Uranium uptake and translocation by the arbuscular mycorrhizal fungus, Glomus intraradices, under root-organ culture conditions

Arbuscular mycorrhizal fungi promote arsenic accumulation in Pteris vittata L. through arsenic solubilization in rhizosphere soil and arsenic uptake by hyphae

The introduction of arbuscular mycorrhizal fungi (AMF) is considered an effective strategy for improving the arsenic phytoremediation efficiency of Pteris vittata L. (P. vittata). However, how hyphae take up arsenic and translocate it to the root cells of P. vittata in the symbiotic mycorrhizal structure is currently unclear. In this study, the role of hyphae in arsenic enrichment in P. vittata and the mechanism of arsenic species transformation in the rhizosphere were studied via a compartmented cultivation setup. After Claroidoglomus etunicatum (C. etunicatum) colonization, the arsenic content of P. vittata increased by 234%. Hyphae contributed 32% to the accumulation of arsenic in symbionts. C. etunicatum promoted the conversion of iron and aluminum oxides to crystalline states in rhizosphere soil, promoted the desorption of arsenic bound to iron and aluminum oxides, and increased the content of available arsenic in rhizosphere soil by 116%. The transfer of arsenic from arbuscular structures to root cells was confirmed by transmission electron microscopy (TEM)/scanning electron microscopy- energy dispersive X-ray spectroscopy (SEMEDS) analysis. This study demonstrated that C. etunicatum inoculation enhances the phytoremediation efficiency of P. vittata in arsenic-contaminated soils through hyphal uptake, plant growth promotion, and alteration of the rhizosphere environment.

Isolation and identification of arbuscular mycorrhizal fungi from an abandoned uranium mine and their role in soil-to-plant transfer of radionuclides and metals

Arbuscular mycorrhizal fungi were recovered from soil samples from the naturally radioactive soil at the long-abandoned South Terras uranium mine in Cornwall, UK. Species of Rhizophagus, Claroideoglomus, Paraglomus, Septoglomus, and Ambispora were recovered, and pot cultures from all except Ambispora were established. Cultures were identified to species level using morphological observation and rRNA gene sequencing combined with phylogenetic analysis. These cultures were used in pot experiments designed with a compartmentalised system to assess the contribution of fungal hyphae to the accumulation of essential elements, such as copper and zinc, and non-essential elements, such as lead, arsenic, thorium, and uranium into root and shoot tissues of Plantago lanceolata. The results indicated that none of the treatments had any positive or negative impact on shoot and root biomass. However, Rhizophagus irregularis treatments showed higher accumulation of copper and zinc in shoots, while R. irregularis and Septoglomus constrictum enhanced arsenic accumulation in roots. Moreover, R. irregularis increased uranium concentration in roots and shoots of the P. lanceolata plant. This study provides useful insight into fungal-plant interactions that determine metal and radionuclide transfer from soil into the biosphere at contaminated sites such as mine workings.

Increasing flavonoid concentrations in root exudates enhance associations between arbuscular mycorrhizal fungi and an invasive plant

Many invasive plants have enhanced mutualistic arbuscular mycorrhizal (AM) fungal associations, however, mechanisms underlying differences in AM fungal associations between introduced and native populations of invasive plants have not been explored. Here we test the hypothesis that variation in root exudate chemicals in invasive populations affects AM fungal colonization and then impacts plant performance. We examined flavonoids (quercetin and quercitrin) in root exudates of native and introduced populations of the invasive plant Triadica sebifera and tested their effects on AM fungi and plant performance. We found that plants from introduced populations had higher concentrations of quercetin in root exudates, greater AM fungal colonization and higher biomass. Applying root exudates more strongly increased AM fungal colonization of target plants and AM fungal spore germination when exudate donors were from introduced populations. The role of root exudate chemicals was further confirmed by decreased AM fungal colonization when activated charcoal was added into soil. Moreover, addition of quercetin into soil increased AM fungal colonization, indicating quercetin might be a key chemical signal stimulating AM fungal associations. Together these results suggest genetic differences in root exudate flavonoids play an important role in enhancing AM fungal associations and invasive plants' performance, thus considering root exudate chemicals is critical to unveiling mechanisms governing shifting plant-soil microbe interactions during plant invasions.

Microbial interaction with and tolerance of radionuclides: underlying mechanisms and biotechnological applications

Radionuclides (RNs) generated by nuclear and civil industries are released in natural ecosystems and may have a hazardous impact on human health and the environment. RN-polluted environments harbour different microbial species that become highly tolerant of these elements through mechanisms including biosorption, biotransformation, biomineralization and intracellular accumulation. Such microbial-RN interaction processes hold biotechnological potential for the design of bioremediation strategies to deal with several contamination problems. This paper, with its multidisciplinary approach, provides a state-of-the-art review of most research endeavours aimed to elucidate how microbes deal with radionuclides and how they tolerate ionizing radiations. In addition, the most recent findings related to new biotechnological applications of microbes in the bioremediation of radionuclides and in the long-term disposal of nuclear wastes are described and discussed.

