Soil inoculation with the biocontrol agent Clonostachys rosea and the mycorrhizal fungus Glomus intraradices results in mutual inhibition, plant growth promotion and alteration of soil microbial communities

Interactions between the biocontrol fungus Clonostachys rosea IK 726 and a tomato/Glomus intraradices BEG87 symbiosis were examined with and without wheat bran, which served as a food base for C. rosea. In soil without wheat bran amendment, inoculation with C. rosea increased plant growth and altered shoot nutrient content resulting in an increase and decrease in P and N content, respectively. Inoculation with G. intraradices had no effect on plant growth, but increased the shoot P content. Dual inoculation with G. intraradices and C. rosea followed the pattern of C. rosea in terms of plant growth and nutrient content. Wheat bran amendment resulted in marked plant growth depressions, which were counteracted by both inoculants and dual inoculation increased plant growth synergistically. Amendment with wheat bran increased the population density of C. rosea and reduced mycorrhizal fungus colonisation of roots. The inoculants were mutually inhibitory, which was shown by a reduction in root colonisation with G. intraradices in treatments with C. rosea and a reduction in colony-forming units (cfu) of C. rosea in treatments with G. intraradices, irrespective of wheat bran amendment. Moreover, both inoculants markedly influenced soil microbial communities examined with biomarker fatty acids. Inoculation with G. intraradices increased most groups of microorganisms irrespective of wheat bran amendment, whereas the influence of C. rosea on other soil microorganisms was affected by wheat bran amendment. Overall, inoculation with C. rosea increased and decreased most groups of microorganisms without and with wheat bran amendment, respectively. In conclusion, despite mutual inhibition between the two inoculants this interaction did not impair their observed plant growth promotion. Both inoculants also markedly influenced other soil microorganisms, which should be further studied in relation to their plant growth-promoting features.     

Pre-symbiotic and symbiotic interactions between Glomus intraradices and two Paenibacillus species isolated from AM propagules. In vitro and in vivo assays with soybean (AG043RG) as plant host

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.     

The mycorrhizal fungus Glomus intraradices and rock phosphate amendment influence plant growth and microbial activity in the rhizosphere of Acacia holosericea

Plants inoculated with arbuscular mycorrhizal (AM) fungi utilize more soluble phosphorus from soil mineral phosphate than non-inoculated plants. However, there is no information on the response of soil microflora to mineral phosphate weathering by AM fungi and, in particular, on the catabolic diversity of soil microbial communities.The AM fungus, Glomus intraradices was examined for (i) its effect on the growth of Acacia holosericea, (ii) plant-available phosphate and (iii) soil microbial activity with and without added rock phosphate.After 4-months culture, AM fungal inoculation significantly increased the plant biomasses (by 1.78× and 2.23× for shoot and root biomasses, respectively), while mineral phosphate amendment had no effect in a sterilized soil. After 12-months culture, the biomasses of A. holosericea plants growing in a non-sterilized soil amended with mineral phosphate were significantly higher than those recorded in the control treatment (by 2.5× and 5× for shoot and root biomasses, respectively). The fungal inoculation also significantly stimulated plant growth, which was significantly higher than that measured in the mineral phosphate treatment. When G. intraradices and mineral phosphate were added together to the soil, shoot growth were significantly stimulated over the single treatments (inoculation or amendment) (1.45×). The P leaf mineral content was also higher in the G. intraradices+mineral phosphate treatment than in G. intraradices or rock phosphate amendment. Moreover, the number of fluorescent pseudomonads has been significantly increased when G. intraradices and/or mineral phosphate were added to the soil. By using a specific type of multivariate analysis (co-inertia analysis), it has been shown that plant growth was positively correlated to the metabolization of ketoglutaric acid, and negatively linked to the metabolisation of phenylalanine and other substrates, which shows that microbial activity is also affected.G. intraradices inoculation is highly beneficial to the growth of A. holosericea plants in controlled conditions. This AM symbiosis optimises the P solubilization from the mineral phosphate and affects microbial activity in the hyphosphere of A. holosericea plants.     

