Patterns of endorhizal fungal associations in fruit crops of southern India

A survey on the endorhizal status of 39 fruit crops of 25 families, indicated that 22 fruit crops had arbuscular mycorrhizal (AM)–, four had dark septate endophyte (DSE)–fungal association, and 13 had dual colonization of AM and DSE fungi. Fruit crops were capable of forming Arum‐, Paris‐, or intermediate‐types of AM morphologies of which intermediate‐type was common. To our knowledge, we report for the first time AM in 10 fruit crops and DSE‐fungal association in 17 fruit crops. The extent of AM‐ and DSE‐fungal colonization ranged from 41% to 98% and < 1% to 89.9%, respectively, in different fruit crops. Arbuscular mycorrhizal–fungal spore numbers in the rhizosphere ranged from 6 to 61 spores per 25 g of soil. Arbuscular mycorrhizal–fungal spores belonging to Acaulospora, Glomus, and Scutellospora were isolated from the rhizosphere soil.     

Involvement of autoregulation in the interaction between rhizobial nodulation and AM fungal colonization in soybean roots

Soybean plants autoregulate to suppress excessive nodulation. It has been revealed recently that the autoregulation of various legumes controls both nodulation and arbuscular mycorrhizal (AM) fungal colonization. We investigated the involvement of autoregulation in the interaction between rhizobial nodulation and AM fungal colonization. We used a wild-type soybean cv. Enrei and its hypernodulating mutant Kanto100, defective in the autoregulation. We included four different treatments: an uninoculated control, inoculation with rhizobium Bradyrhizobium japonicum alone, inoculation with AM fungus Gigaspora rosea alone, and dual inoculation with rhizobium and AM fungus. In both Enrei and Kanto100, AM fungal colonization enhanced the weight and N₂ fixation of nodules, suggesting that autoregulation of host plant is not involved in the stimulatory effect of AM fungal colonization on rhizobial nodulation. In plants with the AM fungus alone, the AM fungal colonization of Enrei was comparable to that of Kanto100. In plants with dual inoculation, however, this was significantly (P?<?0.05) lower than in Kanto100. To confirm the control of AM fungal colonization by the autoregulation of host plant, a reciprocal grafting experiment was performed between Enrei and Kanto100. In plants with the AM fungus alone, AM fungal colonization was comparable among Enrei (shoot)/Enrei (root), Enrei/Kanto100, Kanto100/Enrei, and Kanto100/Kanto100 grafts. In plants with dual inoculation, however, AM fungal colonization of Enrei/Enrei and Enrei/Kanto100 grafts was significantly (P?<?0.05) lower than that of Kanto100/Enrei and Kanto100/Kanto100. These results indicate that rhizobial nodulation suppresses AM fungal colonization, and the autoregulation of host plant, initiated by nodulation, is involved in this phenomenon.     

Arbuscular mycorrhizal fungi mitigates heavy metal toxicity adverse effects in sewage water contaminated soil on Tagetes erecta L

The aim of this experiment was to evaluate the impact of colonization with arbuscular mycorrhizal (AM) fungus Glomus constrictum on the biomass production, flower quality, chlorophyll content, macronutrients and heavy metals content of marigold (Tagetes erecta L.) planted under uncontaminated soil and watered with various rates of sewage water. Sewage water utilization significantly decreased biomass production, characters of flower, nutrient concentration and rates of mycorrhizal colonization of mycorrhizal (M) and non-mycorrhizal (NM) marigold as compared to control untreated plants especially at the higher rates, but the reduction rate was proportionally higher in non-AM treatments. Mycorrhizal plants had significantly greater yield, relative chlorophyll content, leaf area, flower quality and element (P, N, K and Mg) content compared to non-inoculated marigold plants irrigated with or without sewage water. Furthermore, AM inoculation had highly decreased heavy metal (Zn, Co, Mn, Cu) content in tissues as compared to equivalent non-inoculated plants grown under sewage water application. Growing marigold with AM inoculum can reduce toxicity of heavy metals and enhance biomass production and P uptake. The results support the view that AM have a protective function for the host plant, hence playing a potential function in soil polluted immobilization processes, and thus are of assessing the potential of phytoremediation of heavy metals in sewage water contaminated soil.     