Factors that influence the absorption of uranium by indigenous plants on the spoil tip of an abandoned mine in western Spain

The purpose of this work was to study the factors affecting the absorption of U by plants growing on the spoil tip of an abandoned mine in western Spain. The plant species were selected based on how palatable they were to livestock and were sampled for four consecutive years during which, we also recorded rainfall data. The factors related to the plants studied were the leaf size and the percentage and characteristics of the arbuscular mycorrhizae (AM) fungi present in their roots. Our results showed a correlation between the annual rainfall and the U concentration in the plants. The percentage of mycorrhization and AM vesicles is a predominant factor in the uptake of U by plants. Spergularia rubra (L.) J.Presl & C.Presl, which is resistant to mycorrhization, contained higher U concentrations relative to the plants that grew with AM mycorrhization. The absorption curves of the different plants studied indicated that these plants were tolerant to ²³⁸U from 875 Bq kg^-1 (70 mg kg^-1), with a hormesis effect below that concentration. The annual U removal was 0.068%, suggesting that AM are responsible for limiting the incorporation of U into the food chain, favouring its retention in the soil and preventing its dispersion.

The Mycorrhizal Donor Plant (MDP) In Vitro Culture System for the Efficient Colonization of Whole Plants

The mycorrhizal donor plant (MDP) in vitro culture system allows the fast and homogeneous colonization of a wide range of photosynthetically active plants. Here we detailed the setup of the system and its potential applications for basic studies as well as mass production and applied purposes.

Opportunities and challenges in the remediation of metal-contaminated soils by using tobacco (Nicotiana tabacum L.): a critical review

The successful phytoextraction of potentially toxic elements (PTEs) from polluted soils can be achieved by growing non-food and industrial crops. Tobacco (Nicotiana tabacum L.) is one of the main industrial crops and is widely grown in many countries. Tobacco can uptake high concentrations of PTEs especially in aboveground biomass without suffering from toxicity. This review highlighted the potential of tobacco for the phytoextraction of heavy metals and tolerance mechanisms under metal stress. Different management practices have been discussed which can enhance the potential of this plant for metal extraction. Finally, suitable options for the management/disposal of biomass enriched in excess metal have been elaborated to prevent secondary pollution.

Multiple environmental factors influence 238U, 232Th and 226Ra bioaccumulation in arbuscular mycorrhizal-associated plants

Ecological consequences of low-dose radioactivity from natural sources or radioactive waste are important to understand but knowledge gaps still remain. In particular, the soil transfer and bioaccumulation of radionuclides into plant roots is poorly studied. Furthermore, better knowledge of arbuscular mycorrhizal (AM) fungi association may help understand the complexities of radionuclide bioaccumulation within the rhizosphere. Plant bioaccumulation of uranium, thorium and radium was demonstrated at two field sites, where plant tissue concentrations reached up to 46.93 μg g^{-1 238}U, 0.67 μg g^{-1 232}Th and 18.27 kBq kg^{-1 226}Ra. High root retention of uranium was consistent in all plant species studied. In contrast, most plants showed greater bioaccumulation of thorium and radium into above-ground tissues. The influence of specific soil parameters on root radionuclide bioaccumulation was examined. Total organic carbon significantly explained the variation in root uranium concentration, while other soil factors including copper concentration, magnesium concentration and pH significantly correlated with root concentrations of uranium, radium and thorium, respectively. All four orders of Glomeromycota were associated with root samples from both sites and all plant species studied showed varying association with AM fungi, ranging from zero to >60% root colonisation by fungal arbuscules. Previous laboratory studies using single plant-fungal species association had found a positive role of AM fungi in root uranium transfer, but no significant correlation between the amount of fungal infection and root uranium content in the field samples was found here. However, there was a significant negative correlation between AM fungal infection and radium accumulation. This study is the first to examine the role of AM fungi in radionuclide soil-plant transfer at a community level within the natural environment. We conclude that biotic factors alongside various abiotic factors influence the soil-plant transfer of radionuclides and future mechanistic studies are needed to explain these interactions in more detail.

