Search

The aim of this work was to evaluate arbuscular mycorrhizal (AM) fungi as soil indicators and the mycorrhization of native and exotic tree species planted in the Acarau basin, a transition area from the coast to the Brazilian semiarid region. Plots with 6-year-old trees of four native and three non-native species as well as one non-forested area were evaluated in terms of the diversity of AM fungi in the mycorrhizosphere and the root colonization by AM and ectomycorrhizal (EcM) fungi. Twenty-four AM fungi were identified; Claroideoglomus etunicatum, Glomus sinuosum, Paraglomus albidum, Acaulospora laevis, and Acaulospora brasiliensis were abundant in the forest soil. Diversity, dominance, evenness and richness indices of AM fungi were higher in plots with native trees. All root samples were colonized by AM fungi and only Anadenanthera colubrina, Acacia mangium, Casuarina equisetifolia and Eucalyptus urophylla formed associations with EcM fungi. Acaulospora morphotypes served as soil indicators for coverings with the native species Astronium fraxinifolium and Colubrina glandulosa. Exotic species may favor the proliferation of rarer AM fungi. These fungi–plant relationships may be important in the management of forest systems, and the evidence with mycorrhizal associations allows the inclusion of Brazilian species in tropical reforestation.

Four phosphate rock-solubilizing bacteria (PRSB) (Asaia lannensis Vb1, Pseudomonas sp. Vr14, Rahnella sp. Sr24, and Pantoea sp. Vr25) isolated from the mycorrhizosphere of maize were evaluated as inoculants, individually or in different combinations, together with Rhizophagus irregularis DAOM 197198, under greenhouse conditions. To create an effective bacterial mix for P solubilization and growth promotion, parameters such as the ability of the strains (individually or in mixture) to mobilize phosphate rock (PR), their biocompatibility, and their capacity for biofilm formation over PR particles and root colonization were tested. Accompanying PR solubilization, the bacteria produced organic acids and reduced the pH of the medium, produced exopolysaccharides, and had variable capacity to colonize the roots of maize plantlets. In low-phosphorus soil amendment with PR, all strains, regardless of their capacity to solubilize PR, increased dry weight, nutrient (N, P, and K) uptake, and the percentage of indigenous arbuscular mycorrhizae (iAM) root colonization in maize plants, compared to the non-inoculated control. However, mixed inoculation of the strains showed significantly better results. Addition of Rhizophagus irregularis resulted in further growth stimulation. Overall results showed that two combinations—Rahnella sp. Sr24 + Pantoea sp. Vr25 + R. irregularis, and Pantoea sp. Vr25 + Pseudomonas sp. Vr14 + R. irregularis—were better inoculants. We concluded that an effective PRSB combination of biocompatible strains with capacity for PR solubilization, successful biofilm formation, effective root colonization, different growth promotion traits, and the addition of AM has potential as inoculants for more sustainable agriculture in low-phosphorus soils amended with low-reactivity PR.

Aims In the Swedish sub-Arctic, mountain birch (Betula pubescens ssp. czerepanovii) forests mediate rapid soil C cycling relative to adjacent tundra heaths, but little is known about the role of individual trees within forests. Here we investigate the spatial extent over which trees influence soil processes. Methods We measured respiration, soil C stocks, root and mycorrhizal productivity and fungi:bacteria ratios at fine spatial scales along 3 m transects extending radially from mountain birch trees in a sub-Arctic ecotone forest. Root and mycorrhizal productivity was quantified using in-growth techniques and fungi:bacteria ratios were determined by qPCR. Results Neither respiration, nor root and mycorrhizal production, varied along transects. Fungi:bacteria ratios, soil organic C stocks and standing litter declined with increasing distance from trees. Conclusions As 3 m is half the average size of forest gaps, these findings suggest that forest soil environments are efficiently explored by roots and associated mycorrhizal networks of B. pubescens. Individual trees exert influence substantially away from their base, creating more uniform distributions of root, mycorrhizal and bacterial activity than expected. However, overall rates of soil C accumulation do vary with distance from trees, with potential implications for spatio-temporal soil organic matter dynamics and net ecosystem C sequestration.

Phosphate chemical fertilizers are costly and raise concerns about environmental pollution through industrial production. The use of phosphate rock (PRs) emerges as a more sustainable alternative for agriculture. The aim of this work was to isolate phosphate rock-solubilizing bacteria (PRSB) from maize mycorrhizosphere, having growth promoting traits and that will be arbuscular mycorrhizae fungi (AMF) compatible. Bacteria were isolated from the mycorrhizosphere of maize and tested for rock phosphate solubilization, production of organic acids, phosphatases and phytase activities, and growth promotion traits. The capacity of some strains to enhance the dry weight of maize plants was determined in a growth chamber experiment. The compatibility of the selected strains with Rhizophagus irregularis under in vitro conditions was also tested. Out of 118 isolates from maize, eight belonging to Asaia lannaensis, Rahnella sp., Pantoea sp., and Pseudomonas sp. were found to be the best PRSB. On solid media, all strains mobilized P from tricalcium phosphate, hydroxyapatite, and PRs. A. lannaensis was the only PRSB showing visible solubilization of AlPO4 and FePO4. All the PRSBs solubilized PR by producing D-gluconic acid and 2-ketogluconic acid and by lowering the pH. Most strains presented IAA and siderophore production and different biofilm formation and motility capacities. All strains improved the dry weight of maize seedlings compared with non-inoculated plants. The results proved that PRSBs were able to grow on R. irregularis hyphae as the sole in vitro C source. Bacteria isolated from the mycorrhizosphere of maize shows effective solubilization of phosphorus from PR with different reactivity levels. The traits of these bacteria as growth promoters and their biocompatibility with AMF show their potential as inoculants. Improvement of the agronomic effectiveness of PRs is relevant for developing countries that use PRs directly as P-fertilizers (less expensive than soluble P-fertilizers) for sustainable agriculture.

9th International Conference on Mycorrhiza, Prague, Czech Republic, August 2017; International audience; This Mycorrhiza issue groups topical papers based on presentations and discussions at the Mycorrhizal Microbiomes session at 9th International Conference on Mycorrhiza, Prague, Czech Republic, August 2017. The five articles that appear in this special issue advance the field of mycorrhizal microbiomes, not simply by importing ideas from an emerging area, but by using them to inform rich and methodologically grounded research. The aim of this special issue is to explore the interactions between mycorrhizal fungi and surrounding complex environments from a distinct but complementary point of view, highlighting the large spectrum of unknowns that still need to be explored. In this editorial, we first introduce the level of knowledge in this thematic area, then describe major results from the five manuscripts and characterise their importance to mycorrhizal research, and finally discuss the developing topics in this rapidly emerging thematic area.

Ectomycorrhizal (ECM) fungi are obligate symbionts of dominant vascular plants, liverworts and hornworts. There are reports of about 20,000 to 25,000 ECM fungi that promote plant growth by facilitating enhanced water and nutrient absorption, and provide tolerance to environmental stresses. These below-ground fungi play a key role in terrestrial ecosystems as they regulate plant diversity, nutrient and carbon cycles, and influence soil structure and ecosystem multifunctionality. Because ECM fungi are obligate root symbionts, host plant can have a strong effect on ECM species richness and community composition. The biogeographic pattern and detailed functioning and regulation of these mycorrhizosphere processes are still poorly understood and require detailed study. More recent researches have placed emphasis on a wider, multifunctional perspective, including the effects of ectomycorrhizal symbiosis on plant and microbial communities, and on ecosystem processes. Over the years the main focus in ECM research has been on the study of diversity and specificity of ECM strains, the role of ECM in regeneration of degraded ecosystem, the growth and establishment of seedlings through nutrient acquisition and the mediation of plant responses to various types of stress. In this review, recent progresses in ectomycorrhizal biology are presented, especially the potential role of ECM symbioses in resistance or tolerance to various biotic and abiotic stresses, and in maintinance of plant diversity for proper ecosystem functioning.