Effects of indigenous and introduced arbuscular mycorrhizal fungi on the growth of Allium fistulosum under field conditions

ABSTRACT

Enhanced phosphorus (P) uptake from the soil and increased plant growth related to arbuscular mycorrhizal (AM) fungi in pot culture, using sterilized soil, are well-known phenomena. However, these enhancements are not widely observed under field conditions because field sterilization is difficult. The aim of this study was to investigate the effects of AM fungi on P uptake and the growth of Allium fistulosum in non-fumigated and fumigated fields, under different levels of P availability. Plants were inoculated with the AM fungus Glomus R-10 and grown in fumigated soil. For the uninoculated treatment, a sterilized inoculum was applied directly. The field was fumigated using dazomet. Superphosphate was applied to the field at the rates of 0 (P0) or 500 (P500) kg P₂O₅ ha^?1. The inoculated and uninoculated plants were transplanted into the fields and sampled three times to measure AM fungal colonization, shoot P concentration, and shoot dry weight of the plants. At the transplanting stage, AM fungal colonization was observed in the inoculated plants (>70%) but not in the uninoculated plants. At the third sampling, irrespective of P treatment, AM fungal colonization was observed both in the uninoculated and inoculated plants in the non-fumigated field, and there was no difference in shoot P content and shoot dry weight between the inoculated and uninoculated plants. AM fungal colonization in the fumigated field was higher in the inoculated than uninoculated plants, irrespective of P treatment; shoot P content and shoot dry weight were both higher in the inoculated plants than in the uninoculated plants with P0. These results suggest that the responses of A. fistulosum to AM fungal inoculation under the low-P and fumigated conditions are similar to those observed in sterilized pot culture conditions.     

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

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

接种丛枝菌根真菌和巨大芽孢杆菌的红三叶草对难溶磷的利用

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

Effects of Arbuscular Mycorrhizal Colonization on Microbial Community in Rhizosphere Soil and Fusarium Wilt Disease in Tomato

Fusarium wilt is caused by soil-borne pathogen Fusarium oxysporum. Tomato (Lycopersicon esculentum Mill.) is susceptible to Fusarium oxysporum f. sp. lycopersici race 1 and was infected with wilt disease. A pot experiment was conducted to investigate effects of inoculating arbuscular mycorrhizal (AM) fungus (Glomus etunicatium) on the microbial community in the rhizosphere soil and Fusarium wilt in tomato (cv. Oogatafukuju). The results indicated that AM fungal inoculation suppressed the Fusarium number in the rhizosphere soil of tomato and decreased the Fusarium wilt disease index. Compared to the control, AM fungal inoculation increased the actinomycete number but increased bacterial number. Bacterial and fungal numbers were high but actinomycetes number was low when tomato basal stems became discolored brown. Fusarium inoculation significantly suppressed development of AM colonization and decreased polyphenol oxidase (PPO) activity in leaves and roots of tomato. Inoculation with AM fungi and Fusarium maintained high PPO activity in leaves and roots. The AM colonization increased root growth of tomato, whereas Fusarium inoculation had no significant effect on tomato growth. These findings suggest that because AM fungal inoculation changes microbial communities and enhances PPO activity, it should suppress occurrence of Fusarium wilt in tomato.     