Physiological characteristics (SDH and ALP activities) of arbuscular mycorrhizal colonization as affected by Bacillus thuringiensis inoculation under two phosphorus levels

The effect of Bacillus thuringiensis (B.t.) inoculation on plant growth and on the intra- and extraradical mycorrhizal development of lettuce roots colonized by Glomus mosseae or Glomus intraradices was examined in an inert, soil-less substrate. Histochemical determination of succinate dehydrogenase (SDH) and alkaline phosphatase (ALP) activities which indicate active fungal metabolism was carried out at two phosphorus (P) levels. The presence of B.t. increased extra- and intraradical colonization [measured as frequency (%F), intensity (%I) and percentage of arbuscules (%A)] for both arbuscular mycorrhizal fungi (AMF) rather than plant growth or nutrition regardless P level. Under the lowest level of P fertilization, B.t. enhanced to a similar extent the extra- and intraradical development of both endophytes, but the proportion of fungal tissue showing SDH or ALP was increased in G. intraradices-colonized plants. [SDH: 458% (M) and 512% (A); ALP: 358% (M) and 300% (A)]. P supply decreased G. intraradices colonization to a higher extent than G. mosseae. Nevertheless, the totality of G. intraradices structures developed in P-amended medium showed intraradical o extraradical activity, while in G. mosseae-colonized roots, SDH and ALP activities highly decreased relative to fungal tissue determined by TB staining as affected by P. Our results show that bacterial inoculation compensates the negative effect of P on the intraradical fungal growth and vitality. P amendment reduced in a higher extent G. intraradices infection intensity (non-vital and vital staining) and G. mosseae activity (ALP staining). Thus, big differences in the proportion of SDH-active infection showing ALP activity in mycelium developed by each endophyte were noted at the highest P level. Physiological plant parameters such as photosynthetic activity did not explain specific changes on each arbuscular-mycorrhizal fungus as affected by P or B.t. inoculation. The increased extraradical mycelium development and metabolic fungal activity as a result of B.t. inoculation positively affected N and P plant content and photosynthetic rate in G. intraradices-colonized plants under the lowest P conditions. In general, the increased metabolically active fungal biomass in co-inoculated plants was irrespective of P level and was not related to the P plant uptake from the inert soil-less substrate. These results show the bacterial effect increasing the physiological and metabolic status of AM endophytes, which not only confirms but also extends previous findings on arbuscular mycorrhizae-bacteria interactions. The present study emphasizes the ecological and practical importance of rhizosphere free-living bacteria as mycorrhizae-helper microorganisms.     

The mycorrhizal fungus (Glomus intraradices) affects microbial activity in the rhizosphere of pea plants (Pisum sativum)

Pea plants were grown in γ-irradiated soil in pots with and without addition of the AM fungus Glomus intraradices at sufficient N and limiting P. Depending on the growth phase of the plant presence of AM had negative or positive effect on rhizosphere activity. Before flowering during nutrient acquisition AM decreased rhizosphere respiration and number of protozoa but did not affect bacterial number suggesting top-down regulation of bacterial number by protozoan grazing. In contrast, during flowering and pod formation AM stimulated rhizosphere respiration and the negative effect on protozoa decreased. AM also affected the composition of the rhizosphere bacterial community as revealed from DNA analysis (DGGE). With or without mycorrhiza, rhizosphere respiration was P-limited on very young roots, not nutrient limited at more mature roots and C-limited at withering. This suggests changes in the rhizosphere community during plant growth also supported by changes in the bacteria (DGGE).     

Interactions between a plant growth-promoting rhizobacterium,an AM fungus and a phosphate-solubilising fungus in the rhizosphere of Lactuca sativa

This study evaluated the interactions between the inoculation with an arbuscular mycorrhizal fungus, Glomus intraradices Schenck & Smith, a plant growth-promoting rhizobacterium, Bacillus subtilis, and a filamentous soil fungus, Aspergillus niger, with respect to their effects on growth of lettuce plants and on indicators of biological soil quality (microbial biomass C, water-soluble C and carbohydrates and dehydrogenase, urease, acid phosphatase and benzoyl argininamide hydrolyzing protease activities). Water-soluble carbohydrates and microbial biomass were increased only in the rhizosphere soil of G. intraradices-plants. Rhizosphere soil from all microbial inoculation treatments had significantly higher dehydrogenase activity than the control soil, particularly in the soil inoculated with B. subtilis (about 21% higher than control soil). Inoculation with A. niger or B. subtilis increased significantly the urease, protease and phosphatase activities of the rhizosphere soil of the lettuce plants. The foliar P and K contents increased significantly with the B. subtilis or G. intraradices inoculation, alone or in combination. The most effective co-inoculation was observed in the combined treatment of inoculation with G. intraradices and B. subtilis, which synergistically increased plant growth compared with singly inoculated (about 77% greater with respect to the control plants).     

