Interactions between the arbuscular mycorrhizal fungus Glomus intraradices and the plant growth promoting rhizobacteria Paenibacillus polymyxa and P. macerans in the mycorrhizosphere of Cucumis sativus

Interactions between the arbuscular mycorrhizal (AM) fungus Glomus intraradices and bacteria from the genus Paenibacillus (P. macerans and P. polymyxa) were examined in a greenhouse pot experiment with Cucumis sativus with and without organic matter amendment (wheat bran). P. polymyxa markedly suppressed AM fungus root colonization irrespective of wheat bran amendment, whereas P. macerans only suppressed AM fungus root colonization in combination with wheat bran amendment. Dual inoculation with P. macerans and G. intraradices in combination with wheat bran amendment also caused severe plant growth suppression. Inoculation with G. intraradices was associated with increased levels of dehydrogenase activity and available P in the growth substrate suggesting that mycorrhiza formation accelerated the decomposition of organic matter resulting in mobilization of phosphorus. Inoculation with both Paenibacillus species increased all measured microbial fatty acid biomarkers in the cucumber rhizosphere, except for the AM fungus biomarker 16:1ω5, which was reduced, though not significantly. Similarly, inoculation with G. intraradices increased all measured microbial fatty acid biomarkers in the cucumber rhizosphere, except for the Gram-positive bacteria biomarker 15:0 anteiso, which was overall decreased by G. intraradices inoculation. In combination with wheat bran amendment G. intraradices inoculation caused a 39% reduction in the amount of 15:0 anteiso in the treatment with P. polymyxa, suggesting that G. intraradices suppressed P. polymyxa in this treatment. In conclusion, plant growth promoting species of Paenibacillus may have suppressive effects of AM fungi and plant growth, especially in combination with organic matter amendment. The use of an inert plant growth media in the present study allowed us to study rhizosphere microbial interactions in a relative simple substrate with limited interference from other soil biota. However, the results obtained in the present work mainly show potential interactions and should not be directly extrapolated to a soil situation.     

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.     

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 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).     

Nickel-tolerant Brevibacillus brevis and arbuscular mycorrhizal fungus can reduce metal acquisition and nickel toxicity effects in plant growing in nickel supplemented soil

The growth of clover (Trifolium repens ) and its uptake of N, P and Ni were studied following inoculation of soil with Rhizobium trifolii, and combinations of two Ni-adapted indigenous bacterial isolates (one of them was Brevibacillus brevis) and an arbuscular mycorrhizal (AM) fungus (Glomus mosseae). Plant growth was measured in a pot experiment containing soil spiked with 30 (Ni I), 90 (Ni II) or 270 (Ni III) mg kg⁻¹ Ni-sulphate (corresponding to 11.7, 27.6 and 65.8 mg kg⁻¹ available Ni on a dry soil basis). Single inoculation with the most Ni-tolerant bacterial isolate (Brevibacillus brevis) was particularly effective in increasing shoot and root biomass at the three levels of Ni contamination in comparison with the other indigenous bacterial inoculated or control plants. Single colonisation of G. mosseae enhanced by 3 fold (Ni I), by 2.4 fold (Ni II) and by 2.2 fold (Ni III) T. repens dry weight and P-content of the shoots increased by 9.8 fold (Ni I), by 9.9 fold (Ni II) and by 5.1 fold (Ni III) concomitantly with a reduction in Ni concentration in the shoot compared with non-treated plants. Coinoculation of G. mosseae and the Ni-tolerant bacterial strain (B. brevis) achieved the highest plant dry biomass (shoot and root) and N and P content and the lowest Ni shoot concentration. Dual inoculation with the most Ni-tolerant autochthonous microorganisms (B. brevis and G. mosseae) increased shoot and root plant biomass and subtantially reduced the specific absorption rate (defined as the amount of metal absorbed per unit of root biomass) for nickel in comparison with plants grown in soil inoculated only with G. mosseae. B. brevis increased nodule number that was highly depressed in Ni I added soil or supressed in Ni II and Ni III supplemented soil. These results suggest that selected bacterial inoculation improved the mycorrhizal benefit in nutrients uptake and in decreasing Ni toxicity. Inoculation of adapted beneficial microorganisms (as autochthonous B. brevis and G. mosseae) may be used as a tool to enhance plant performance in soil contaminated with Ni.     

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.     

