoculated with one of three different strains of Sinorhizobium medicae (WSM540, WSM688) or S. meliloti (NA39) or left uninoculated and sown in two fields of pH (CaCl2) 5.9 and 7.2 of differing soil rhizobial backgrounds (11 and 7.1 × 104 cells/g soil, respectively). Nodulation was assessed in years 1 and 3 of the rotation. At the site with a small rhizobial background, M. polymorpha nodulated poorly when inoculated with the acid-sensitive strain NA39 but nodulated well when inoculated with acid-tolerant strains WSM688 and WSM540. M. truncatula had a similar extent of nodulation with each of the rhizobial inoculants. At the site with a large rhizobial background all treatments had greater than 85% of plants nodulated. Nodule occupancies, assessed by PCR, provided further insight: at the site with a small rhizobial background both medic species successfully nodulated with the acid-tolerant strains WSM540 and WSM688 and these strains persisted to year 3. However, at the site with large rhizobial background, only one strain, WSM688, was identified from M. truncatula nodules in year 3. This study highlights the importance of edaphic constraints and plant–rhizobia interactions to the successful development of nodulation in a field environment.  相似文献   

18.
We show that Pseudomonas fluorescens strain P13, a plant growth-promoting bacterium, enhanced the growth of corn in uncontaminated soil but not in contaminated soil, perhaps because of its inability to reduce phytotoxicity. Another bacterial strain, Pseudomonas aeruginosa strain SZH16, showed in situ phenol-degrading activity and contained a plasmid loaded with a gene encoding for catechol 2, 3-dioxygenase, an important enzyme in the degradation pathway of aromatic compounds. We implanted this biodegradation ability into strain P13, using horizontal gene transfer techniques using strain SZH16 as the donor and P13 as the recipient, to generate a phenol-degrading transconjugant which obtained the effective plasmid from strain SZH16. Introduction of the transconjugant P13 strain into an artificially phenol-spiked soil promoted the growth of corn and in situ phenol degradation, and the increase in plant biomass correlated with the decrease in soil phenol content. Furthermore, the transconjugant P13 strain was also found to stimulate corn growth and reduce phenol concentration in water containing phenol and in historically contaminated field soils, indicating that the transconjugant strain could promote plant growth in both contaminated and uncontaminated environments. The transconjugant P13 strain was more efficient than either strain P13 or SZH16, and shows how plant growth-promoting bacteria which show no, or only limited, ability to degrade organic pollutants may be modified. This technique is attractive for many environmental remediation and agronomic applications.  相似文献   

19.
It has been established that arbuscular mycorrhizal (AM) fungi are involved in the conservation of soil structure. However, the effect of ectomycorrhizal (EM) fungi alone or in interaction with AM fungi in soil structure has been much less studied. This experiment evaluated EM and AM fungi effects on soil aggregation and plant growth. Ash plants (Fraxinus uhdei) were grown in pots, and were inoculated with Glomus intraradices and Pisolithus tinctorius separately but also in combination. Our results showed that F. uhdei established a symbiotic association with EM and AM fungi, and that these organisms, when interacting, showed synergistic and additive effects on plant growth compared to singly inoculated treatments. EM and AM fungi prompted changes in root morphology and increased water-stable aggregates. AM fungi affect mainly small-sized macroaggregates, while EM and EM-AM fungi interaction mainly affected aggregates bigger than 0.5 mm diameter. These results suggest that ectomyccorrhizal as well as arbuscular mycorrhizal fungi should be considered in restoration programs with Fraxinus plants.  相似文献   

20.
The biocontrol agents Coniothyrium minitans and Bacillus subtilis MBI 600 were added separately to three soil types that had been either sterilised, pasteurised or left non-sterile. Applied as a conidial suspension of 1×106 cfu g−1 soil, C. minitans showed good survival in all sterilised, pasteurised and non-sterile soils, remaining at the numerical level at which it was applied for the duration of the 30 d experiment. Applied at a lower rate of 1×103 cfu g−1 soil, C. minitans proliferated in sterilised soil to numbers slightly over 1×106 cfu g−1 soil, whereas no increase was seen in pasteurised or non-sterile soils from this lower application rate. However, although C. minitans was not easily recovered on plates from non-sterile soil, it did survive at the lower numerical level in pasteurised soil, and was recoverable throughout the experiment at the rate at which it was applied. B. subtilis MBI 600 survived well following introduction as a cell suspension into sterilised soil at a rate of 1×106 cfu g−1 soil. Spores were formed rapidly and, after 14 d, the introduced microorganism survived in this form rather than as vegetative cells. However, in non-sterile soil, the introduced microorganism did not compete well and decreased in number, with spores being formed in low numbers. Survival of B. subtilis MBI 600 in pasteurised soil was variable, but resembled the survival seen in non-sterile soil more than that seen in sterilised soil. More B. subtilis MBI 600 spores were formed in pasteurised soil than in non-sterile soil, however, and may have been important for survival in pasteurised soil. In conclusion, this work has shown that the biocontrol agent C. minitans can survive well in soil irrespective of whether the soil has been pasteurised or not and shows good promise as a soil inoculant for control of Sclerotinia sclerotiorum. Although soil pasteurisation does improve establishment of B. subtilis MBI 600 compared to non-sterile soil, survival is relatively poor when applied as cells. The best survival of B. subtilis MBI 600 occurred as spores in sterilised soil, and spore applications to pasteurised soil in an integrated control strategy may allow sufficient establishment of the biocontrol agent to target pathogens causing damping-off.  相似文献   

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