Community structure of arbuscular mycorrhizal fungi associated with Robinia pseudoacacia in uncontaminated and heavy metal contaminated soils

The significance of arbuscular mycorrhizal fungi (AMF) in soil remediation has been widely recognized because of their ability to promote plant growth and increase phytoremediation efficiency in heavy metal (HM) polluted soils by improving plant nutrient absorption and by influencing the fate of the metals in the plant and soil. However, the symbiotic functions of AMF in remediation of polluted soils depend on plant–fungus–soil combinations and are greatly influenced by environmental conditions. To better understand the adaptation of plants and the related mycorrhizae to extreme environmental conditions, AMF colonization, spore density and community structure were analyzed in roots or rhizosphere soils of Robinia pseudoacacia. Mycorrhization was compared between uncontaminated soil and heavy metal contaminated soil from a lead–zinc mining region of northwest China. Samples were analyzed by restriction fragment length polymorphism (RFLP) screening with AMF-specific primers (NS31 and AM1), and sequencing of rRNA small subunit (SSU). The phylogenetic analysis revealed 28 AMF group types, including six AMF families: Glomeraceae, Claroideoglomeraceae, Diversisporaceae, Acaulosporaceae, Pacisporaceae, and Gigasporaceae. Of all AMF group types, six (21%) were detected based on spore samples alone, four (14%) based on root samples alone, and five (18%) based on samples from root, soil and spore. Glo9 (Rhizophagus intraradices), Glo17 (Funneliformis mosseae) and Acau3 (Acaulospora sp.) were the three most abundant AMF group types in the current study. Soil Pb and Zn concentrations, pH, organic matter content, and phosphorus levels all showed significant correlations with the AMF species compositions in root and soil samples. Overall, the uncontaminated sites had higher species diversity than sites with heavy metal contamination. The study highlights the effects of different soil chemical parameters on AMF colonization, spore density and community structure in contaminated and uncontaminated sites. The tolerant AMF species isolated and identified from this study have potential for application in phytoremediation of heavy metal contaminated areas.     

Bioavailability of heavy metals and abundance of arbuscular mycorrhiza in a soil polluted by atmospheric deposition from a smelter

The bioavailability of heavy metals (Cd, Zn, Pb, Cu) and the abundance of arbuscular mycorrhiza (AM) were studied in two agricultural fields close to a Pb-Zn smelter and three fields outside the pollution zone all cultivated with maize (Zea mays L.). Metal extractability with ethylenediaminetetraacetic acid (EDTA)-NH₄OAc and Ca(NO₃)₂, plant metal uptake, and mycorrhizal parameters (spore number, root colonization) were assessed at two growth stages (six-leaf and maturity). Despite regular liming, the availability of Cd, Zn, and Pb was markedly higher in the two metal-polluted fields than in the three uncontaminated fields. However, the AM abundance was not correlated with metal availability. Root colonization and spore numbers in the metal polluted fields were relatively high, though at plant maturity the former was significantly lower than in one of the uncontaminated fields. The very low AM abundance in the two other unpolluted fields was related to other factors, particular soil and plant P status and soil pH. AM root colonization did not substantially prevent plant metal accumulation, since the metal concentrations in maize grown on the polluted fields strongly exceeded normal values, and for Cd and Pb reached the limits of toxicity for animal feed.     

Native soil organic matter as a decisive factor to determine the arbuscular mycorrhizal fungal community structure in contaminated soils

The present study of arbuscular mycorrhizal (AM) fungi is focused on the identification of AM ecotypes associated with different plants species (Poa annua, Medicago polymorpha, and Malva sylvestris) growing in three contaminated soils with different organic matter, phosphorus, and trace element (TE; Cu, Cd, Mn, and Zn) contents. Soils were amended with biosolid and alperujo compost. Shifts in AM fungal community structure, diversity, richness, root colonization, and plant TE uptake were evaluated. Soil properties and plant species had a significant effect on AM fungal community composition as well as on root colonization. However, AM fungal diversity and richness were only affected by soil properties and especially by soil organic matter that was a major driver of AM fungal community. As soil quality increased, Glomeraceae decreased in favor of Claroideoglomeraceae in the community, AM fungal diversity and richness increased, and root colonization decreased. No effect due to amendment (exogenous organic matter) addition was found either in AM fungal parameters measured or TE plant uptake. Our results revealed that the role of TE contamination was secondary for the fungal community behavior, being the native organic matter content the most significant factor.     