Our study was intended to elucidate the involvement of three species of arbuscular mycorrhizal fungi (AMF) in the formation and stabilization of aggregates in semiarid soils with different textures and calcium carbonate contents. We used a root-hyphae compartment approach to compare the effect of three AMF (Rhizophagus irregularis, Septoglomus deserticola, and Gigaspora gigantea) on the structural stability of the hyphosphere (root-free hyphae) and mycorrhizosphere (hyphae + root) soil of Olea europaea plants grown in two soils differing in their texture (sandy loam and silty loam) and calcium carbonate content. Only the R. irregularis strain significantly increased the percentage of stable aggregates in both types of soil, being the increases higher in the hyphosphere compartment (on average, about 30 % compared to non-inoculated soil). In the hyphosphere compartment of both soils, the hyphal length developed by plants inoculated with R. irregularis was 81 % greater than that of non-inoculated plants. The effect of the AMF on soil aggregation was mediated by mechanical entanglement of mycorrhizal fungal hyphae but without a contribution of labile carbohydrates. The ability of extraradical hyphae to improve soil structure was independent of the soil texture and content of carbonates.

Tricholoma matsutake is an ectomycorrhizal basidiomycete that produces prized, yet uncultivable, "matsutake" mushrooms along densely developed mycelia, called "shiro," in the rhizosphere of coniferous forests. Pinus densiflora is a major host of this fungus in Japan. Measuring T. matsutake biomass in soil allows us to determine the kinetics of fungal growth before and after fruiting, which is useful for analyzing the conditions of the shiro and its surrounding mycorrhizosphere, predicting fruiting timing, and managing forests to obtain better crop yields. Here, we document a novel method to quantify T. matsutake mycelia in soil by quantifying a single-copy DNA element that is uniquely conserved within T. matsutake but is absent from other fungal species, including close relatives and a wide range of ectomycorrhizal associates of P. densiflora. The targeted DNA region was amplified quantitatively in cultured mycelia that were mixed with other fungal species and soil, as well as in an in vitro co-culture system with P. densiflora seedlings. Using this method, we quantified T. matsutake mycelia not only from shiro in natural environments but also from the surrounding soil in which T. matsutake mycelia could not be observed by visual examination or distinguished by other means. It was demonstrated that the core of the shiro and its underlying area in the B horizon are predominantly composed of fungal mycelia. The fungal mass in the A or A0 horizon was much lower, although many white mycelia were observed at the A horizon. Additionally, the rhizospheric fungal biomass peaked during the fruiting season.

Sixty-four native bacterial colonies were isolated from mycorrhizal roots of Helianthemum almeriense colonized by Terfezia claveryi, mycorrhizosphere soil, and peridium of T. claveryi to evaluate their effect on mycorrhizal plant production. Based on the phylogenetic analysis of the 16S rDNA partial sequence, 45 different strains from 17 genera were gathered. The largest genera were Pseudomonas (40.8 % of the isolated strains), Bacillus (12.2 % of isolated strains), and Varivorax (8.2 % of isolated strains). All the bacteria were characterized phenotypically and by their plant growth-promoting rhizobacteria (PGPR) traits (auxin and siderophore production, phosphate solubilization, and ACC deaminase activity). Only bacterial combinations with several PGPR traits or Pseudomonas sp. strain 5, which presents three different PGPR traits, had a positive effect on plant survival and growth. Particularly relevant were the bacterial treatments involving auxin release, which significantly increased the root-shoot ratio and mycorrhizal colonization. Moreover, Pseudomonas mandelii strain 29 was able to considerably increase mycorrhizal colonization but not plant growth, and could be considered as mycorrhiza-helper bacteria. Therefore, the mycorrhizal roots, mycorrhizosphere soil, and peridium of desert truffles are environments enriched in bacteria which may be used to increase the survival and mycorrhization in the desert truffle plant production system at a semi-industrial scale.

Arbuscular mycorrhizal (AM) hyphae represent an important route for input of plant-derived C to soil, but impacts of these inputs on microbial communities and processes are poorly understood. In this study we characterised pathways of C-flow through microbial communities associated with AM hyphae and quantified impacts on mineralisation of native SOM. Continuous, steady-state 13CO2 labelling was applied throughout the growth period (60 d) of Lolium perenne. Exclusion meshes were used to control access of roots and AM hyphae to soil, and plant-derived C was quantified within microbial PLFA and NLFA, and soil CO2 efflux was partitioned into plant- and soil organic matter (SOM) derived components. Pathways of C-flow through hyphosphere and mycorrhizosphere communities were distinct, as was the fate of plant-derived C from AM hyphae accessing soil through 37 and 1 μm meshes. Mineralisation of native SOM was increased in all treatments, relative to unplanted controls, and this priming effect was largest for AM hyphae accessing soil through the 1 μm mesh size. We demonstrated that AM hyphae can strongly increase mineralisation of native SOM and identified distinct pathways of C-flow through hyphosphere communities. Our results suggest that, in addition to affecting rates of litter decomposition, AM hyphae may have a significant influence on turnover of native SOM.

The rhizosphere is a hot-spot for biogeochemical cycles, including production of greenhouse gases, as microbial activity is stimulated by rhizodeposits released by roots and mycorrhizae. The biogeochemical cycle of nitrogen (N) in soil is complex, consisting of many simultaneously occurring processes. In situ studies investigating the effects of roots and mycorrhizae on gross N turnover rates are scarce. We conducted a 15N tracer study under field conditions in a spruce forest on organic soil, which was subjected to exclusion of roots and roots plus ectomycorrhizae (ECM) for 6 years by trenching. The forest soil had, over the 6-year period, an average emission of nitrous oxide (N2O) of 5.9 ± 2.1 kg N2O ha−1 year−1. Exclusion of roots + ECM nearly tripled N2O emissions over all years, whereas root exclusion stimulated N2O emission only in the latest years and to a smaller extent. Gross mineralization–ammonium (NH4 +) immobilization turnover was enhanced by the presence of roots, probably due to high inputs of labile carbon, stimulating microbial activity. We found contrasting effects of roots and ECM on N2O emission and mineralization, as the former was decreased but the latter was stimulated by roots and ECM. The N2O emission was positively related to the ratio of gross NH4 + oxidation (that is, autotrophic nitrification) to NH4 + immobilization. Ammonium oxidation was only stimulated by the presence of ECM, but not by the presence of roots. Overall, we conclude that plants and their mycorrhizal symbionts actively control soil N cycling, thereby also affecting N2O emissions from forest soils. Consequently, adapted forest management with permanent tree cover avoiding clearcutting could be a means to reduce N2O emissions and potential N leaching; despite higher mineralization in the presence of roots and ECM, N2O emissions are decreased as the relative importance of NH4 + oxidation is decreased, mainly due to a stimulated microbial NH4 + immobilization in the mycorrhizosphere.

Aims To investigate the importance of ectomycorrhizal (ECM) extraradical mycelia and soil substrate in shaping specific mycorrhizosphere microbial communities.

Belowground biological interactions that occur among plant roots, microorganisms and animals are dynamic and substantially influence ecosystem processes. Among these interactions, the ectomycorrhizal (ECM) symbiosis is remarkable but unfortunately these associations have mainly been considered within the rather narrow perspective of their effects on the uptake of dissolved mineral nutrients by individual plants. More recent research has placed emphasis on a wider, multifunctional perspective, including the effects of ectomycorrhizal symbiosis on plant and microbial communities, and on ecosystem processes. This includes mobilization of N and P from organic polymers, release of nutrients from mineral particles or rock surfaces via weathering, effects on carbon cycling, interactions with mycoheterotrophic plants, mediation of plant responses to stress factors such as drought, soil acidification, toxic metals, and plant pathogens, rehabilitation and regeneration of degraded forest ecosystems, as well as a range of possible interactions with groups of other soil microorganisms. Ectomycorrhizas are almost invariably characterized by a Hartig net composed of highly branched hyphae which entirely surround the outer root cortical cells. The Hartig net is the place of massive bidirectional exchanges of nutrients between the host and the fungus. Through these branched hyphae ectomycorrhizal fungi connect their plant hosts to the heterogeneously distributed nutrients required for their growth, enabling the flow of energy-rich compounds required for nutrient mobilization whilst simultaneously providing conduits for the translocation of mobilized products back to their hosts. In addition to increasing the nutrient absorptive surface area of their host plant root systems, the extraradical mycelium of ectomycorrhizal fungi provides a direct pathway for translocation of photosynthetically derived carbon from their hosts to microsites in the soil and a large surface area for interaction with other soil micro-organisms. The detailed functioning and regulation of these mycorrhizosphere processes is still poorly understood and needs detailed molecular approach to study these mycorrhizosphere processes but recent progress in ectomycorrhizal associations is reviewed and potential benefits of improved understanding of mycorrhizosphere interactions are discussed.