A Model of Uranium Uptake by Plant Roots Allowing for Root-Induced Changes in the soil

We develop a model with which to study the poorly understood mechanisms of uranium (U) uptake by plants. The model is based on equations for transport and reaction of U and acids and bases in the rhizosphere around cylindrical plant roots. It allows for the speciation of U with hydroxyl, carbonate, and organic ligands in the soil solution; the nature and kinetics of sorption reactions with the soil solid; and the effects of root-induced changes in rhizosphere pH. A sensitivity analysis showed the importance of soil sorption and speciation parameters as influenced by pH and CO₂ pressure; and of root geometry and root-induced acid-base changes linked to the form of nitrogen taken up by the root. The root absorbing coefficient for U, relating influx to the concentration of U species in solution at the root surface, was also important. Simplified empirical models of U uptake by different plant species and soil types need to account for these effects.

Chromium immobilization by extra- and intraradical fungal structures of arbuscular mycorrhizal symbioses

Arbuscular mycorrhizal (AM) fungi can enhance plant Cr tolerance through immobilizing Cr in mycorrhizal roots. However, the detailed processes and mechanisms are unclear. The present study focused on cellular distribution and speciation of Cr in both extraradical mycelium (ERM) and mycorrhizal roots exposed to Cr(VI) by using field emission scanning electron microscopy equipped with energy dispersive X-ray spectrometer (FE-SEM-EDS), scanning transmission soft X-ray microscopy (STXM) and X-ray absorption fine structure (XAFS) spectroscopy techniques. We found that amounts of particles (possibly extracellular polymeric substances, EPS) were produced on the AM fungal surface upon Cr(VI) stress, which contributed greatly to Cr(VI) reduction and immobilization. With EDS of the surface of AM fungi exposed to various Cr(VI) levels, a positive correlation between Cr and P was revealed, suggesting that phosphate groups might act as counter ions of Cr(III), which was also confirmed by the XAFS analysis. Besides, STXM and XAFS analyses showed that Cr(VI) was reduced to Cr(III) in AM fungal structures (arbuscules, intraradical mycelium, etc.) and cell walls in mycorrhizal roots, and complexed possibly with carboxyl groups or histidine analogues. The present work provided evidence of Cr immobilization on fungal surface and in fungal structures in mycorrhizal roots at a cellular level, and thus unraveled the underlying mechanisms by which AM symbiosis immobilize Cr.

Rhizophagus irregularis MUCL 41833 can colonize and improve P uptake of Plantago lanceolata after exposure to ionizing gamma radiation in root organ culture

Long-lived radionuclides such as (90)Sr and (137)Cs can be naturally or accidentally deposited in the upper soil layers where they emit β/γ radiation. Previous studies have shown that arbuscular mycorrhizal fungi (AMF) can accumulate and transfer radionuclides from soil to plant, but there have been no studies on the direct impact of ionizing radiation on AMF. In this study, root organ cultures of the AMF Rhizophagus irregularis MUCL 41833 were exposed to 15.37, 30.35, and 113.03 Gy gamma radiation from a (137)Cs source. Exposed spores were subsequently inoculated to Plantago lanceolata seedlings in pots, and root colonization and P uptake evaluated. P. lanceolata seedlings inoculated with non-irradiated AMF spores or with spores irradiated with up to 30.35 Gy gamma radiation had similar levels of root colonization. Spores irradiated with 113.03 Gy gamma radiation failed to colonize P. lanceolata roots. P content of plants inoculated with non-irradiated spores or of plants inoculated with spores irradiated with up to 30.35 Gy gamma radiation was higher than in non-mycorrhizal plants or plants inoculated with spores irradiated with 113.03 Gy gamma radiation. These results demonstrate that spores of R. irregularis MUCL 41833 are tolerant to chronic ionizing radiation at high doses.

Transformation and Immobilization of Chromium by Arbuscular Mycorrhizal Fungi as Revealed by SEM-EDS, TEM-EDS, and XAFS

Arbuscular mycorrhizal fungi (AMF), ubiquitous soil fungi that form symbiotic relationships with the majority of terrestrial plants, are known to play an important role in plant tolerance to chromium (Cr) contamination. However, the underlying mechanisms, especially the direct influences of AMF on the translocation and transformation of Cr in the soil-plant continuum, are still unresolved. In a two-compartment root-organ cultivation system, the extraradical mycelium (ERM) of mycorrhizal roots was treated with 0.05 mmol L(-1) Cr(VI) for 12 days to investigate the uptake, translocation, and transformation of Cr(VI) by AMF using inductively coupled plasma mass spectrometry (ICP-MS), scanning electron microscopy equipped with energy-dispersive spectroscopy (SEM-EDS), transmission electron microscopy equipped with energy-dispersive spectroscopy (TEM-EDS), and X-ray-absorption fine structure (XAFS) technologies. The results indicated that AMF can immobilize quantities of Cr via reduction of Cr(VI) to Cr(III), forming Cr(III)-phosphate analogues, likely on the fungal surface. Besides this, we also confirmed that the extraradical mycelium (ERM) can actively take up Cr [either in the form of Cr(VI) or Cr(III)] and transport Cr [potentially in the form of Cr(III)-histidine analogues] to mycorrhizal roots but immobilize most of the Cr(III) in the fungal structures. Based on an X-ray absorption near-edge spectroscopy analysis of Cr(VI)-treated roots, we proposed that the intraradical fungal structures can also immobilize Cr within mycorrhizal roots. Our findings confirmed the immobilization of Cr by AMF, which plays an essential role in the Cr(VI) tolerance of AM symbioses.