Effect of excess zinc and arbuscular mycorrhizal fungus on bioproduction and trace element nutrition of Tomato (Solanum lycopersicum L. cv. Micro-Tom)

ABSTRACT

The effect of excess Zn and arbuscular mycorrhizal (AM) fungus on bioproduction and trace element nutrition were investigated in tomato. In a completely randomized factorial design, the experimental treatments – Zn addition at 0 (normal) and 300 (excess) mg Zn kg^?1 soil, and AM inoculation (non-AM and Rhizophagus irregularis) – were set up in a growth chamber for 10 weeks. Generally, AM effects on the available Zn, Mn, Cu, and Fe in the rhizosphere soil were in tandem with the effects in host tissues. Under normal Zn condition, AM enhanced Cu availability in the rhizosphere, optimized the Cu:Zn balance in shoots, and increased the host biomass production. Excess Zn reduced mycorrhizal colonization in AM plants and the total plant biomass in both AM and non-AM plants. Although AM decreased the Zn concentrations in soil and host tissues under excess Zn, the distortions in host TE balance were not significantly ameliorated by the fungus. While Zn in fruit was within the safety threshold, Mn deficiency in the fruit was observed under excess Zn, alongside increased root-to-fruit Fe and Cu translocations. Mycorrhizal reductions in soil and tissue Mn concentrations were considered a minus in terms of probable symbiont amelioration of Mn:Zn in-balance under excess Zn. Additional microbe(s) that can enhance Mn homeostasis might be helpful in tomato under elevated soil Zn.     

Impact of soil nitrogen concentration on Glomus spp.-Sinorhizobium interactions as affecting growth, nitrate reductase activity and protein content of Medicago sativa

Our objective was to evaluate how increasing levels of N in the medium (0, 4, 8 and 16 mmol N added kg^-1 soil) affect the interaction between Sinorhizobium and arbuscular mycorrhiza (AM) fungi in the tripartite symbiosis with Medicago sativa. Growth response, nutrient acquisition, protein content, and nitrate reductase (NR) activity were measured both in plant shoots and roots. Results showed that N levels in soil did not affect mycorrhizal colonization but they strongly influenced nodulation, particularly of mycorrhizal plants. Mycorrhizal colonization was required for a proper nodulation when no N was applied to soil. In contrast, the addition of 4 mmol N kg^-1 soil reduced nodulation only in mycorrhizal plants and 8 mmol N added kg^-1 soil allowed nodule formation only in non-mycorrhizal plants. Nodulation was totally inhibited in all treatments with the addition of 16 mmol N added kg^-1 soil. N addition enhanced NR activity in all the treatments, while AM colonization increased the proportion of NR allocated to roots. This effect was more pronounced under the lowest N levels in the medium. The two AM fungal species showed different distribution pattern of enzymatic activities in plant tissues indicating specific physiological traits. Protein content as well as the relative proportion of protein in roots were greatly increased after mycorrhizal colonization. Glomus intraradices-colonized plants had the highest protein content in shoot and root. Mycorrhizal effects on growth, N acquisition and biochemical variables cannot be interpreted as an indirect P-mediated effect since P content was lower in mycorrhizal plants than in those which were P fertilized. Mycorrhizal colonization increased the N content in plants irrespective of the N level, but the effectiveness of AM fungi on plant N acquisition depended on the AM fungus involved, G. intraradices being the most effective, particularly at the highest N rate. N₂ fixation, enhanced by AM colonization, contributed to N acquisition when a moderate N quantity was available in the soil. Nevertheless, under a high N amount the nodulating process and/or fixing capacity by Sinorhizobium was reduced in AM plants. In contrast, the AM fungal mycelium from a particular mycorrhizal fungus may continue to contribute efficiently to the N uptake from the soil even at high N levels. These results demonstrate the particular sensitivity of AM fungal species in terms of their growth and/or function to increasing N amounts in the medium. A selection of AM fungi used to address specific environmental conditions, such as N fertilization regimes comparable to those used in agronomic practices, is required for a better use of N applied to soil.     