Arbuscular mycorrhizal (AM) fungi known to cause plant growth depressions in tomato were examined for their biocontrol effects against root rot caused by Pythium aphanidermatum. The main hypothesis was that plant growth suppressive AM fungi would elicit a defence response in the host plant reducing Pythium root rot development. To test this hypothesis a fully factorial experiment was performed with AM fungi (Glomus intraradices, G. mosseae, G. claroideum or nonmycorrhizal), Pythium (± P. aphanidermatum) and harvest (7 and 14 days after pathogen inoculation (dapi)) as the main factors. Two weeks after AM fungi inoculation, roots were challenged with P. aphanidermatum. Variables evaluated at each harvest were root colonization levels of the interacting fungi, plant growth responses, and expression of a plant pathogenesis related protein gene (PR-1). All of the tested AM fungi caused marked growth suppressions, but did not affect PR-1 gene expression or the phosphorous concentration in the host plant. Plants singly inoculated with P. aphanidermatum had an increased PR-1 expression and phosphorous concentration. Among the AM fungi included in the study only G. intraradices reduced the pathogen root infection level, measured both in terms of Pythium ELISA and by recovery on selective media and only at the first harvest. Likewise, P. aphanidermatum root infection reduced colonization levels of G. intraradices, but not that of the two other AM fungi. In conclusion, plant growth suppressive AM fungi may offer plant beneficial traits in terms of biocontrol of root cortical pathogens. © 2011 KNPV.

We conducted a mesocosm study to investigate the extent to which the process of hydraulic redistribution of soil water by plant roots is affected by mycorrhizosphere disturbance. We used deuterium-labeled water to track the transfer of hydraulically lifted water (HLW) from well-hydrated donor oaks (Quercus agrifolia Nee.) to drought-stressed receiver seedlings growing together in mycorrhizal or fungicide-treated mesocosms. We hypothesized that the transfer of HLW from donor to receiver plants would be enhanced in undisturbed (non-fungicide-treated) mesocosms where an intact mycorrhizal hyphal network was present. Contrary to expectations, both upper soil and receiver seedlings contained significantly greater proportions of HLW in mesocosms where the abundance of mycorrhizal hyphal links between donor and receiver roots had been sharply reduced by fungicide application. Reduced soil hyphal density and viability likely hampered soil moisture retention properties in fungicide-treated mesocosms, thus leading to faster soil water depletion in upper compartments. The resulting steeper soil water potential gradient between taproot and upper compartments enhanced hydraulic redistribution in fungicide-treated mesocosms. Belowground disturbances that reduce soil hyphal density and viability in the mycorrhizosphere can alter the patterns of hydraulic redistribution by roots through effects on soil hydraulic properties.

Arbuscular mycorrhizal (AM) fungi interact with bacteria (AM fungi-associated bacteria, AMB) in the mycorrhizosphere. We previously identified a set of AMB that enhance AM fungal colonization, plant growth, and inhibit pathogens. Here, we used transformed carrot root cultures in a two-compartment plate system for further in vitro studies on interactions taking place among Glomus irregulare (syn.Glomus intraradices), AMB, and plant pathogens. We found that exudates of G. irregulare stimulated growth of all ten AMB isolates tested in multi-well plates. AMB growth stimulation was observed also during co-cultivation of three of these AMB with G. irregulare in the hyphal compartment. In addition, co-cultivation stimulated growth of G. irregulare hyphae and spore production, as well as G. irregulare root colonization. GC/MS analysis in a preliminary screening of metabolites revealed differences in concentrations of several identified but also unidentified compounds in G. irregulare hyphal exudates. Exudates in presence of three different AMB isolates co-cultivated with G. irregulare contained several additional compounds that differed in amount compared with G. irregulare alone. The results indicate that G. irregulare exudates contain carbohydrates, amino acids, and unidentified compounds that could serve as a substrate to stimulate AMB growth. With regard to effects on plant pathogens, growth inhibition of Rhizoctonia solani, Verticillium dahliae, and Pectobacterium carotovorum ssp. carotovorum was evident in the presence of the AMB isolates tested together with the G. irregulare exudates. These in vitro studies suggest that G. irregulare and AMB stimulate growth of each other and that they together seem to provide an additive effect against growth of both fungal and bacterial pathogens.

Plant growth and productivity under abiotic stresses such as water shortage or pollution are major problems which currently worry scientists in the field of food production and plant health. Since the intensification of livestock production with its associated increased demand for fodder has encouraged farmers to rely more heavily on chemical fertilizers, very often, municipal waste and wastewater sludge is considered a possible source of plant nutrients, although it might carry a significant amount of anthropogeneous pollutants. The extent to which plants react to drought, as well as how pollutants are taken up or how they act on plants, might depend on rhizosphere processes such as mycorrhizal symbioses. Therefore, it seemed timely to investigate plant defense reactions in the presence of arbuscular mycorrhizal fungi (AMF) toward a possible dangerous sewage sludge pollutant under the influence of drought. This study was done with mycorrhizal plants (+AMF) and non-mycorrhizal plants (−AMF) under conditions of water shortage and certain supplements of pharmaceutics. Plant water relations, activity of plant detoxification enzymes, and phylogeny of root mycorrhization were surveyed to investigate beneficial aspects of AMF symbiosis. Plants were exposed to certain drought phases and watered with tap water containing the pharmaceutical paracetamol (acetaminophen) as a model xenobiotic with environmental relevance. Enzymes of interest for the present study were catalase (CAT) and glutathione S-transferase (GST). Assays followed published methods. GST spectrophotometric assays included model substrates 1-chloro-2,4dinitrobenzene, 1,2-dichloro-4-nitrobenzene, and 1,2-dichloro-4-nitrobenzoyl-chloride. Our observations indicate H2O2 accumulation in roots of AMF plants and concomitant increases of catalase activities. Enzyme activities were lower in AMF plants even after exposure to both drought stress and paracetamol. AMF symbiosis seems a strategy to influence plant water tolerance and stress enzymes. Catalase activity seemed to be exhausted at the end of 56 days of drought treatment and pharmaceutics incubation, as indicated by high H2O2 levels at the end of the experiment. GST reactions differed between drought or paracetamol-stressed plants. A combination of both stressors resulted in lowered GST activities. Both catalase and GST activities decreased in the presence of paracetamol in the AMF roots after drought treatment. When soil water contents declined, AMF symbiosis supported leaf water content and production of biomass. During dry cycles, AMF roots achieved lower activities of cytosolic GST than that as compared to non-mycorrhizal roots. However, under drought and pharmaceutics supplements, water relations of mycorrhizal plants were more balanced than in non-mycorrhizal plants. In addition, roots showed an enhanced mycorrhization rate in those circumstances. We propose that the adjustment of CAT and GST activities in mycorrhizal roots under drought and xenobiotics might be caused by a decreased oxidative damage to rhizosphere because AMF plants had consistently better plant water relations and biomass production. Additional drought avoidance mechanisms indicated by the arbuscular mycorrhizal symbiosis might also contribute to the lower oxidative stress in mycorrhizal plants. Since the role of mycorrhiza in the fate of organic pollutants has not yet been understood, further ecotoxicological studies in mycorrhizosphere needs to be achieved as cooperation between plant–microbe specialists and ecotoxicologists.