Radioactivity and the environment: technical approaches to understand the role of arbuscular mycorrhizal plants in radionuclide bioaccumulation

Phytoaccumulation of radionuclides is of significant interest with regards to monitoring radionuclide build-up in food chains, developing methods for environmental bioremediation and for ecological management. There are many gaps in our understanding of the characteristics and mechanisms of plant radionuclide accumulation, including the importance of symbiotically-associated arbuscular mycorrhizal (AM) fungi. We first briefly review the evidence that demonstrates the ability of AM fungi to enhance the translocation of (238)U into plant root tissues, and how fungal association may prevent further mobilization into shoot tissues. We then focus on approaches that should further advance our knowledge of AM fungi-plant radionuclide accumulation. Current research has mostly used artificial cultivation methods and we consider how more ecologically-relevant analysis might be performed. The use of synchrotron-based X-ray fluorescence imaging and absorption spectroscopy techniques to understand the mechanisms of radionuclide transfer from soil to plant via AM fungi is evaluated. Without such further knowledge, the behavior and mobilization of radionuclides cannot be accurately modeled and the potential risks cannot be accurately predicted.

On the possible role of macrofungi in the biogeochemical fate of uranium in polluted forest soils

Interactions of macrofungi with U, Th, Pb and Ag were investigated in the former ore mining district of Příbram, Czech Republic. Samples of saprotrophic (34 samples, 24 species) and ectomycorrhizal (38 samples, 26 species) macrofungi were collected from a U-polluted Norway spruce plantation and tailings and analyzed for metal content. In contrast to Ag, which was highly accumulated in fruit-bodies, concentrations of U generally did not exceed 3mg/kg which indicates a very low uptake rate and efficient exclusion of U from macrofungi. In ectomycorrhizal tips (mostly determined to species level by DNA sequencing), U contents were practically identical with those of the non-mycorrhizal fine spruce roots. These findings suggest a very limited role of macrofungi in uptake and biotransformation of U in polluted forest soils. Furthermore, accumulation of U, Th, Pb and Ag in macrofungal fruit-bodies apparently does not depend on total content and chemical fractionation of these metals in soils (tested by the BCR sequential extraction in this study).

Cementation of Bioproducts Generated from Biodegradation of Radioactive Cellulosic-Based Waste Simulates by Mushroom

The current work was devoted to study the solidification of bioproducts originated from the bioremediation of mixture of solid cellulose-based radioactive waste simulates using a mushroom (Pleurotus pulmonarius), in Portland cement. The obtained solidified waste form was subjected to mechanical integrity qualification after curing periods of 28 and 90 days. Chemical performance of the cement-waste form was also evaluated in different leachant media during 540 days. The results obtained gave useful information about the mechanical, physical, and chemical performances of the final cement-waste form incorporated the radioactive bioproducts. Moreover, it indicated that cement can provide a highly durable form that ensures a long-term stability of the solidified waste material and can act as a first barrier against the release of radiocontaminants from radioactive wastes to the surrounding environment.

Extraradical mycelium network of arbuscular mycorrhizal fungi allows fast colonization of seedlings under in vitro conditions

Actively growing extraradical hyphae extending from mycorrhizal plants are an important source of inoculum in soils which has seldom been considered in vitro to inoculate young plantlets. Seedlings of Medicago truncatula were grown in vitro in the extraradical mycelium network extending from mycorrhizal plants. After 3, 6, 9, 12, and 15 days of contact with the mycelium, half of the seedlings were harvested and analyzed for root colonization. The other half was carefully transplanted in vitro on a suitable growth medium and mycelium growth and spore production were evaluated for 4 weeks. Seedlings were readily colonized after 3 days of contact with the mycelium. Starting from 6 days of contact, the newly colonized seedlings were able to reproduce the fungal life cycle, with the production of thousands of spores within 4 weeks. The fast mycorrhization process developed here opens the door to a broad range of in vitro studies for which either homogenous highly colonized seedlings or mass-produced in vitro inoculum is necessary.