Interaction between AM fungi and a liquid organic amendment with respect to enhancement of the performance of the leguminous shrub Retama sphaerocarpa

This study examined the interactions between the inoculation with three arbuscular mycorrhizal fungi, namely, Glomus intraradices, Glomus deserticola and Glomus mosseae, and the addition of a liquid organic amendment at different rates (0, 50, 100 or 300 mg C of liquid amendment per kilogram soil) obtained by alkaline extraction of composted dry olive residue with respect to their effects on growth of Retama sphaerocarpa seedlings and on some microbiological and physical properties of soil. One year after planting, both mycorrhizal inoculation treatments and the addition of amendment had increased plant growth and dehydrogenase, urease and benzoyl argininamide hydrolysing activities. The inoculation with G. mosseae increased plant growth to a greater extent than the addition of the amendment (about 35% greater than plants grown in the amended soil and about 79% greater than control plants) and both treatments produced similar increases in soil aggregate stability (about 31% higher than control soil). The organic amendment produced a very significant decrease in the levels of microbial biomass C and a strong increase in soil dehydrogenase and urease activities, which were proportional to the amendment rate. Only the combined treatment involving the addition of a medium dose of amendment (100 mg C kg⁻¹ soil) and the mycorrhizal inoculation with G. intraradices or G. deserticola produced an additive effect on the plant growth with respect to the treatments applied individually (about 77% greater than plants grown in the amended soil and about 63% greater than inoculated plants).     

Interaction between the soil yeast Rhodotorula mucilaginosa and the arbuscular mycorrhizal fungi Glomus mosseae and Gigaspora rosea

The effect of the soil yeast, Rhodotorula mucilaginosa LBA, on Glomus mosseae (BEG n°12) and Gigaspora rosea (BEG n°9) was studied in vitro and in greenhouse trials. Hyphal length of G. mosseae and G. rosea spores increased significantly in the presence of R. mucilaginosa. Exudates from R. mucilaginosa stimulated hyphal growth of G. mosseae and G. rosea spores. Increase in hyphal length of G. mosseae coincided with an increase in R. mucilaginosa exudates. No stimulation of G. rosea hyphal growth was detected when 0.3 and 0.5 ml per petri dish of yeast exudates was applied. Percentage root length colonization by G. mosseae in soybean (Glycine max L. Merill) and by G. rosea in red clover (Trifolium pratense L. cv. Huia) was increased only when the soil yeast was inoculated before G. mosseae or G. rosea was introduced. Beneficial effects of R. mucilaginosa on arbuscular mycorrhizal (AM) colonization were found when the soil yeast was inoculated either as a thin agar slice or as a volume of 5 and 10 ml of an aqueous solution. R. mucilaginosa exudates (20 ml per pots) applied to soil increased significantly the percentage of AM colonization of soybean and red clover.     

Field survival of the phytostimulator Azospirillum lipoferum CRT1 and functional impact on maize crop, biodegradation of crop residues, and soil faunal indicators in a context of decreasing nitrogen fertilisation

Heavy nitrogen fertilisation is often implemented in maize cropping systems, but it can have negative environmental effects. Nitrogen-fixing, phytohormone-producing Azospirillum plant growth-promoting rhizobacteria (PGPR) have been proposed as crop inoculants to maintain high yield when decreasing nitrogen fertilisation. In this context, agronomic and ecological effects of the inoculation of maize seeds with the PGPR Azospirillum lipoferum CRT1 were studied in two consecutive years. The inoculant was recovered from maize at 10⁵ CFU g⁻¹ root or higher. Inoculation enhanced root growth and development based on results of root biomass, rooting depth and/or parameters describing root system architecture, and a transient positive effect on shoot height was observed in the first year. Inoculation did not increase yield, but reducing mineral nitrogen fertilisation had only a minor effect on yield. This suggests that the lack of positive effect of the PGPR on yield was due to the fact that the whole field was heavily fertilised in years prior to the start of the experiment. Soil nitrogen levels decreased during the 2 years of the study, and the inoculant had no effect on residual soil nitrogen levels at harvest. Inoculation had no impact on Fusarium symptoms and concentration of the mycotoxin deoxynivalenol in maize kernels, but both were influenced by the interaction between inoculation and nitrogen fertilisation level. Inoculation did not influence meso/macrofaunal soil populations, but had a small but significant effect (smaller than the effect of added nitrogen) on decomposition, nitrogen mineralisation and mesofaunal colonisation of maize leaves (in litter bags). Overall, the ecological impact of seed inoculation with the PGPR A. lipoferum CRT1 was small, and its magnitude was smaller than that of chemical nitrogen fertilisation.     

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.     

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.     

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.     