The aim was to study the influence of abiotic (elevated ozone) or biotic stress (Phytophthora citricola) or their combination on soil biological components and processes in the rhizosphere of young beech trees. Ectomycorrhizal and overall microbial community composition was studied at two soil depths in a lysimeter experiment with 7 year old trees of Fagus sylvatica. As a functional parameter, potential enzyme activities were measured in mycorrhizosphere soil and on excised mycorrhizal tips. The degree of mycorrhization, structure and potential enzymatic activities of mycorrhizal communities were only slightly influenced by treatments. Soil enzyme activities were depressed under elevated ozone and stimulated by P. citricola under ambient but not under elevated ozone. Overall microbial community composition (PLFA) and ectomycorrhizal diversity changed with depth. PLFA analyses not only suggested a reaction of the microbial community to elevated ozone but also indicated an increase in plant stress related components. No influence of the biotic stress on ectomycorrhizal or overall microbial community structure was detected. Changes in the mycorrhizosphere community structure and function due to ozone may be explained by the quality of plant derived carbon.

Growth and enzymatic activities of extraradical mycelia (ERM) of native mycorrhizal symbionts associated with three orchid species, Dactylorhiza fuchsii, D. majalis and Platanthera bifolia, were studied. ERM extracted from the mycorrhizosphere of these species showed features typical for fungi that form orchid mycorrhiza. In the first pot experiment, three different treatments were applied on tubers of D. fuchsii transplanted from a natural site: control (no specific treatment), reinoculated (surface-sterilized tubers reinoculated with mycorrhizal fungi-colonised roots), and benomyl (nonsterilized tubers treated with fungicide). However, no significant differences in ERM growth and intensity of root mycorrhizal colonisation at harvest were observed among these treatments. ERM associated with reinoculated D. fuchsii plants showed significantly higher alkaline phosphatase (ALP) enzymatic activity at week 36 than at week 24, but no differences were observed for NADH diaphorase activity. Benomyl application significantly reduced ALP activity in comparison with reinoculated plants at week 36. In the second experiment, plants of all three species were either untreated (control), or repeatedly treated with benomyl. Similarly to the results of the first experiment, benomyl application did not reduce the ERM growth of mycorrhizal symbionts associated with D. majalis and D. fuchsii. The low ERM growth associated with benomyl-treated P. bifolia was probably caused by poor root system development in this treatment. Significantly higher mycorrhizal colonisation was found for D. fuchsii compared to P. bifolia in control treatments at the end of cultivation. The ERM of native symbionts of the three orchid species studied seemed to have a different growth pattern over time and responded differently to fungicide application.

In a Robinia-pseudoacacia-dominated coastal forest in Tottori prefecture Japan, the growth and survival of Pinus thunbergii seedlings and the natural regeneration of P. thunbergii was disturbed by R. pseudoacacia. In order to improve the growth of P. thunbergii seedling in the Tottori sand dune, we tried to find a mycorrhiza helper bacteria (MHB) from P. thunbergii mycorrhizosphere in a Tottori sand dune. Two MHB, Ralstonia sp. and Bacillus subtilis, were selected from the nine bacterial species isolated from the mycorrhizosphere of P. thunbergii. The bacterial effect on the ectomycorrhizal fungus Suillus granulatus was investigated by confrontation assay and a microcosm experiment. The confrontation assay showed that Ralstonia sp. promoted the hyphal growth of S. granulatus. Moreover, the S. granulatus–P. thunbergii symbiosis was significantly stimulated by Ralstonia sp. and B. subtilis. Ralstonia sp. and B. subtilis were regarded as MHB associated with P. thunbergii. This is the first report of Ralstonia sp. as an MHB.

The effects of 17 Paenibacillus strains on root colonization by Glomus intraradices or Glomus mosseae and plant growth parameters (shoot and root weight) of mycorrhizal cucumber plants were examined. The Paenibacillus strains were originally isolated from mycorrhizal (G. intraradices) and non-mycorrhizal cucumber rhizosphere and/or hyphosphere, except for strain EJP73, which originated from a Pinus sylvestris-Lactarius rufus ectomycorrhiza. Root colonization of cucumber plants by G. intraradices or G. mosseae was unaffected by all seven strains of Paenibacillus polymyxa, but was decreased or increased by four strains of Paenibacillus macerans and strain EJP73 of Paenibacillus sp. Overall, shoot dry weight of cucumber grown in symbioses with either G intraradices or G. mosseae was unaffected by inoculation with all of the Paenibacillus strains, except for strain MB02-429 of P. macerans, which increased the shoot dry weight in the cucumber-G. mosseae symbiosis. On the other hand, several Paenibacillus strains caused altered root growth. Three strains of P. polymyxa and four strains of P. macerans increased the root fresh weight of the cucumber–G. intraradices symbiosis, whereas three strains of P. polymyxa and one strain of P. macerans as well as Paenibacillus sp. EJP73, decreased the root fresh weight of the cucumber–G. mosseae symbiosis. In conclusion, our results show that bacteria from several species of Paenibacillus differentially affect cucumber mycorrhizas.

Recent studies have established the controlling influence of rhizospheric biota, especially arbuscular mycorrhizal fungi (AMF), on colonization and spread of some alien plants in their introduced range. But how AMF from different geographical sources influence traits that contribute to invasiveness, particularly in presence of neighbouring plants of other species, has been rarely investigated. Thus, we compared the influence of some local (Kashmir Himalayan isolates) and non-local (isolates from Rajasthan, India) AMF isolates of Glomus moseae, G. fasciculatum and Gigaspora margarita on vegetative and reproductive attributes of Mayweed Chamomile (Anthemis cotula L.), a highly invasive species in the Kashmir Himalaya, India. We also examined whether or not the neighbouring plant species, namely Daucus carota L. (Apiaceae) alters the mutualistic interaction between the AMF and A. cotula. Pot experiments revealed greater positive impact of the local than the non-local AMF on vegetative as well as reproductive attributes of A. cotula. Experimental field studies showed that the incidence of highly prevalent Arum-type mycorrhizal colonization in natural populations of A. cotula was reduced in presence of D. carota. Besides, the local AMF significantly promoted growth of A. cotula more than D. carota under mixed-culture conditions. These results suggest that the facilitation of some alien plant invasions by AMF needs to be considered together with plant–plant interactions and invasion-induced changes in the soil microbial community.

We present a field study on the drought effects on total soil respiration (SRt) and its components, i.e., “autotrophic” (SRa: by roots/mycorrhizosphere) and “heterotrophic” respiration (SRh: by microorganisms and soil fauna in bulk soil), in a mature European beech/Norway spruce forest. SRa and SRh were distinguished underneath groups of beech and spruce trees using the root exclusion method. Seasonal courses of SRa and SRh were studied from 2002 to 2004, with the summer of 2003 being extraordinarily warm and dry in Central Europe. We (1) analyzed the soil temperature (T s) and moisture sensitivity of SRa and SRh underneath both tree species, and (2) examined whether drought caused differential decline of SRa between spruce and beech. Throughout the study period, SRa of beech accounted for 45–55% of SRt, independent of the soil water regime; in contrast, SRa was significantly reduced during drought in spruce, and amounted then to only 25% of SRt. In parallel, fine-root production was decreased during 2003 by a factor of six in spruce (from 750 to 130 mg l−1 a−1), but remained at levels similar to those in 2002 in beech (about 470 mg l−1 a−1). This species-specific root response to drought was related to a stronger decline of SRa in spruce (by about 70%) compared to beech (by about 50%). The sensitivity of SRa and SRh to changing T s and available soil water was stronger in SRa than SRh in spruce, but not so in beech. It is concluded that SRa determines the effect of prolonged drought on the C efflux from soil to a larger extent in spruce than beech, having potential implications for respective forest types.

Over the last two decades, much information has been gathered on the ectomycorrhizal fungus community composition of plant associations of boreal, temperate, and tropical regions. Worldwide, Tomentella ectomycorrhizas (ECM) are often common and dominant in the mycorrhizosphere of coniferous and deciduous forests. They are present under different environmental conditions and associate with diverse plant hosts. Tomentella sporocarps, however, are rarely found aboveground, so Tomentella species are often missing from fungus community studies based on fruit-body presence. Tomentella is a resupinate genus of Thelephoraceae (Basidiomycota) forming black-brown, brown, yellow, or ochre ECM on the roots of gymnosperm and angiosperm trees, distinguished by typical morphological-anatomical characteristics (clamped hyphae, angular mantle, surface network, special rhizomorphs and cystidia). In this paper, we review the taxonomic position and morphological-anatomical characteristics of Tomentella ECM. A short summary of the microscopic features used for distinguishing tomentelloids during morphotyping and identification is presented in order to support molecular and ecological studies of ectomycorrhizal fungus communities.