Arbuscular mycorrhizas contribute to phytostabilization of uranium in uranium mining tailings

Uranium (U) tailings pose environmental risks and call for proper remediation. In this paper medic and ryegrass plants were used as host plants to examine whether inoculation with an AM fungus, Glomus intraradices, would help phytostabilization of U tailings. The need of amending with uncontaminated soil for supporting plant survival was also examined by mixing soil with U tailing at different mixing ratios. Soil amendment increased plant growth and P uptake. Ryegrass produced a more extensive root system and a greater biomass than medic plants at all mixing ratios. Medic roots were extensively colonized by G. intraradices whereas ryegrass were more sparsely colonized. Plant growth was not improved by mycorrhizas, which, however, improved P nutrition of medic plants. Medic plants contained higher U concentrations and showed higher specific U uptake efficiency compared to ryegrass. In the presence of U tailing, most U had been retained in plant roots, and this distribution pattern was further enhanced by mycorrhizal colonization. The results suggest a role for AM fungi in phytostabilization of U tailings.

Arbuscular mycorrhiza reduces phytoextraction of uranium, thorium and other elements from phosphate rock

Uptake of metals from uranium-rich phosphate rock was studied in Medicago truncatula plants grown in symbiosis with the arbuscular mycorrhizal fungus Glomus intraradices or in the absence of mycorrhizas. Shoot concentrations of uranium and thorium were lower in mycorrhizal than in non-mycorrhizal plants and root-to-shoot ratio of most metals was increased by mycorrhizas. This protective role of mycorrhizas was observed even at very high supplies of phosphate rock. In contrast, phosphorus uptake was similar at all levels of phosphate rock, suggesting that the P was unavailable to the plant-fungus uptake systems. The results support the role of arbuscular mycorrhiza as being an important component in phytostabilization of uranium. This is the first study to report on mycorrhizal effect and the uptake and root-to-shoot transfer of thorium from phosphate rock.

Impact of arbuscular mycorrhizal fungi on uranium accumulation by plants

Contamination by uranium (U) occurs principally at U mining and processing sites. Uranium can have tremendous environmental consequences, as it is highly toxic to a broad range of organisms and can be dispersed in both terrestrial and aquatic environments. Remediation strategies of U-contaminated soils have included physical and chemical procedures, which may be beneficial, but are costly and can lead to further environmental damage. Phytoremediation has been proposed as a promising alternative, which relies on the capacity of plants and their associated microorganisms to stabilize or extract contaminants from soils. In this paper, we review the role of a group of plant symbiotic fungi, i.e. arbuscular mycorrhizal fungi, which constitute an essential link between the soil and the roots. These fungi participate in U immobilization in soils and within plant roots and they can reduce root-to-shoot translocation of U. However, there is a need to evaluate these observations in terms of their importance for phytostabilization strategies.

Fungal transformations of uranium oxides

The biogeochemical activities of free-living and symbiotic fungi must be acknowledged in attempts to understand uranium cycling and dispersal in the environment. Although the near-surface geochemistry of uranium is very complex and a wide variety of mineral phases is known, uranium trioxide (UO3) and triuranium octaoxide (U(3)O(8)) can be used as well characterized models in the study of biotransformations. We have used a complex methodological approach involving advanced solid state speciation and scanning electron microscopy to study the ability of saprotrophic, ericoid and ectomycorrhizal fungi to transform these model oxides. This study has revealed that fungi exhibit a high uranium oxide tolerance, and possess the ability to solubilize UO3 and U(3)O(8) and to accumulate uranium within the mycelium to over 80 mg (g dry weight)(-1) biomass. X-ray absorption spectroscopy of uranium speciation within the biomass showed that in most of the fungi the uranyl ion was coordinated to phosphate ligands, but in ectomycorrhizal fungi mixed phosphate/carboxylate coordination was observed. Abundant uranium precipitates associated with phosphorus were found in the mycelium and encrusted the hyphae. Some of the fungi caused the biomineralization of well-crystallized uranyl phosphate minerals of the meta-autunite group. This is the first experimental evidence for fungal transformations of uranium solids and the production of secondary mycogenic uranium minerals.

Can we predict uranium bioavailability based on soil parameters? Part 2: soil solution uranium concentration is not a good bioavailability index

The present study aimed to quantify the influence of soil parameters on uranium uptake by ryegrass. Ryegrass was established on eighteen distinct soils, spiked with (238)U. Uranium soil-to-plant transfer factors (TF) ranged from 0.0003 to 0.0340kgkg(-1). There was no significant relation between the U soil-to-plant transfer (or total U uptake or flux) and the uranium concentration in the soil solution or any other soil factor measured, nor with the U recovered following selective soil extractions. Multiple linear regression analysis resulted in a significant though complex model explaining up to 99% of variation in TF. The influence of uranium speciation on uranium uptake observed was featured: UO(2)(+2), uranyl carbonate complexes and UO(2)PO(4)(-) seem the U species being preferentially taken up by the roots and transferred to the shoots. Improved correlations were obtained when relating the uranium TF with the summed soil solution concentrations of mentioned uranium species.