Defoliation effects on Plantago lanceolata resource allocation and soil decomposers in relation to AM symbiosis and fertilization

Plants can mediate interactions between aboveground herbivores and belowground decomposers as both groups depend on plant-provided organic carbon. Most vascular plants also form symbiosis with arbuscular mycorrhizal fungi (AMF), which compete for plant carbon too. Our aim was to reveal how defoliation (trimming of plant leaves twice to 6 cm above the soil surface) and mycorrhizal infection (inoculation of the fungus Glomus claroideum BEG31), in nutrient poor and fertilized conditions, affect plant growth and resource allocation. We also tested how these effects can influence the abundance of microbial-feeding animals and nitrogen availability in the soil. We established a 12-wk microcosm study of Plantago lanceolata plants growing in autoclaved soil, into which we constructed a simplified microfood-web including saprotrophic bacteria and fungi and their nematode feeders. We found that fertilization, defoliation and inoculation of the mycorrhizal fungus all decreased P. lanceolata root growth and that fertilization increased leaf production. Plant inflorescence growth was decreased by defoliation and increased by fertilization and AMF inoculation. These results suggest a negative influence of the treatments on P. lanceolata belowground biomass allocation. Of the soil organisms, AMF root colonization decreased with fertilization and increased with defoliation. Fertilization decreased numbers of bacterial-feeding nematodes, probably because fertilized plants produced less root mass. On the other hand, bacterial feeders were more abundant when associated with defoliated than non-defoliated plants despite defoliated plants having less root mass. The AMF inoculation per se increased the abundance of fungal feeders, but the reduced and increased root AM colonization rates of fertilized and defoliated plants, respectively, were not reflected in the numbers of fungal feeders. We found no evidence of plant-mediated effects of the AM fungus on bacterial feeders, and against our prediction, soil inorganic nitrogen concentrations were not positively associated with the concomitant abundances of microbial-feeding animals. Altogether, our results suggest that (1) while defoliation, fertilization and AMF inoculation all affect plant resource allocation, (2) they do not greatly interact with each other. Moreover, it appears that (3) while changes in plant resource allocation due to fertilization and defoliation can influence numbers of bacterial feeders in the soil, (4) these effects may not significantly alter mineral N concentrations in the soil.     

Interactive effect between Cu‐adapted arbuscular mycorrhizal fungi and biotreated agrowaste residue to improve the nutritional status of Oenothera picensis growing in Cu‐polluted soils

The interactive effect of sugar beet (SB) agrowaste and arbuscular mycorrhizal (AM) fungi inoculation in response to increasing Cu levels was evaluated in the metallophyte Oenothera picensis. Plants were grown in a Cu‐added soil (0, 100, or 500 mg Cu kg^?1), in presence or absence of SB, and inoculated with: (1) indigenous Cu adapted mycorrhiza (IM) isolated from Cu‐polluted soils; (2) Claroideoglomus claroideum (CC); or (3) maintained uninoculated (control). Sugar beet application produced an increase in shoot biomass of 2 to 7 times, improving plant nutritional status and allowing their survival at the highest Cu concentrations. Moreover, AM fungi utilization had a positive effect promoting the plant establishment; nevertheless, Cu plant concentration as well as the mycorrhizal development in terms of AM colonization, AM spore density, and glomalin production were strictly dependent of the AM fungi strains used. Remarkable differences between AM fungi strains were observed at the highest soil Cu level where only plants colonized by IM were able to survive and grow when no SB residue was added. An interactive effect between AM fungi and SB produced a higher plant growth than plants without the amendment application, improving the plant establishment and allowing their survival at highest copper concentrations, suggesting that this combination could be used as a biotechnological tool for the phytoremediation of Cu‐polluted soils.     

Clonal and seasonal shifts in communities of saprotrophic microfungi and soil enzyme activities in the mycorrhizosphere of Salix spp.