Interactions between the external mycelium of the mycorrhizal fungus Glomus intraradices and other soil microorganisms as affected by organic matter

The influence of organic matter on the interactions between external mycelium of the arbuscular mycorrhizal (AM) fungus Glomus intraradices, the bacterium Burkholderia cepacia and other soil microorganisms was studied in a root-free sand environment. Organic matter amendment, in terms of ground barley leaves, markedly increased the growth of the external mycelium of G. intraradices as estimated both with the fatty acid biomarker 16:1ω5 and hyphal length measurements. Mycelial proliferation of G. intraradices in sand with organic matter was unaffected by both inoculation with B. cepacia and a soil filtrate containing a mixed population of indigenous microorganisms. On the other hand, in the absence of organic matter, both inoculation with B. cepacia and the soil filtrate reduced the growth of G. intraradices, as estimated with measurements of 16:1ω5. In contrast, B. cepacia inoculation increased hyphal length density of G. intraradices in the absence of organic matter. Overall, the presence of external mycelium of G. intraradices increased the bacterial biomass and counteracted a suppressive effect of B. cepacia on the growth of saprotrophic fungi.     

Interactions between phosphorus availability and an AM fungus (Glomus intraradices) and their effects on soil microbial respiration, biomass and enzyme activities in a calcareous soil

The interactions between soil P availability and mycorrhizal fungi could potentially impact the activity of soil microorganisms and enzymes involved in nutrient turnover and cycling, and subsequent plant growth. However, much remains to be known of the possible interactions among phosphorus availability and mycorrhizal fungi in the rhizosphere of berseem clover (Trifolium alexandrinum L.) grown in calcareous soils deficient in available P. The primary purpose of this study was to look at the interaction between P availability and an arbuscular mycorrhizal (AM) fungus (Glomus intraradices) on the growth of berseem clover and on soil microbial activity associated with plant growth. Berseem clover was grown in P unfertilized soil (−P) and P fertilized soil (+P), inoculated (+M) and non-inoculated (−M) with the mycorrhizal fungus for 70 days under greenhouse conditions. We found an increased biomass production of shoot and root for AM fungus-inoculated berseem relative to uninoculated berseem grown at low P levels. AM fungus inoculation led to an improvement of P and N uptake. Soil respiration (SR) responded positively to P addition, but negatively to AM fungus inoculation, suggesting that P limitation may be responsible for stimulating effects on microbial activity by P fertilization. Results showed decreases in microbial respiration and biomass C in mycorrhizal treatments, implying that reduced availability of C may account for the suppressive effects of AM fungus inoculation on microbial activity. However, both AM fungus inoculation and P fertilization affected neither substrate-induced respiration (SIR) nor microbial metabolic quotients (qCO₂). So, both P and C availability may concurrently limit the microbial activity in these calcareous P-fixing soils. On the contrary, the activities of alkaline phosphatase (ALP) and acid phosphatase (ACP) enzymes responded negatively to P addition, but positively to AM fungus inoculation, indicating that AM fungus may only contribute to plant P nutrition without a significant contribution from the total microbial activity in the rhizosphere. Therefore, the contrasting effects of P and AM fungus on the soil microbial activity and biomass C and enzymes may have a positive or negative feedback to C dynamics and decomposition, and subsequently to nutrient cycling in these calcareous soils. In conclusion, soil microbial activity depended on the addition of P and/or the presence of AM fungus, which could affect either P or C availability.     