Effects of AM colonization on "wild tobacco" plants grown in zinc-contaminated soil

This greenhouse study aimed to determine the effect of colonization by the arbuscular mycorrhizal (AM) fungus (Glomus intraradices Schenck & Smith) on the "wild" tobacco (Nicotiana rustica L. var. Azteca), under soil-zinc (Zn) conditions. Plants of N. rustica were grown in AM or non-AM inoculated substrate and subjected to four soil-[Zn] concentrations (0, 50, 100, and 250 mg Zn kg(-1) dry soil). The AM root colonization increased markedly from 14 to 81% with the increasing soil-[Zn] and the mycorrhizal structures were significantly more abundant at the highest soil-[Zn], suggesting that Zn may be involved directly or indirectly in AM root colonization. In addition, total Zn content or Zn concentrations in shoots and roots were shown to increase as soil-[Zn] increased in both AM and non-AM plants. As for the growth parameters studied, there were no significant differences between treatments despite the increase in Zn content or concentration. The AM roots subjected to the highest soil-[Zn] had a significant reduction by about 50% of total Zn content and Zn concentration compared to non-AM roots. Still, the relative extracted Zn percentage decreased dramatically as soil-[Zn] increased. Soil pH was significantly lower in non-AM than AM treatments at the highest soil-[Zn]. In summary, AM plants (particularly roots) showed lower Zn content and concentration than non-AM plants. In this regard, the AM fungi have a protective role for the host plant, thus playing an important role in soil-contaminant immobilization processes; and, therefore, are of value in phytoremediation, especially when heavy metals approach toxic levels in the soil.

Effects of arbuscular mycorrhizal inoculation on uranium and arsenic accumulation by Chinese brake fern (Pteris vittata L.) from a uranium mining-impacted soil

A glasshouse experiment was conducted to investigate U and As accumulation by Chinese brake fern, Pteris vittata L., in association with different arbuscular mycorrhizal fungi (AMF) from a U and As contaminated soil. The soil used contains 111 mg U kg(-1) and 106 mg As kg(-1). P. vittata L. was inoculated with each of three AMF, Glomus mosseae, Glomus caledonium and Glomus intraradices. Two harvests were made during plant growth (two and three months after transplanting). Mycorrhizal colonization depressed plant growth particularly at the early stages. TF (transfer factor) values for As from soil to fronds were higher than 1.0, while those for roots were much lower. Despite the growth depressions, AM colonization had no effect on tissue As concentrations. Conversely, TF values for U were much higher for roots than for fronds, indicating that only very small fraction of U was translocated to fronds (less than 2%), regardless of mycorrhizal colonization. Mycorrhizal colonization significantly increased root U concentrations at both harvests. Root colonization with G. mosseae or G. intraradices led to an increase in TF values for U from 7 (non-inoculation control) to 14 at the first harvest. The highest U concentration of 1574 mg kg(-1) was recorded in roots colonized by G. mosseae at the second harvest. The results suggested that P. vittata in combination with appropriate AMF would play very important roles in bioremediation of contaminated environments characterized by a multi-pollution.

Response to cadmium of Daucus carota hairy roots dual cultures with Glomus intraradices or Gigaspora margarita

Ri T-DNA-transformed carrot roots were cultivated in two experiments either non-inoculated or inoculated with the arbuscular mycorrhizal (AM) fungi Glomus intraradices or Gigaspora margarita. The influence of two concentrations of cadmium (Cd) in the medium (2 mg l(-1), 4 mg l(-1)) on both root and mycelium growth was tested. Both parameters were estimated at 10-day intervals for 70 or 100 days for G. intraradices and Gi. margarita, respectively. In the first experiment, G. intraradices showed a rapid spread of extraradical mycelium (ERM) and reached average densities per treatment of about 90 cm cm(-2) agar medium after 70 days. At the higher Cd level, the growth of ERM was delayed in comparison to the treatment without Cd addition. Root growth was inhibited by both Cd levels; the inhibition was, however, significantly lower in the treatments inoculated with G. intraradices compared to the non-inoculated control. In the second experiment, the ERM of Gi. margarita started to grow after a period of 50 days and reached average densities per treatment of only up to 27 cm cm(-2) by the end of the cultivation. The growth of Gi. margarita mycelium was not inhibited by Cd. No differences in root growth were observed between the Gi. margarita inoculated and non-inoculated treatments. The inhibitory effect of Cd on root growth differed between the non-inoculated treatments in both experiments. The study has shown that the AM fungus Glomus intraradices can alleviate Cd-induced growth inhibition to carrot hairy roots. The potential and limits of the monoxenic system in studying the interaction between AM fungi and heavy metals are discussed.