Colonisation of seminal roots of wheat and barley by egg-parasitic nematophagous fungi and their effects on Gaeumannomyces graminis var. tritici and development of root-rot

This study provides evidence that egg-parasitic nematophagous fungi, Pochonia chlamydosporia, Pochonia rubescens and Lecanicillium lecanii, can also reduce root colonisation and root damage by a fungal pathogen. Interactions of nematophagous fungi with the take-all fungus, Gaeumannomyces graminis var. tritici (Ggt), and their influence on severity of the root disease it causes were studied in laboratory and pot experiments. In Petri dish experiments the three nematophagous fungi reduced colonisation of barley roots by Ggt and also reduced necrotic symptoms. On the contrary, root colonisation by nematophagous fungi was unaffected by Ggt. In growth tube experiments, the three nematophagous fungi again reduced Ggt root colonisation and increased effective root length of barley seedlings. This was true for both simultaneous and sequential inoculation of nematophagous fungi versus Ggt. In the pot experiments the inoculum of the tested fungi in soil was applied in the same pot, as a mixture or in layers, or in coupled pots used for wheat grown with a split-root system. The nematophagous fungi P. chlamydosporia (isolate 4624) and L. lecanii (isolate 4629), mixed with Ggt or in split root systems with the pathogen, promoted growth of wheat (i.e. increased shoot weight), although no disease reduction was found. In split root systems, lower levels of peroxidase activity were found in seedlings inoculated with Ggt in combination with the nematophagous isolates 4624 and 4629 than when the take-all fungus was applied alone.Our results show that nematophagous fungi reduce root colonisation by Ggt, root damage and stress induced senescence in Ggt-inoculated plants.     

Belowground interactions between intercropped wheat and Brassicas in acidic and alkaline soils

Our previous studies showed that, under P-limiting conditions, growth and P uptake were lower in the wheat genotype Janz than in three Brassica genotypes when grown in monoculture. The present study was conducted to answer the question if P mobilised by the Brassicas is available to wheat; leading to improved growth of wheat when intercropped with Brassicas compared to monocropped wheat. To assess if the interactions between the crops depend on soil type, the wheat genotype Janz and three Brassica genotypes (two canolas and one mustard) were grown for 6 weeks in monoculture or wheat intercropped with each Brassica genotype in an acidic and an alkaline soil with low P availability (with two plants per pot). Wheat grew equally well in the two soils, but the Brassicas grew better in the acidic than in the alkaline soil. In the acidic soil, monocropped Brassicas had a 3 to 4 fold greater plant dry weight (dw) and P uptake than wheat; plant dw and P uptake in wheat were decreased or not affected by intercropping and increased in the Brassicas. In the alkaline soil, dw and P uptake of the Brassicas was twice as high as in wheat, with intercropping having no effect on these parameters. The contribution of wheat to the total shoot dw and P uptake per pot was 4-21% and 32-40% in acidic and alkaline soil, respectively. Mycorrhizal colonisation was low in all genotypes in the acidic soil (1-6%). In the alkaline soil, mycorrhizal colonisation of monocropped wheat was 62%, but only 43-47% in intercropped wheat. Intercropping decreased P availability in the rhizosphere of wheat in the acidic soil but had no effect on rhizosphere P availability in the alkaline soil. Intercropping had a variable effect on rhizosphere microbial community composition (assessed by fatty acid methylester analysis (FAME) and ribosomal intergenic spacer amplification (RISA)), ranging from intercropping having no effect on the rhizosphere communities to intercropping resulting in a new and similar rhizosphere community composition in both genotypes. The results of this study show that intercropping with Brassicas does not improve growth and P uptake of wheat; thus there is no indication that P mobilised by the Brassicas is available to wheat.     

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.     

Growth promotion and increased potassium uptake of cotton and rape by a potassium releasing strain of Bacillus edaphicus

A potassium-releasing bacterial strain Bacillus edaphicus NBT was examined for plant-growth-promoting effects and nutrient uptake on cotton and rape in K-deficient soil in pot experiments. Inoculation with bacterial strain B. edaphicus NBT was found to increase root and shoot growth of cotton and rape. Strain NBT was able to mobilize potassium efficiently in both plants when illite was added to the soil. In cotton and rape growing in soils treated with insoluble potassium and inoculated with strain NBT, the potassium content was increased by 30 and 26%, respectively. Bacterial inoculation also resulted in higher N and P contents of above ground plant components. The bacterial isolate was also able to colonize and develop in the rhizosphere soil of cotton and rape after root inoculation.     