The species‐specific microbial root and rhizosphere colonization contributes essentially to the plant nutrient supply. The species number and colonization densities of cultivable saprotrophic microfungi and the activities of nutrient‐releasing soil enzymes (protease, acid and alkaline phosphatase, arylsulfatase) were investigated in the rhizosphere of one low mycorrhizal (Salix viminalis) and one higher mycorrhizal (S. × dasyclados) willow clone at a Eutric Cambisol in N Germany. After soil washing, in total 32 and 28 saprotrophic microfungal species were isolated and identified microscopically from the rhizosphere of S. viminalis and S. × dasyclados, respectively. The fungal species composition changed within the growing season but the species number was always lower under S. × dasyclados than under S. viminalis. Under both willow clones, the fungal colonization density was largest in spring, and the species number was largest in autumn. Acid‐phosphatase activity (p < 0.001) and protease activity (p < 0.003) were significantly affected by the Salix clone, whereas arylsulfatase and alkaline‐phosphatase activities did not show clone‐specific differences. All enzyme activities reached their maxima in the summer sampling. Rhizosphere colonization with Acremonium butyri, Cladosporium herbarum, and Penicillium janthinellum contributed significantly to explain the activities of acid phosphatase. Rhizosphere colonization with Cylindrocarpon destructans, Penicillium spinulosum, Plectosphaerella cucumerina, and Trichoderma polysporum contributed significantly to explain the arylsulfatase activities. Effects of the saprotrophic fungal colonization densities on the protease activities in the rhizosphere were low. Acid‐ and alkaline‐phosphatase and arylsulfatase activities in the rhizosphere soil were stronger affected by the composition of the saprotrophic fungal communities than by the Salix clone itself. In conclusion, the colonization density of some saprotrophic microfungi in the rhizosphere contributed to explain shifts in soil‐enzyme activities of the P and S cycles under different willow clones.     

Influence of arbuscular mycorrhizal fungi and salinity on growth,biomass, and mineral nutrition of<Emphasis Type="Italic"> Acacia auriculiformis</Emphasis>

The effect of salinity on the efficacy of two arbuscular mycorrhizal fungi, Glomus fasciculatum and G. macrocarpum, alone and in combination was investigated on growth, development and nutrition of Acacia auriculiformis. Plants were grown under different salinity levels imposed by 0.3, 0.5 and 1.0 S m^-1 solutions of 1 M NaCl. Both mycorrhizal fungi protected the host plant against the detrimental effect of salinity. The extent of AM response on growth as well as root colonization varied with fungal species, and with the level of salinity. Maximum root colonization and spore production was observed with combined inoculation, which resulted in greater plant growth at all salinity levels. AM fungal inoculated plants showed significantly higher root and shoot weights. Greater nutrient acquisition, changes in root morphology, and electrical conductivity of soil in response to AM colonization was observed, and may be possible mechanisms to protect plants from salt stress.     

Survival of inocula and native AM fungi species associated with shrubs in a degraded Mediterranean ecosystem

Reconstitution of the potential of soil mycorrhizal inoculum is a key step in revegetation programs for semiarid environments. We tested the effectiveness of inoculation with native arbuscular mycorrhizal (AM) fungi or with an allochthonous AM fungus, Glomus claroideum, with respect to the growth of four shrub species, the release of mycorrhizal propagules in soil, within and outside the canopy, and the improvement of soil structural stability. Two years after outplanting, the mixture of native endophytes was more effective than, for Olea europaea subsp. sylvestris, Retama sphaerocarpa and Rhamnus lycioides, or equally as effective as, for Pistacia lentiscus, the non-native AM fungus Glomus claroideum, with respect to increasing shoot biomass and foliar NPK contents. The increases in glomalin concentration and structural stability produced by inoculation treatments in the rhizosphere soil of the all shrub species, except R. lycioides, ranged from about 55 to 173% and 13 to 21%, respectively. The mixture of native AM fungi produced the highest levels of mycorrhizal propagules in soil from the center of the canopy of P. lentiscus and R. lycioides, while plants of O. europaea and R. sphaerocarpa inoculated with G. claroideum had more mycorrhizal propagules than did those inoculated with the mixture of native fungi. The number of mycorrhizal propagules in soil outside the canopy of the four shrub species was 5-35 times higher in inoculation treatments than in soil of the non-inoculated plants.     