The hyphae of ectomycorrhizal and ericoid mycorrhizal fungi proliferate in nitrogen (N)-limited forests and tundra where the availability of inorganic N is low; under these conditions the most common fungal species are those capable of protein degradation that can supply their host plants with organic N. Although it is widely understood that these symbiotic fungi supply N to their host plants, the transfer is difficult to quantify in the field. A novel approach uses the natural 15N:14N ratios (expressed as δ15N values) in plants, soils, and mycorrhizal fungi to estimate the fraction of N in symbiotic trees and shrubs that enters through mycorrhizal fungi. This calculation is possible because mycorrhizal fungi discriminate against 15N when they create compounds for transfer to plants; host plants are depleted in 15N, whereas mycorrhizal fungi are enriched in 15N. The amount of carbon (C) supplied to these fungi can be stoichiometrically calculated from the fraction of plant N derived from the symbiosis, the N demand of the plants, the fungal C:N ratio, and the fraction of N retained in the fungi. Up to a third of C allocated belowground, or 20% of net primary production, is used to support ectomycorrhizal fungi. As anthropogenic N inputs increase, the C allocation to fungi decreases and plant δ15N increases. Careful analyses of δ15N patterns in systems dominated by ectomycorrhizal and ericoid mycorrhizal symbioses may reveal the ecosystem-scale effects of alterations in the plant–mycorrhizal symbioses caused by shifts in climate and N deposition.

The corporate R&D banana orchards of Musa paradisiaca (dwarf Cavendish AAA, var. shrimanti) on a medium black alluvial soil with low nutrients harboured diversified species of vesicular-arbuscular mycorrhizal (VAM) fungi. These fungi infected the roots severely (69.2%), showed elevated (69.8 g(-1) soil) spore density, increased soil bacterial density (245 x 10(8) cfu g(-1)), produced siderophores (58.2%) and reduced nematode population (2.3 g(-1)) in the mycorrhizosphere of plants for integrated plant nutrition management (IPNM) system as compared to traditional treatment of applying chemical fertilisers alone and other test treatments. The interactions of plant roots with native VAM and local and applied rhizobacteria in the matrix of soil conditioner enabled proper nourishment and protection of crop in IPNM treatment as compared to traditional way. Hence, exploitation of plant growth promoting rhizobacteria through judiciously designed IPNM system revealed the (a) relatively increased banana productivity (21.6%, 76 MT ha(-1)), (b) least occurrence of fusarial wilt and negligible evidence of Sigatoka, (c) saving of 50% chemical fertilisers and (d) permitted control over soil fertility in producer's favour over traditional cultivation practices. These findings are discussed in detail.

The ectomycorrhizal (ECM) fungi associated with Pinus thunbergii seedlings grown on sand dune were identified by molecular method, and the diversity of bacteria associated with ECM and Extraradical mycelium were examined by Denaturing Gradient Gel Electrophoresis (DGGE) of PCR-amplified 16S rDNA. The mycorrhizal formation rate of 1-year old P. thunbergii seedlings was more than 95%. Cenococcum geophilum was the most dominant ECM fungus, followed by T01, RFLP-8, Russula spp., and Suillus sp. Bacterial community was most diverse with C. geophilum- and RFLP-8-mycorrhiza. Sequencing analysis showed that Burkholderia spp. and Bradyrhizobium spp. were on the surface of ECM short root of seven ECM. The fungi detected as extraradical mycelium using DGGE of 18S rDNA were Suillus bovinus and RFLP-8-mycorrhiza. Bacterial community on the extraradical mycelium was more diverse than those on ECM root tip. Burkholderia spp. and Bradyrhizobium spp. were found also on extraradical mycelium.

Excess manganese (Mn) in soil is toxic to crops, but in some situations arbuscular mycorrhizal fungi (AMF) alleviate the toxic effects of Mn. Besides the increased phosphorus (P) uptake, mycorrhiza may affect the balance between Mn-reducing and Mn-oxidizing microorganisms in the mycorrhizosphere and affect the level of extractable Mn in soil. The aim of this work was to compare mycorrhizal and non-mycorrhizal plants that received extra P in relation to alleviation of Mn toxicity and the balance between Mn-oxidizing and Mn-reducing bacteria in the mycorrhizosphere. A clayey soil containing 508 mg kg−1 of extractable Mn was fertilized with 30 mg kg−1 (P1) or 45 mg kg−1 (P2) of soluble P. Soybean (Glycine max L. Merrill, cv. IAC 8-2) plants at P1 level were non-inoculated (CP1) or inoculated with either Glomus etunicatum (GeP1) or G. macrocarpum (GmP1), while plants at P2 level were left non-inoculated (CP2). Plants were grown in a greenhouse and harvested after 80 days. In the mycorrhizosphere of the GmP1 and GeP1 plants a shift from Mn-oxidizing to Mn-reducing bacteria coincided with higher soil extractability of Mn and Fe. However, the occurrence of Mn-oxidizing/reducing bacteria in the (mycor)rhizosphere was unrelated to Mn toxicity in plants. Using 16S rDNA sequence homologies, the Mn-reducing isolates were consistent with the genus Streptomyces. The Mn-oxidizers were homologous with the genera Arthrobacter, Variovorax and Ralstonia. While CP1 plants showed Mn toxicity throughout the whole growth period, CP2 plants never did, in spite of having Fe and Mn shoot concentrations as high as in CP1 plants. Mycorrhizal plants showed Mn toxicity symptoms early in the growth period that were no longer visible in later growth stages. The shoot P concentration was almost twice as high in mycorrhizal plants compared with CP1 and CP2 plants. The shoot Mn and Fe concentrations and contents were lower in GmP1 and GeP1 plants compared with the CP2 treatment, even though levels of extractable metals increased in the soil when plants were mycorrhizal. This suggests that mycorrhiza protected its host plant from excessive uptake of Mn and Fe. In addition, higher tissue P concentrations may have facilitated internal detoxification of Mn in mycorrhizal plants. The exact mechanisms acting on alleviation of Mn toxicity in mycorrhizal plants should be further investigated.

Optimizing the turnover and recycling of nutrients, a fundamental issue for the sustainability and productivity of agro-ecosystems is depending on the functionality of a framework of plant-soil interactions where microbial populations are involved. Both mutualistic symbionts and saprophytic microorganisms living at the root-soil interfaces, the rhizosphere, or in the plant-associated soil, are recognized as essential drivers of nutrient cycling, availability and capture. Among the mutualistic symbionts, arbuscular mycorrhizal (AM) fungi are one of the most influential groups of soil biota because after establishing the AM symbiosis with most plant species they enhance plant nutrient uptake properties. Saprophytic microorganisms are recognized for their abilities to propel nitrogen (N) fixation and/or phosphorus (P) mobilization, two fundamental processes for sustain plant productivity. Mycorrhiza establishment changes the biological and physical-chemical properties of the rhizosphere, developing the so-called mycorrhizosphere. Particularly relevant is the mycorrhizosphere of legume plants since it also involves the symbiosis with N2-fixing nodulating rhizobial bacteria. In this overview of mycorrhizosphere interactions related to nutrient cycling, after describing the protagonist microorganisms, the mechanisms responsible for nutrient acquisition by AM-plants are first analyzed. Then, the processes involved in mycorrhizosphere establishment and functions are described. Finally, the achievements derived from managing selected AM fungi and beneficial bacteria interactions (mycorrhizosphere tailoring) are discussed. The use of 15N and 32P to elucidate the contribution of the mycorrhizosphere components to plant nutrient acquisition is detailed.