Arbuscular mycorrhizal fungus enhances crop yield and P-uptake of maize (Zea mays L.): A field case study on a sandy loam soil as affected by long-term P-deficiency fertilization

The P efficiency, crop yield, and response of maize to arbuscular mycorrhizal fungus (AMF) Glomus caledonium were tested in an experimental field with long-term (18-year) fertilizer management. The experiment included five fertilizer treatments: organic amendment (OA), half organic amendment plus half mineral fertilizer (1/2 OM), mineral fertilizer NPK, mineral fertilizer NK, and the control (without fertilization). AMF inoculation responsiveness (MIRs) of plant growth and P-uptake of maize were estimated by comparing plants grown in unsterilized soil inoculated with G. caledonium and in untreated soil containing indigenous AMF. Soil total P, available P, microbial biomass P, alkaline phosphatase activity, plant biomass, crop yield and total P-uptake of maize were all significantly increased (P < 0.05) by the application of OA, 1/2 OM, and NPK, but not by the application of NK. Specifically, the individual crop yield of maize approached zero in the NK-fertilized soils, as well as in the control soils. All maize plants were colonized by indigenous AMF, and the root colonization at harvest time was not significantly influenced by fertilization. G. caledonium inoculation increased mycorrhizal colonization significantly (P < 0.05) only with the NK treatment, and produced low but demiurgic crop yield in the control and NK-fertilized soils. Compared to the inoculation in balanced-fertilized soils, G. caledonium inoculation in either the NK-fertilized soils or the control soils had significantly greater (P < 0.05) impacts on soil alkaline phosphatase activity, stem length, plant biomass, and total P-uptake of maize, indicating that AMF inoculation was likely more efficient in extremely P-limited soils. These results also showed that balanced mineral fertilizers and organic amendments did not differ significantly in their effects on MIRs in these soils.     

Susceptibility and effectiveness of vesicular-arbuscular mycorrhizae in wheat cultivars under different growing conditions

Summary Wheat cultivars assumed to be non-susceptible to vesicular-arbuscular (VA) mycorrhizae became colonized, and this effect persisted under different growth conditions. Colonization of all cultivars was similar regardless of the amount of inoculum and the time interval of inoculation. Different plant growth temperatures and the support given by the culture media, inoculation with different endophytes, and inoculation with sterilized and unsterilized spores affected VA colonization levels, although the level of colonization reached in cv. Champlein was similar to that reached in cv. 7-Cerros under each condition. VA mycorrhizal colonization was also affected by different plant growth conditions. After VA reinoculation, the plant dry weight of Castan and 7-Cerros increased, but not Negrillo and Champlein cultivars. VA mycorrhizae increased the shoot dry weight of 7-Cerros only, but not of Champlein, when grown at 35/24°C, and had no effect on the dry weight of either cultivar grown at 18/12°C and 42/24°C. Inoculation with Glomus mosseae increased the dry weight of the cultivars more than inoculation with G. fasciculatum or G. agregatum. The effect on the plant dry weight was greater in plants grown in soil than in sand/vermiculite pots. Inoculation with sterilized and unsterilized spores of G. mosseae, either in soil pots or in sand/vermiculite tubes, did not increase the plant dry weight. Our results indicate that there was no close relationship between the level of root colonization and the effect on plant growth. The effects of accompanying microorganisms in the VA inoculum on VA mycorrhizal symbiosis are discussed.     

Inoculating maize fields with earthworms (Aporrectodea trapezoides) and an arbuscular mycorrhizal fungus (Rhizophagus intraradices) improves mycorrhizal community structure and increases plant nutrient uptake

Earthworms and arbuscular mycorrhizal fungi (AMF) are important macrofauna and microorganisms of the rhizosphere. The effect of the inoculation of soil with earthworms (Aporrectodea trapezoides) and mycorrhiza (Rhizophagus intraradices) on the community structure of mycorrhizal fungi and plant nutrient uptake was determined with split plots in a maize field. Maize plants were inoculated or not inoculated with AMF, each treated with or without earthworms. Wheat straw was added as a feed source for earthworms. Inoculating AMF significantly increased maize yield (p?<?0.05), and the yield was further enhanced by the addition of earthworms. Alkaline phosphomonoesterase activities, soil microbial biomass carbon (SMBC) and nitrogen (SMBN) increased with the addition of both earthworms and AMF. Soil inorganic N and available K were positively affected by earthworms, while available P showed a negative relationship with AMF. Treatment with both AMF and earthworms increased shoot and root biomass as well as their N and P uptake by affecting soil phosphomonoesterase and urease activities, SMBC, SMBN, and the content of available nutrients in soil. The applied fungal inoculants were successfully traced by polymerase chain reaction with novel primers (AML1 and AML2) which target the small subunit rRNA gene. The amplicons were classified by restriction fragment length polymorphism and sequencing. Moreover, field inoculation with inocula of non-native isolates of R. intraradices appeared to have stimulated root colonization and yield of maize. Adding earthworms might influence native AMF community, and the corresponding abundance increased after earthworms were inoculated, which has positive effects on maize growth.     

Fraxinus-Glomus-Pisolithus symbiosis: Plant growth and soil aggregation effects

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.