Solubilization of insoluble inorganic zinc compounds by ericoid mycorrhizal fungi derived from heavy metal polluted sites

Ericoid mycorrhizal fungi increase the ability of their host plants to colonize soils polluted with toxic metals, although the underlying mechanisms are not clearly understood. Two mycorrhizal strains of Oidiodendron maius isolated from contaminated soil were previously shown to tolerate high concentrations of toxic metals. We investigated further the biological mechanisms that may explain metal tolerance, focussing on the interactions between insoluble metal species and extracellular fungal metabolites. In particular, we demonstrate that fungal strains derived from polluted and unpolluted soils mobilize insoluble inorganic zinc compounds to different extents. Strains from polluted soils showed in fact little ability to solubilize Zn from both ZnO and Zn₃(PO₄)₂, whereas strains from unpolluted soils showed a higher solubilization potential. This different behaviour was confirmed when the solubilization abilities of a wider range of fungal strains (25 isolates) was examined. Induction of organic acids (malate and citrate) by the metal compounds was at least in part responsible for metal solubilization. Our results suggest that ericoid mycorrhizal strains from polluted and unpolluted soils may interact differently with metal compounds. We speculate that this may reflect specific strategies to maintain homeostasis of essential metals under different soil conditions.     

Molecular characterization and glomalin production of arbuscular mycorrhizal fungi colonizing a heavy metal polluted ash disposal island, downtown Venice

In this work the arbuscular mycorrhizal (AM) fungal communities colonizing a polluted ash dump island, downtown Venice, were studied by using a multimodal approach. The island, Sacca San Biagio, was covered with a thick layer of municipal solid waste residues produced by an incinerator operating from 1973, to 1984. Such residues contained high levels of heavy metals (Cu, Pb and Zn). We characterized the AMF communities present in soils on Sacca San Biagio island by using molecular methods. Nine AM fungal sequence types were detected in the roots of three plant species, representative of the dominant flora, by using partial SSU ribosomal RNA genes. The most abundant sequence types corresponded to Glomus intraradices/Glomus fasciculatum, and to Glo18, a sequence detected so far only in planta. Two sequences were new to science. Glomalin-related soil protein (GRSP), extracted from rhizosphere soil of dominant plant species, ranged from 1.6 to 2.3 mg g⁻¹. The occurrence of an active AM fungal community able to live in such harsh environment was evinced by the correlation between mycorrhizal colonization and GRSP content.     

The influence of tillage on the structure of rhizosphere and root-associated arbuscular mycorrhizal fungal communities

Soil environmental factors affect the structure of arbuscular mycorrhizal (AM) fungal communities present in soil. However, it is not understood to which degree management practices such as tillage lead to dissimilarities between intra- and extraradical AM fungal communities. This study aims to assess the influence of two different soil management practices (conventional tillage and no-till) on the diversity of AMF communities, both in rhizosphere soil and inside corn roots. We hypothesized that under no-till, roots are colonized as they grow through the undisturbed fungal mycelia left from the previous crop whereas under conventional tillage they are colonized by those propagules that survived disturbance and can re-establish in their new relocated and mixed environment. We predicted that the degree of similarity of AM fungal communities inside versus outside the roots would be greater under no-till than under tillage. Using terminal restriction fragment length polymorphism (T-RFLP) and denaturing gradient gel electrophoresis we observed a different AM fungal community present in roots under no-till than under conventional tillage. Moreover, the communities present in the rhizosphere soil were different than in the roots of the corn plants. These results suggest that soil management does alter the diversity of AM fungal communities associated with corn roots and that plants influence the structure of the AMF community colonizing their roots. Sequencing results indicated that the majority of AMF species found in this agricultural soil was Glomus spp. However, further work is required to determine the extent to which AM fungal genotypic alterations by soil management influences competitive relationships.