Interactions between the plant and its microbial communities in the rhizosphere control microbial polycyclic aromatic hydrocarbons (PAH) biodegradation processes. Arbuscular mycorrhizal (AM) fungi can influence plant survival and PAH degradation in polluted soil. This work was aimed at studying the contribution of the mycorrhizosphere to PAH biodegradation in the presence of ryegrass (Lolium perenne L., cv. Barclay) inoculated with Glomus mosseae (BEG 69) by taking into account the structure and activity of bacterial communities, PAH degrading culturable bacteria as a function of the distance from roots. Ryegrass was grown in compartmentalized systems designed to harvest successive sections of rhizosphere in lateral compartments polluted or not with phenanthrene (PHE). Colonization of roots by G. mosseae (BEG 69) modified the structure and density of bacterial populations in the mycorrhizosphere, compared to the rhizosphere of non-mycorrhizal plants. G. mosseae increased the density of culturable heterotrophic and PAH degrading bacteria beyond the immediate rhizosphere in the presence of PHE, and increased the density of PAH degraders in the absence of the pollutant. Biodegradation was not significantly increased in the mycorrhizosphere, compared to control non-mycorrhizal plants, where PHE biodegradation already reached 92% after 6 weeks. However, dioxygenase transcriptional activity was found to be higher in the immediate mycorrhizosphere in the presence of G. mosseae (BEG 69).

Mycorrhizal fungus colonization of roots may modify plant metal acquisition and tolerance. In the present study, the contribution of the extraradical mycelium of an arbuscular mycorrhizal (AM) fungus, Glomus mosseae (BEG 107), to the uptake of metal cations (Cu, Zn, Cd and Ni) by cucumber (Cucumis sativus) plants was determined. The influence of the amount of P supplied to the hyphae on the acquisition and partitioning of metal cations in the mycorrhizal plants was also investigated. Pots with three compartments were used to separate root and root-free hyphal growing zones. The shoot concentration of Cd and Ni was decreased in mycorrhizal plants compared to non-mycorrhizal plants. In contrast, shoot Zn and Cu concentrations were increased in mycorrhizal plants. High P supply to hyphae resulted in decreased root Cu concentrations and shoot Cd and Ni concentrations in mycorrhizal plants. These results confirm that some elements required for plant growth (P, Zn, Cu) are taken up by mycorrhizal hyphae and are then transported to the plants. Conversely, Cd and Ni were transported in much smaller amounts by hyphae to the plant, so that arbuscular mycorrhizal fungus colonization could partly protect plants from toxic effects of these elements. Selective uptake and transport of plant essential elements over non-essential elements by AM hyphae, increased growth of mycorrhizal plants, and metal accumulation in the root may all contribute to the successful growth of mycorrhizal plants on metal-rich substrates. These effects are stimulated when hyphae can access sufficient P in soil.

The hypothesis tested in this present study was that the ectomycorrhizosphere effect on the bacterial community was not root-growth-dependent. The impacts of ectomycorrhizal infection (Pisolithus albus COI007) and a chemical fertilization to reproduce the fungal effect on root growth were examined on (1) the structure of bacterial community and (2) fluorescent pseudomonad and actinomycete populations in the mycorrhizosphere of Acacia auriculiformis using both culture-independent and culture-dependent methods. A. auriculiformis plants were grown in disinfested soil in pots with or without addition of the ectomycorrhizal fungus or N/P/K fertilization (to reproduce the fungal effect on root growth) for 4 months and then transferred to 20-L pots filled with nondisinfested sandy soil. The fungal and fertilizer applications significantly improved the plant growth after 4-month culture in the disinfested soil. In the nondisinfested cultural substrate, these positive effects on plant growth were maintained. The total soil microbiota was significantly different within the treatments as revealed from DNA analysis [denaturing gradient gel electrophoresis (DGGE)]. The structure of fluorescent pseudomonad populations was also affected by fungal and fertilizer applications. In contrast, no qualitative effect was observed for the actinomycete communities within each treatment, but fungal inoculation significantly decreased the number of actinomycetes compared to the fertilizer application treatment. These results show that the mycorrhizosphere effect is not root-growth-dependent but is mainly due to the presence of the ectomycorrhizal fungus and more particularly to the extramatrical mycelium.

We investigated the impact of arbuscular mycorrhizae fungi on common cattails (Typha latifolia), a ubiquitous wetland plant species. The mycorrhizal relationship, which involves the exchange of fungal-acquired nutrients and plant-produced carbon, has been shown to elicit a range of physiological and biochemical responses in host plants. Growth, photosynthetic activity, biomass accumulation, and nutrition were compared between seedlings inoculated with viable mycorrhizal fungal spores and control seedlings given a sterilized inoculum. Plants were grown in inundated soils at three levels of phosphorous availability under glasshouse conditions for 11 weeks. The presence of arbuscules and hyphae was confirmed in all inoculated plants with levels of infection reaching 23.1 and 34.1%, respectively. Control plants were taller and had greater above- and below-ground biomass. Photosynthetic rates measured at week 11 of the experiment were significantly higher in mycorrhizal plants than in control plants. Mycorrhizal plants had higher concentrations of N, P, and C in their shoots and higher concentrations of N in their roots. Our results suggest that under greenhouse conditions, the fungus acts to reduce host plant growth despite increased mineral nutrition and photosynthetic activity.

Arbuscular mycorrhizal fungi are key components of soil microbiota and obviously interact with other microorganisms in the rhizosphere, i.e. the zone of influence of plant roots on microbial populations and other soil constituents. Mycorrhiza formation changes several aspects of plant physiology and some nutritional and physical properties of the rhizospheric soil. These effects modify the colonization patterns of the root or mycorrhizas (mycorrhizosphere) by soil microorganisms. The rhizosphere of mycorrhizal plants, in practice a mycorrhizosphere, harbors a great array of microbial activities responsible for several key ecosystem processes. This paper summarizes the main conceptual principles and accepted statements on the microbial interactions between mycorrhizal fungi and other members of rhizosphere microbiota and discusses current developments and future trends concerning the following topics: (i) effect of soil microorganisms on mycorrhiza formation; (ii) mycorrhizosphere establishment; (iii) interactions involved in nutrient cycling and plant growth; (iv) interactions involved in the biological control of plant pathogens; and (v) interactions to improve soil quality. The main conclusion is that microbial interactions in the rhizosphere of mycorrhizal plants improve plant fitness and soil quality, critical issues for a sustainable agricultural development and ecosystem functioning.

This review discusses the role arbuscular mycorrhizal fungi play in the regulation of plant nutrient transporter genes. Many plant nutrient transporter genes appear to be transcriptionally regulated by a feed-back mechanism that reduces their expression when the plant reaches an optimal level of nutrition. Their down-regulation in mycorrhizal roots, therefore, would be predicted as a result of symbiotic function. A variety of studies on Pi- Zn- and ammonium- or nitrate-transporter genes from two plant species indirectly support this model. For example, one study showed that the expression of the high-affinity Pi-transporter MtPT2 within mycorrhizal roots of Medicago truncatulawas inversely correlated with the concentration of P within the shoots, which suggested that P supply from the fungus influenced this gene's expression. However, there is some evidence that these plant nutrient transporters are also down-regulated within mycorrhizal roots by a mechanism not involving nutrient supply by the fungus. Carbon demand by the fungus may be one factor that can influence their expression within mycorrhizal roots.

A simple "inserted membrane technique" (IMT) for sampling mycorrhizal extraradical mycelium (ERM) was developed as an alternative to the commonly used membrane filtration technique (MFT). The ERM of two types of mycorrhiza, orchid and arbuscular, was extracted by insertion of cellulose nitrate or cellulose acetate membrane filters (0.45–0.6 µm pore size) into the mycorrhizosphere of host plants. The membranes with adhered ERM were removed at harvest and stained with trypan blue for estimation of total hyphal length and with enzyme stains to indicate the viability of the ERM. There are two apparent advantages of the IMT over the MFT: (1) Samples were cleaner and easier to observe, particularly when the hyphae were stained for enzyme activities and (2) the ERM remained intact and, thus, was also suitable for observation of ERM morphology. There were statistically significant correlations between the lengths of ERM extracted from clinoptinolite using MFT and IMT for both mycorrhizal types tested, orchid mycorrhiza (r=0.63) and arbuscular mycorrhiza (r=0.80). Linear regression analysis indicated the best fit for the data obtained (P

Mycorrhizal symbioses were found in the roots of 45 out of 59 species of pteridophytes collected in Korea. The mycorrhizal fungi were colonized in the root cortical cells, primarily in terrestrial species, but rarely in epiphytic or aquatic pteridophytes. Mycorrhizae that are typically found in orchid colonized the roots of the epiphytic pteridophytes, but not in other species. These were the first observations of orchid mycorrhizae in pteridophytes. Arbuscular mycorrhizal fungi were examined after staining, then confirmed with PCR, using a specific primer. This is the first report of arbuscular mycorrhizal colonization in the roots of pteridophyte species in Asia.