Effects of arbuscular mycorrhizal fungi on the root uptake and translocation of radiocaesium

Because mycorrhizal fungi are intimately associated with plant roots, their importance in radionuclide (RN) recycling and subsequent dispersion into the biosphere has received an increasing interest. Recently, the capacity of arbuscular mycorrhizal fungi to take up and translocate radiocaesium to their host was demonstrated. However, the relative contribution of these processes in comparison to the ones of roots remains unknown. Here, the respective contributions of the hyphae of a Glomus species and the transformed carrot (Daucus carota L.) roots on radiocaesium uptake and translocation were compared and quantified. We observed that radiocaesium uptake by hyphae was significantly lower as compared to that of the roots, while the opposite was noted for radiocaesium translocation/uptake ratio. We also observed that the intraradical fungal structures might induce a local accumulation of radiocaesium and concurrently reduce its translocation within mycorrhizal roots. We believe that intraradical fungal structures might induce the down-regulation of radiocaesium channels involved in the transport processes of radiocaesium towards the xylem.

Phosphate uptake, transport and transfer by the arbuscular mycorrhizal fungus Glomus intraradices is stimulated by increased carbohydrate availability

* The influence of carbohydrate availability to mycorrhizal roots on uptake, metabolism and translocation of phosphate (P) by the fungus was examined in axenic cultures of transformed carrot (Daucus carota) roots in symbiosis with Glomus intraradices. * 14C-labelled carbohydrates, 33P-phosphate and energy dispersive X-ray microanalysis were used to follow the uptake and transfer of C and P in the arbuscular mycorrhizal (AM) symbiosis. * The uptake of P by the extraradical mycelium (ERM) and its translocation to the mycorrhizal roots was stimulated and the metabolic and spatial distribution of P within the fungus were altered in response to increased carbohydrate availability. Sucrose supply resulted in a decrease of polyphosphates and an increased incorporation into phospholipids and other growth-related P pools and also caused elevated cytoplasmic P levels in the intraradical mycelium (IRM) within the root and higher cytoplasmic P levels in the root cortex. * These findings indicate that the uptake of P by the fungus and its transfer to the host is also stimulated by the transfer of carbon from plant to fungus across the mycorrhizal interface.

Mycorrhiza and root hairs in barley enhance acquisition of phosphorus and uranium from phosphate rock but mycorrhiza decreases root to shoot uranium transfer

Some phosphate rocks (PR) contain high concentrations of uranium (U), which are potentially toxic via accumulation in soils and food chains, and plant uptake of U is likely to be influenced by characteristics of roots and associated microorganisms. The relative importance of root hairs and mycorrhiza in U uptake from PR was studied using a root hairless barley (Hordeum vulgare) mutant (Brb) and its wild type (WT). Both plant genotypes were grown in pots with Glomus intraradices BEG 87, or in the absence of mycorrhiza, and three P treatments were included: nil P, 2% (w/w) PR and 50 mg KH(2)PO(4)-P kg(-1) soil. Mycorrhiza markedly increased d. wts and P contents of Brb amended with nil P or PR, but generally depressed d. wts of WT plants, irrespective of P amendments. Mycorrhiza had contrasting effects on U contents in roots and shoots, in particular in Brb where mycorrhiza increased root U concentrations but decreased U translocation from roots to shoots. The experiment supports our understanding of arbuscular mycorrhiza as being multifunctional by not only improving the utilization of PR by the host plant but also by contributing to the phytostabilization of uranium.

No significant contribution of arbuscular mycorrhizal fungi to transfer of radiocesium from soil to plants

The diffuse pollution by fission and activation products following nuclear accidents and weapons testing is of major public concern. Among the nuclides that pose a serious risk if they enter the human food chain are the cesium isotopes 137Cs and 134Cs (with half-lives of 30 and 2 years, respectively). The biogeochemical cycling of these isotopes in forest ecosystems is strongly affected by their preferential absorption in a range of ectomycorrhiza-forming basidiomycetes. An even more widely distributed group of symbiotic fungi are the arbuscular mycorrhizal fungi, which colonize most herbaceous plants, including many agricultural crops. These fungi are known to be more efficient than ectomycorrhizas in transporting mineral elements from soil to plants. Their role in the biogeochemical cycling of Cs is poorly known, in spite of the consequences that fungal Cs transport may have for transfer of Cs into the human food chain. This report presents the first data on transport of Cs by these fungi by use of radiotracers and compartmented growth systems where uptake by roots and mycorrhizal hyphae is distinguished. Independent experiments in three laboratories that used different combinations of fungi and host plants all demonstrated that these fungi do not contribute significantly to plant uptake of Cs. The implications of these findings for the bioavailability of radiocesium in different terrestrial ecosystems are discussed.