Interactions of arbuscular-mycorrhizal fungi and Bacillus strains and their effects on plant growth,microbial rhizosphere activity (thymidine and leucine incorporation) and fungal biomass (ergosterol and chitin)

The effects of two Bacillus strains (Bacillus pumillus and B. licheniformis) on Medicago sativa plants were determined in single or dual inoculation with three arbuscular-mycorrhizal (AM) fungi and compared to P-fertilization. Shoot and root plant biomass, values of thymidine and leucine incorporation as well as ergosterol and chitin in rhizosphere soil were evaluated to estimate metabolic activity and fungal biomass, respectively, according to inoculation treatments. For most of the plant parameters determined, the effectiveness of AM fungal species was influenced by the bacterial strain associated. Dual inoculation of Bacillus spp. and AM fungi did not always significantly increase shoot biomass compared to single AM-colonized plants. The most efficient treatment in terms of dry matter production was the dual Glomus deserticola plus B. pumillus inoculation, which produced similar shoot biomass and longer roots than P-fertilization and a 715% (shoot) and 190% (root length) increase over uninoculated control. The mycorrhizas were more important for N use-efficiency than for P use-efficiency, which suggests a direct mycorrhizal effect on N nutrition not mediated by P uptake. Both chemical and biological treatments affected thymidine and leucine incorporation in the rhizosphere soil differently. Thymidine was greater in inoculated than in control rhizospheres and B. licheniformis was more effective than B. pumillus in increasing thymidine. Non-inoculated rhizospheres showed the lowest thymidine and leucine values, which shows that indigenous rhizosphere bacteria increased with introduced inocula. The highest thymidine and leucine values found in P-fertilized soils indicate that AM plants are better adapted to compete with saprophytic soil bacteria for nutrients than P-amended plants. Chitin was only increased by coinoculation of B. licheniformis and G. intraradices. B. pumillus increased ergosterol (indicative of active saprophyte fungal populations) in the rhizosphere of AM plants and particularly when colonized by G. mosseae. The different AM fungi have different effects on bacterial and/or fungal saprophytic populations and for each AM fungus, this effect was specifically stimulated or reduced by the same bacterium. This is an indication of ecological compatibilities between microorganisms. Particular Glomus–bacterium interactions (in terms of effect on plant growth responses or rhizosphere population) do not seem to be related to the percentage of AM colonization. The effect on plant growth and stimulation of rhizosphere populations, as a consequence of selected microbial groups, may be decisive for the plant establishment under limiting soil conditions.     

Mycorrhizal association of Agrostis capillaris and Glomus intraradices under heavy metal stress: Combination of plant clones and fungal isolates from contaminated and uncontaminated substrates

The present study aimed to compare the impact of arbuscular mycorrhizal (AM) fungi on plant growth and heavy-metal (HM)-uptake when both plant and fungal symbionts originated either from contaminated or uncontaminated sites. HM-tolerance of six clones of the grass Agrostis capillaris and three isolates of the AM fungus Glomus intraradices of different origin was first tested separately. Plant clones from the vicinity of a lead smelter showed a consistently high HM-tolerance, whereas the control clones varied in their ability to cope with HMs. The AM isolate from the contaminated substrate also performed better under HM-stress than the isolates from uncontaminated soils. All A. capillaris clones were then grown in a contaminated substrate, uninoculated or inoculated either with a tolerant or non-tolerant G. intraradices isolate. Clones from the uncontaminated site accumulated considerably more HMs in their shoots and roots, regardless of inoculation. The effect of AM inoculation on plant growth and HM-uptake depended on the particular combination of plant clone and fungal isolate, without clear differences between tolerant and non-tolerant clones.     

Changes in microbial communities and carbohydrate profiles induced by the mycorrhizal fungus (Glomus intraradices) in rhizosphere of olive trees (Olea europaea L.)

The influence of inoculation of olive trees with arbuscular mycorrhizal (AM) fungi, Glomus (G) intraradices, on microbial communities and sugar concentrations, were examined in rhizosphere of olive trees (Olea europaea L.). Analyses of phospholipid and neutral lipid fatty acids (PLFA and NLFA, respectively) were then used to detect changes in microbial community structure in response to inoculation of plantlets with G. intraradices.Microscopic observations studies revealed that the extraradical mycelium of the fungus showed formation of branched absorbing structures (BAS) in rhizosphere of olive tree. Root colonization with the AM fungi G. intraradices induced significant changes in the bacterial community structure of olive tree rhizosphere compared to non-mycorrhizal plants. The largest proportional increase was found for the fatty acid 10Me18:0, which indicated an increase in the number of actinomycetes in mycorrhizal rhizosphere soil, whereas the PLFAs i15:0, a15:0, i16:0, 16:1ω7 and cy17:0 which were used as indicators of bacteria decreased in mycorrhizal treatment compared to non-mycorrhizal control treatment. A highest concentration of glucose and trehalose and a lowest concentration of fructose, galactose, sucrose, raffinose and mannitol were detected in mycorrhizal rhizosphere soil. This mycorrhizal effect on rhizosphere communities may be a consequence of changes in characteristics in the environment close to mycorrhizal roots.     