Mycorrhizal effectiveness depends on the compatibility between fungus and plant. Therefore, genetic variation in plant and fungal species affect the effectiveness of the symbiosis. The importance of mycorrhizal plant and mycorrhizal fungus origin was investigated in two experiments. In the first experiment, clones (cuttings) of Salix repens L. from three different origins (two coastal dune and one inland ecosystem) were inoculated with three mycorrhizal fungi (the ectomycorrhizal fungi Hebeloma leucosarx and Paxillus involutus; the arbuscular mycorrhizal fungus Glomus mosseae) on two soils with different nutrient availabilities. In the second experiment, homogeneous plant material (one clone of S. repens) was inoculated with four isolates of H. leucosarx, all found in association with S. repens. Plant origin had a large effect on symbiotic effectiveness, assessed by three different criteria (shoot biomass, shoot N-content, shoot P-content). This effect was probably mediated through genetic differences in root parameters. There was also a large plant origin×soil type interaction, with the plants from the nutrient-poor habitat performing better on the poorest soil. The plants from the more nutrient-rich habitat performed better on the relatively rich soil. Effects of plant origin on symbiotic effectiveness were also fungal-species specific. Fungal origin had only a minor effect on symbiotic effectiveness. Studies concerning the ecological significance of mycorrhizal symbiosis should be more explicit about both mycorrhizal plant and fungus origin and compatibility.

A pot experiment has been carried out under controlled conditions to study the possibility of applying the technique ofin vivo staining for acid phosphatase activity on the roots of mycorrhizal plants and arbuscular mycorrhizal hyphae. The pots had 5 compartments. The central root compartment was separated from the two adjacent hyphal compartments using nylon nets of 30 μm mesh, and the two hyphal compartments were separated from the two outermost compartments with 0.45 μm membranes. Red clover was grown in the root compartment and was either inoculated with the arbuscular mycorrhizal fungus (AMF)Glomus mosseae or unlnoculated. Sodium phytate was applied to all compartments. The results show that AMF can increase acid phosphatase activity of clover roots. The plant roots acquired deep red “mycorrhizal prints”. The external hyphae also had obvious “hyphal prints” on the test papers, indicating the ability of mycorrhizal hyphae to release acid phosphatase.

Polycyclic aromatic hydrocarbons (PAH) can be degraded in the rhizosphere but may also interact with vegetation by accumulation in plant tissues or adsorption on root surface. Previous studies have shown that arbuscular mycorrhizal (AM) fungi contribute to the establishment and maintenance of plants in a PAH contaminated soil. We investigated the fate of PAH in the rhizosphere and mycorrhizosphere including biodegradation, uptake and adsorption. Experiments were conducted with ryegrass inoculated or not with Glomus mosseae P2 (BEG 69) and cultivated in pots filled with soil spiked with 5 g kg−1 of anthracene or with 1 g kg−1 of a mixture of 8 PAH in a growth chamber. PAH were extracted from root surfaces, root and shoot tissue and rhizosphere soil and were analysed by GC-MS. In both experiments, 0.006 – 0.11‰ of the initial extractable PAH concentration were adsorbed to roots, 0.003 – 0.16‰ were found in root tissue, 0.001‰ in shoot tissue and 36 – 66% were dissipated, suggesting that the major part of PAH dissipation in rhizosphere soil was due to biodegradation or biotransformation. With mycorrhizal plants, anthracene and PAH were less adsorbed to roots and shoot tissue concentrations were lower than with non mycorrhizal plants, which could contribute to explain the beneficial effect of AM fungi on plant survival in PAH contaminated soils.

Castanospermum australe A. Cunn. & C. Fraser is the only species of the genus Castanospermum (the Moreton Bay chestnut or black bean) native to NE Australia. One constituent of the plant, castanospermine, can inhibit the AIDS virus. The present study investigated possible symbioses between its roots and arbuscular mycorrhizal (AM) fungi. The effects of mycorrhizal fungi on the growth of the plant and yield of alkaloid castanospermine were also studied. The mycorrhizosphere soil and roots of C. australe collected from various sites in and around Sydney, Australia showed AM symbiotic associations with roots, with arbuscules and vesicles in the root cortices. Wet sieving and decanting yielded AM fungal spores, mainly Glomus spp. A positive correlation was found between AM fungal infection and the castanospermine content of seeds of field-grown trees. Field study results were confirmed by growing seedlings under greenhouse conditions and inoculating them with Glomus intraradices Schenck and Smith (INVAM isolate KS906) and Gigaspora margarita Becker & Hall (INVAM isolate BR444–2). The AM fungi increased the growth and P contents of plants and the yield of castanospermine in the leaves, irrespective of the P treatment. No correlation was found between the alkaloid contents of leaves from mycorrhizal seedlings and from non-mycorrhizal plants which received P. No significant difference in the production of castanospermine was found between P treatments when G. margarita was used as inoculum.

The influence of an arbuscular mycorrhizal (AM) fungus on phosphorus (P) and iron (Fe) uptake of peanut (Arachis hypogea L.) and sorghum (Sorghum bicolor L.) plants was studied in a pot experiment under controlled environmental conditions. The plants were grown for 10 weeks in pots containing sterilised calcareous soil with two levels of Fe supply. The soil was inoculated with rhizosphere microorganisms only or with rhizosphere microorganisms together with an AM fungus (Glomus mosseae [Nicol. & Gerd.] Gerdemann & Trappe). An additional small soil compartment accessible to hyphae but not roots was added to each pot after 6 weeks of plant growth. Radiolabelled P and Fe were supplied to the hyphae compartment 2 weeks after addition of this compartment. After a further 2 weeks, plants were harvested and shoots were analysed for radiolabelled elements. In both plant species, P uptake from the labelled soil increased significantly more in shoots of mycorrhizal plants than non-mycorrhizal plants, thus confirming the well-known activity of the fungus in P uptake. Mycorrhizal inoculation had no significant influence on the concentration of labelled Fe in shoots of peanut plants. In contrast, 59Fe increased in shoots of mycorrhizal sorghum plants. The uptake of Fe from labelled soil by sorghum was particularly high under conditions producing a low Fe nutritional status of the plants. These results are preliminary evidence that hyphae of an arbuscular mycorrhizal fungus can mobilise and/or take up Fe from soil and translocate it to the plant.

Soil aggregation is a dynamic process in which plants and the soil microbiota play a major role. This experiment was conducted to determine whether the effects of mycorrhizae on the stability of water-stable soil aggregates (WSA) and on selected groups of soil microorganisms are interrelated. Soil containers consisting of four compartments were utilized. Two compartments on each side of a solid barrier were separated by a 43 μm screen that permitted the passage of hyphae, but not of roots. The roots of Sorghum bicolor plants were split over the center barrier, and the roots on one side were inoculated with an arbuscular-mycorrhizal (AM) fungus. This design produced mycorrhizosphere soils (M) by AM roots or hyphosphere (H) soils by AM hyphae in the two compartments on the one side of the barrier, and rhizosphere soils (R) by nonAM roots or root- and hypha-free bulk soil (S) in the two compartments on the other side. At harvest (10 wk), there were significant differences in WSA between soils in the order: M>R>H>S, and WSA stability was significantly correlated with root or hyphal length. Numbers of colony-forming units of the microflora (total bacteria, actinomycetes, anaerobes, P solubilizers, and nonAM fungi) were in general not correlated with root or hyphal length, but in some cases were significantly correlated with WSA. Bacteria isolated from the water-stable soil-aggregate fraction tended to be more numerous than from the unstable fraction. The difference was significant in the M soil for total bacteria and P solubilizing bacteria. NonAM fungi were more numerous in the unstable fraction of the M soil. The data show that the root and fungal components of mycorrhizae enhance WSA stability individually and additively in concert, and suggest that they affect microorganism numbers indirectly by providing a favorable and protective habitat through the creation of habitable pore space in the WSA.