Arbuscular mycorrhizal fungi can decrease the uptake of uranium by subterranean clover grown at high levels of uranium in soil

Subterranean clover inoculated or not with the arbuscular mycorrhizal (AM) fungus Glomus intraradices was grown on soil containing six levels of 238U in the range 0-87 mg kg(-1). Increasing U concentration in soil enhanced the U concentration in roots and shoots of both mycorrhizal and nonmycorrhizal plants but had no significant effects on plant dry matter production or root AM colonization. Mycorrhizas increased the shoot dry matter and P concentration in roots and shoots, while in most cases, it decreased the Ca, Mg and K concentrations in plants. The AM fungus influenced U concentration in plants only in the treatment receiving 87 mg U kg(-1) soil. In this case, U concentration in shoots of nonmycorrhizal plants was 1.7 times that of shoots of mycorrhizal plants. These results suggested that mycorrhizal fungi can limit U accumulation by plants exposed to high levels of U in soil.

Comparison of 233U and 33P uptake and translocation by the arbuscular mycorrhizal fungus Glomus intraradices in root organ culture conditions

This study aimed to quantify and compare 233U and 33P uptake and translocation by hyphae of the arbuscular mycorrhizal (AM) fungus Glomus intraradices in root organ culture conditions with transformed carrot (Daucus carota L.) roots as host. Mycorrhizal roots were grown in two-compartment Petri dishes to spatially separate a root compartment (RC) and a hyphal compartment (HC). The HC was labelled with 8.33 Bq 233U ml(-1) and 13.33 Bq 33P ml(-1). After 2 weeks contact between hyphae and the labelled solution, 233U and 33P activities were measured in the RC and in the HC. 233U and 33P were taken up by the extraradical AM mycelium grown in the HC and this uptake represented 4.4% and 16% of the initial isotope supply, respectively. The translocation into roots developing in the RC via hyphae accounted for 5.9% and 72% of the initial isotope supply, respectively. Thus, both uptake and translocation were much higher for 33P than for 233U. This suggests (1) the existence in hyphal tissues of efficient mechanisms limiting the uptake and translocation of non-essential elements such as U, and (2) that the hyphae have a higher sequestration than translocation function for U, and the converse for P.

Uranium mill tailings: nuclear waste and natural laboratory for geochemical and radioecological investigations

Uranium mill tailings (UMT) are a high volume, low specific activity radioactive waste typically disposed in surface impoundments. This review focuses on research on UMT and related earth materials during the past decade relevant to the assessment of: (1) mineral hosts of radionuclides; (2) the use of soil analogs in predicting long-term fate of radionuclides; (3) microbial and diagenetic processes that may alter radionuclide mobility in the surficial environment; (4) waste-management technologies to limit radionuclide migration; and (5) the impact of UMT on biota.

Extraradical mycelium of the arbuscular mycorrhizal fungus Glomus lamellosum can take up, accumulate and translocate radiocaesium under root-organ culture conditions

Radiocaesium enters the food chain when plants absorb it from soil, in a process that is strongly dependent on soil properties and plant and microbial species. Among the microbial species, arbuscular mycorrhizal (AM) fungi are obligate symbionts that colonize the root cortex of many plants and develop an extraradical mycelial (ERM) network that ramifies in the soil. Despite the well-known involvement of this ERM network in mineral nutrition and uptake of some heavy metals, only limited data are available on its role in radiocaesium transport in plants. We used root-organ culture to demonstrate that the ERM of the AM fungus Glomus lamellosum can take up, possibly accumulate and unambiguously translocate radiocaesium from a 137Cs-labelled synthetic root-free compartment to a root compartment and within the roots. The accumulation of 137Cs by hyphae in the root-free compartment may be explained by sequestration in the hyphae or by a bottleneck effect resulting from a limited number of hyphae crossing the partition between the two compartments. Uptake and translocation resulted from the incorporation of 137Cs into the fungal hyphae, as no 137Cs was detected in mycorrhizal roots treated with formaldehyde. The importance of the translocation process was indicated by the correlation between 137Cs measured in the roots and the total hyphal length connecting the roots with the labelled compartment. 137Cs may be translocated via a tubular vacuolar system or by cytoplasmic streaming per se.