Growth and nitrate reductase activity in Juniperus oxycedrus subjected to organic amendments and inoculation with arbuscular mycorrhizae

The effectiveness of reforestation programs on degraded soils in the Mediterranean region is frequently limited by a low soil availability and a poor plant uptake and assimilation of nutrients. While organic amendments can improve the nutrient supply, inoculation with mycorrhizal fungi can enhance plant nutrient uptake. A pot experiment was conducted in 2004 to study the influence of inoculation with an arbuscular mycorrhizal (AM) fungus (Glomus intraradices Schenck & Smith) or with a mixture of three AM fungi (G. intraradices, G. deserticola Trappe, Bloss. & Menge, and G. mosseae (Nicol & Gerd.) Gerd. & Trappe) and of an addition of composted sewage sludge or Aspergillus niger–treated dry‐olive‐cake residue on plant growth, nutrient uptake, mycorrhizal colonization, and nitrate reductase (NR) activity in shoot and roots of Juniperus oxycedrus L. Six months after planting, the inoculation of the seedlings with G. intraradices or a mixture of three AM fungi was the most effective treatment for stimulating growth of J. oxycedrus. There were no differences between the two mycorrhizal treatments. All treatments increased plant growth and foliar N and P contents compared to the control plants. Mycorrhizal inoculation and organic amendments, particularly fermented dry olive cake, increased significantly the NR activity in roots.     

Hydrolytic enzyme activities in maize (Zea mays) and sorghum (Sorghum bicolor) roots inoculated with Gluconacetobacter diazotrophicus and Glomus intraradices

Two strains of Gluconacetobacter diazotrophicus (Pal 5, UAP5541) and the arbuscular mycorrhizal fungus Glomus intraradices increased both the shoot and root dry weight of sorghum 45 days after inoculation, whereas they had no effect on the shoot and root dry weight of maize. Co-inoculation (Gluconacetobacter diazotrophicus plus Glomus mosseae) did not increase the shoot and root dry weight of either plant. There was a synergistic effect of Gluconacetobacter diazotrophicus on root colonization of maize by Glomus intraradices, whereas an antagonistic interaction was observed in the sorghum root where the number of Gluconacetobacter diazotrophicus and the colonization by Glomus intraradices were reduced. Plant roots inoculated with Gluconacetobacter diazotrophicus and Glomus intraradices, either separately or together, significantly increased root endoglucanase, endopolymethylgalacturonase and endoxyloglucanase activities. The increase varied according to the plant. For example, in comparison with non-inoculated plants, there were higher endoglucanase (+328%), endopolymethylgalacturonase (+180%) and endoxyloglucanase (+125%) activities in 45-day old co-inoculated maize, but not in 45-day old sorghum. The possibility is discussed that hydrolytic enzyme activities were increased as a result of inoculation with Gluconacetobacter diazotrophicus, considering this to be one of the mechanisms by which these bacteria may increase root colonization by AM fungi.     

Cadmium Concentration in Flax Colonized by Mycorrhizal Fungi Depends on Soil Phosphorus and Cadmium Concentrations

Effect of arbuscular mycorrhizal (AM) fungus on cadmium (Cd) concentration in flax was investigated in a pot experiment. Flax inoculated with Glomus intraradices and uninoculated controls were grown in a pasteurized soil that received Cd (0, 2.5, and 10 mg kg^?1) and phosphorus (P; 10 and 50 mg kg^?1) additions. Root colonization was not affected by Cd addition but was reduced by high P addition. Effect of G. intraradices on Cd was evident only at low P supply. Inoculation with G. intraradices decreased shoot Cd at no or low Cd addition, which was attributed to reduced root-to-shoot Cd translocation. In contrast, G. intraradices inoculation increased shoot Cd at high Cd addition, which might be associated with the greater absorption of Cd by extraradical hyphae and lower rhizosphere pH. Our results indicate that a benefit of AM fungus in reducing Cd in crops is achievable at Cd and P concentrations commonly in agricultural soils.     