Roots and mycorrhizal fungi may not associate with soil bacteria randomly, but rather in a hierarchical structure of mutual preferences. Elucidation of such structures would facilitate the management of the soil biota to enhance the stability of the plant-soil system. We conducted an experiment utilizing two isolates of soil bacteria to determine their persistence in distinct mycorrhizal regions of the root zone, and their effects on general rhizosphere populations of fluorescent pseudomonads (FP). Split-root sorghum (Sorghum bicolor L.) plants were grown in four-compartment containers, constructed so that the soils in individual compartments held either (1) roots colonized by the arbuscular-mycorrhizal (AM) fungus Glomus mosseae (M), (2) nonAM roots only (R), (3) hyphae of G. mosseae (H), or (4) no mycorrhizal structures (S). The soils were inoculated (107 cells g-1 dry soil) with antibiotic-resistant (rifampicin, rif; streptomycin, sm) strains of the soil bacteria, Alcaligenes eutrophus (rifr50) or Arthrobacter globiformis (smr250), or were left uninoculated as control. A. eutrophus had been isolated from a specific source (hyphosphere soil of G. mosseae), and A. globiformis from mycorrhizosphere soils of two AM fungi. After 10 wk of growth, the presence of A. eutrophus was barely detectable ( R>H>S. There was a positive correlation of FP numbers with both bacterial inoculants in M and H soils. Numbers of FP changed with root or hyphal lengths, an effect that was related to changes in the numbers of the inoculated bacteria. The results indicate that the hyphosphere-specific A. eutrophus depended on the presence of G. mosseae, but that the nonspecific A. globiformis did not. The mycorrhizal status of soils may selectively influence persistence of bacterial inoculants as well as affecting the numbers of other native bacteria.

This study investigated the impact of mycorrhizal plants, non-mycorrhizal plants and soil organic matter on the relative abundance of soil hyphae perceived to belong to indigenous arbuscular mycorrhizal (AM) plants. The mycorrhizal plants corn (Zea mays L.) and barley (Hordeum vulgare L.) and a non-mycorrhizal plant, canola (Brassica napus L.), were grown in unsterilized soil in pots inoculated with mycorrhizal corn root fragments. The abundance of hyphae was measured after 5 weeks and the response of fungal growth to the addition of corn residues in the absence of plants was assessed. The abundance of hyphae was higher in the presence of the mycorrhizal plants than in the other treatments. AM hyphae present under mycorrhizal plants accounted for more than 83% of the measured hyphae. The levels of root colonization of 32% in corn and 27% in barley confirmed the mycorrhizal status of the experimental plants. Only a few points of entry were observed in canola, the non-host plant. The percentage of mycorrhizal colonization was positively related (R2 = 0.85) to the abundance of soil hyphae, indicating that AM hyphae were the major component of the soil hyphae in the presence of mycorrhizal plants in this study.

The development of vesicular-arbuscular mycorrhizal fungi (VAMF): Glomus mosseae (Nicol and Gerd.) Gerdemann and Trappe, Glomus versiforme (Karsten) Berch, Sclerocystis sinuosa Gerdemann and Bakhi and Verticillium dahliae and the effects of the VAMF on the verticillium wilt of cotton (Gossypium hirsutum L. and Gossypium barbadense L.) were studied with paper pots, black plastic tubes and clay pots under natural growth conditions. All of the tested VAMF were able to infect all the cotton varieties used in the present experiment and typical vesicles and arbuscules were formed in the cortical cells of the cotton roots after inoculation. The cap cells, meristem, differentiating and elongating zones of the root tip were found to be colonized by the VAMF. In the case of most V. dahliae infection, the colonization occurred mostly from the root tip up to 2 cm. VAMF and V. dahliae mutually reduced their percentage of infection when inoculated simultaneously. VAMF inoculation reduced the numbers of germinable microsclerotia in the soil of the mycorrhizosphere, while the quantity of VAM fungal spores in the soil was not influenced by infection of with V. dahliae. The % of arbuscule colonization in roots was negatively correlated with the disease grades, while the numbers of vesicles in roots were not. These results suggest that certain vital competition and antagonistic reactions exist between VAMF and V. dahliae. VAMF reduced the incidence and disease indices of verticillium wilt of cotton during the whole growth phase. It is evident that cotton seedling growth was promoted, flowering was advanced, the numbers of flowers and bolls were increased, and this resulted in an increase in the yield of seed cotton. Among the VAMF species, Glomus versiforme was the most effective, and Sclerocystis sinuosa was inferior. So far as the author is aware, such an effect of VAMF on the increase of cotton wilt tolerance/resistance is reported here far the first time.

Protozoan communities around roots with different types of ectomycorrhizae were distinct. These protozoan communities differed both qualitatively and quantitatively with the host (Pinus ponderosa, Pseudo- tsuga menziesii, Picea sitchensis, Tsuga heterophylla and Abies grandis) and the ectomycorrhizal fungal species. Based on the species identified and the numbers of in- dividuals of each species, six communities of protozoa were found associated with specific ectomycorrhizae. Previous researchers have shown that mycorrhizal colo- nization of roots alters the amounts and types of exu- dates produced by roots, which in turn alters the bacte- rial community present. Most likely, mycorrhizal colo- nization of roots influences the protozoan community around roots by controlling the bacterial community. However, the protozoan community may in turn in- fluence the successional dynamics of ectomycorrhizal fungi on different host root systems by a variety of mechanisms. These mechanisms could include: (1) preying upon individuals and perhaps removing parti- cular species of bacteria from the mycorrhizosphere; and (2) controlling nitrogen mineralization in the rhi- zosphere. Further work needs to be performed to de- termine the interaction between these quadrate (plant- bacteria-fungi-protozoa) associations.

The influence of vesicular-arbuscular mycorrhizal (VAM) fungi on rhizosphere mycoflora was studied together with the possible mechanism involved in this process. Six combinations of VAM fungi and phosphorus fertilizer treatments were applied to Leucaena leucocephala roots and quantitative and qualitative observations were made periodically of the rhizosphere mycoflora and constituents of root exudates. The results obtained indicate that the presence of specific mycoflora in the rhizosphere of mycrorrhizal roots is mediated through root exudates rather than being an outcome of improved P nutrition.

The role of mycorrhizal fungi in acquisition of mineral nutrients by host plants is examined for three groups of mycorrhizas. These are; the ectomycorrhizas (ECM), the ericoid mycorrhizas (EM), and the vesicular-arbuscular mycorrhizas (VAM). Mycorrhizal infection may affect the mineral nutrition of the host plant directly by enhancing plant growth through nutrient acquisition by the fungus, or indirectly by modifying transpiration rates and the composition of rhizosphere microflora.

Nitrogenase activities, measured by acetylene reduction, were detected under microaerophilic field conditions in Douglas-fir tuberculate ectomycorrhizae. Tuberculate ectomycorrhizae consist of densely packed clusters of ectomycorrhizal rootlets enclosed in a supplementary fungal peridium-like layer. Nitrogenase activity was primarily in the external layer and was greatly enhanced with added sucrose. The bacterium isolated, a nitrogen-fixing, spore-forming Bacillus sp., is an aerobe but requires anaerobic conditions for nitrogenase activity. Respiration in the tuberculate complex by the fungus, roots, and associated mycorrhizosphere microbes probably contributes to maintaining a microaerophilic niche where nitrogen fixation can take place. Water extracts of peridium or mycorrhizal root tips enhanced nitrogenase activity of this associative Bacillus sp., thereby indicating a close nutritional relationship between this bacterium and the tuberculate mycorrhizae. Thiamine more significantly enhanced bacterial nitrogenase activity than biotin; no activity was detected with p-aminobenzoic acid. Even though the levels ofnitrogenase activities in the tubercles in situ were low, as measured by the present methods, they may indicate a significant contribution to the nitrogen dynamics of these nitrogen-limited Douglas-fir forests over a long-term period.