Differential effects of soil disturbance and plant residue retention on function of arbuscular mycorrhizal (AM) symbiosis are not reflected in colonization of roots or hyphal development in soil

The effects of soil disturbance and residue retention on the functionality of the symbiosis between medic (Medicago truncatula L.) and arbuscular mycorrhizal fungi (AMF) were assessed in a two-stage experiment simulating a crop rotation of wheat (Triticum aestivum L.) followed by medic. Plants were inoculated or not with the AMF, Glomus intraradices and Gigaspora margarita, separately or together. The contribution of the arbuscular mycorrhizal (AM) pathway for P uptake was determined using ³²P-labeled soil in a small hyphal compartment accessible only to hyphae of AMF. In general AM colonization was not affected by soil disturbance or residue application and disturbance did not affect hyphal length densities (HLDs) in soil. At 4 weeks disturbance had a negative effect on growth and phosphorus (P) uptake of plants inoculated with G. margarita, but not G. intraradices. By 7 weeks disturbance reduced growth of plants inoculated with G. margarita or AMF mix and total P uptake in all inoculated plants. With the exception of plants inoculated with G. margarita in disturbed soil at 4 weeks, the AM pathway made a significant contribution to P uptake in all AM plants at both harvests. Inoculation with both AMF together eliminated the negative effects of disturbance on AM P uptake and growth, showing that a fungus insensitive to disturbance can compensate for loss of contribution of a sensitive one. Application of residue increased growth and total P uptake of plants but decreased ³²P in plants inoculated with the AMF mix in disturbed soil, compared with plants receiving no residue. The AMF responded differently to disturbance and G. intraradices, which was insensitive to disturbance, compensated for lack of contribution by the sensitive G. margarita when they were inoculated together. Colonization of roots and HLDs in soil were not good predictors of the outcomes of AM symbioses on plant growth, P uptake or P delivery via the AM pathway.     

AM真菌对田间山银花生长、矿质营养及绿原酸含量的影响

Lonicera confusa, a traditional Chinese medicine herb for treating cold, flu, acute fever, and so forth, is often grown artificially in acidic soils and suffers from phosphorus (P) deficiency. A five-year field experiment was carried out to study the colonization rate, growth, nutrition, and chlorogenic acid content of Lonicera confusa seedlings inoculated with arbuscular mycorrhizal (AM) fungi, Glomus etunicatum and Glomus intraradices. Before transplanting into a field, both AM-inoculated and uninoculated control plants were cultured in nursery beds. In the plants inoculated with the AM fungi, the colonization rate decreased linearly with time and a greater decrease was observed in the plants inoculated with G. intraradices than with G. etunicatum, while the AM colonization increased from 0% to 12.1% in the uninoculated control plants 5 years after transplanting. Plant height, crown diameter, number of new branches, and flower yield increased significantly by AM inoculation as compared to the uninoculated control. Phosphorus concentrations in leaves and flowers increased, and plant uptake of nutrients, e.g., nitrogen (N), P, and potassium (K), was also enhanced significantly by AM inoculation. The Lonicera confusa seedlings had a better response to inoculation of G. intraradices than G. etunicatum in both growth and chlorogenic acid content in flowers. In contrast, both plant P uptake and P concentrations in leaves and flowers were similar between two fungal inoculations. The positive responses of Lonicera confusa to AM inoculation in growth, nutrient uptake, flowering, and chlorogenic acid content in flowers suggested that AM inoculation in nursery beds could promote the plant growth and increase chlorogenic acid content in flowers of Lonicera confusa when grown on acidic and P-deficient soils.     

Elevated CO2 increases the effect of an arbuscular mycorrhizal fungus and a plant-growth-promoting rhizobacterium on structural stability of a semiarid agricultural soil under drought conditions

In arid and semiarid Mediterranean regions, an increase in the severity of drought events could be caused by rising atmospheric CO₂ concentrations. We studied the effects of the interaction of CO₂, water supply and inoculation with a plant-growth-promoting rhizobacterium (PGPR), Pseudomonas mendocina Palleroni, or inoculation with an arbuscular mycorrhizal (AM) fungus, Glomus intraradices (Schenk & Smith), on aggregate stabilisation of the rhizosphere soil of Lactuca sativa L. cv. Tafalla. The influence of such structural improvements on the growth of lettuce was evaluated. We hypothesised that elevated atmospheric CO₂ concentration would increase the beneficial effects of inoculation with a PGPR or an AM fungus on the aggregate stability of the rhizosphere soil of lettuce plants. Leaf hydration, shoot dry biomass and mycorrhizal colonisation were decreased significantly under water-stress conditions, but this decrease was more pronounced under ambient vs elevated CO₂. The root biomass decreased under elevated CO₂ but only in non-stressed plants. Under elevated CO₂, the microbial biomass C of the rhizosphere of the G. intraradices-colonised plants increased with water stress. Bacterial and mycorrhizal inoculation and CO₂ had no significant effect on the easily-extractable glomalin concentration. Plants grown under elevated CO₂ had a significantly higher percentage of stable aggregates under drought stress than under well-watered conditions, particularly the plants inoculated with either of the assayed microbial inocula (about 20% higher than the control soil). We conclude that the contribution of mycorrhizal fungi and PGPR to soil aggregate stability under elevated atmospheric CO₂ is largely enhanced by soil drying.     

接种丛枝菌根真菌和巨大芽孢杆菌的红三叶草对难溶磷的利用

A pot experiment was conducted to investigate the mobilization of sparingly soluble inorganic andorganic sources of phosphorus (P) by red clover (Trghlium pmtense L.) whose roots were colonized by the arbuscular mycorrhizal (AM) fungus Glomus mosseae and in association with the phosphate-solubilizing (PS) bacterium Bacillus megaterium ACCC10010. Phosphate-solubilizing bacteria and rock phosphate hada synergistic effect on the colonization of plant roots by the AM fungus. There was a positive interaction between the PS bacterium and the AM fungus in mobilization of rock phosphate, leading to improved plant P nutrition. In dual inoculation with the AM fungus and the PS bacterium, the main contribution to plant P nutrition was made by the AM fungus. Application of P to the low P soil increased phosphatase activityin the rhizosphere. Alkaline phosphatase activity was significantly promoted by inoculation with either the PS bacterium or the AM fungus.     

Sugar beet waste and its component ferulic acid inhibits external mycelium of arbuscular mycorrhizal fungus

External arbuscular mycorrhiza (AM) mycelium plays an important role in soil while interacting with a range of biotic and abiotic factors. One example is the soil organic amendment sugar beet waste. The fermented Aspergillus niger-sugar beet waste (ASB) increases growth and P uptake by the AM mycelium in soil whereas non-fermented waste (SB) had a strong inhibitory effect. The underlying mechanisms are not understood.We used gas chromatography-mass spectrometry to identify differences in composition of water extracts of ASB and SB. The chromatograms showed that ferulic acid was present in SB and absent in ASB. We compared the effects of the water extracts of SB and ASB and ferulic acid upon the growth of Glomus intraradices in in vitro monoxenic cultures.Hyphal growth of the AM fungus G. intraradices was extremely reduced in ferulic acid and SB treatments. Moreover, AM hyphae appeared disorganized, undulated and tangled. In contrast, ASB increased hyphal length and numbers of branched absorbing structures and of spores. We conclude that ferulic acid is one compound in SB which is responsible for its inhibition of AM extraradical growth. The relevance of these findings is discussed.     

Contribution of Pseudomonas mendocina and Glomus intraradices to aggregate stabilization and promotion of biological fertility in rhizosphere soil of lettuce plants under field conditions

A field‐plot experiment was undertaken to compare the effect of inorganic fertilizer with that of inoculation with an arbuscular mycorrhizal fungus, Glomus intraradices, or with a plant‐growth‐promoting rhizobacterium, Pseudomonas mendocina, alone or in combination with inorganic fertilizer, on plant growth and nutrient uptake by lettuce. The influence of the microbial inoculation treatments on soil physical, biochemical and biological properties was also assessed. Two months after planting, fertilizer and inoculation with G. intraradices or P. mendocina had significantly increased shoot and root biomass and foliar nutrient contents (P, Fe). The inoculation with G. intraradices or P. mendocina both increased the soil water‐soluble carbohydrates and the percentage of stable aggregates. In this study, we provide the first evidence of the beneficial effect of a plant‐growth‐promoting rhizobacterium on soil aggregate stabilization under field conditions. Only inoculation with P. mendocina had a significant effect on the dehydrogenase and phosphatase activities, 21 and 89%, respectively, compared with the control. Inorganic fertilization alone did not increase aggregate stability or enzyme activities in soil, even though this treatment produced the largest increases in mass of lettuce.