Anastomosis Formation and Nuclear and Protoplasmic Exchange in Arbuscular Mycorrhizal Fungi

We observed anastomosis between hyphae originating from the same spore and from different spores of the same isolate of the arbuscular mycorrhizal fungi Glomus mosseae, Glomus caledonium, and Glomus intraradices. The percentage of contacts leading to anastomosis ranged from 35 to 69% in hyphae from the same germling and from 34 to 90% in hyphae from different germlings. The number of anastomoses ranged from 0.6 to 1.3 per cm (length) of hyphae in mycelia originating from the same spore. No anastomoses were observed between hyphae from the same or different germlings of Gigaspora rosea and Scutellospora castanea; no interspecific or intergeneric hyphal fusions were observed. We monitored anastomosis formation with time-lapse and video-enhanced light microscopy. We observed complete fusion of hyphal walls and the migration of a mass of particles in both directions within the hyphal bridges. In hyphal bridges of G. caledonium, light-opaque particles moved at the speed of 1.8 ± 0.06 μm/s. We observed nuclear migration between hyphae of the same germling and between hyphae belonging to different germlings of the same isolate of three Glomus species. Our work suggests that genetic exchange may occur through intermingling of nuclei during anastomosis formation and opens the way to studies of vegetative compatibility in natural populations of arbuscular mycorrhizal fungi.     

Influence of nitrogen and phosphorus on polyphosphate accumulation in Gigaspora margarita during spore germination

Polyphosphate (polyP) is the form in which phosphorus (P) is transferred from extraradical hyphae into arbuscles in the symbiotic stage of arbuscular mycorrhizal fungi. However, polyP dynamics in the presymbiotic stage are less understood. In this study, we aimed to investigate polyP accumulation in Gigaspora margarita as influenced by nitrogen (N) and/or P supply during germination. Spores of G. margarita were incubated on medium with or without P or N addition. PolyP content in the fungal tissue was monitored using a polyP kinase/luciferase system, and polyP synthetic activity was determined with ³²P labeling. The results showed that both N and P were necessary for polyP accumulation in germ tubes. Nitrate increased the polyP content in germ tubes, but ammonium did not. Along with germination, polyP content decreased in spores, but increased in germ tubes. ³²P labeling indicated that polyP synthetic activity increased in germ tubes along with germination, but was negligible in spores. Our results suggest that, in the presymbiotic stage of G. margarita, uptake of environmental N and P increases polyP content in germ tubes, and that polyP synthesis occurs mainly therein, leading to polyP accumulation. The possible mechanism of transfer of polyP from spores to hyphae remains to be elucidated.     

Mycorrhization alleviates benzo[a]pyrene-induced oxidative stress in an in vitro chicory root model

Among chemicals that are widely spread both in terrestrial and aquatic ecosystems, benzo[a]pyrene is a major source of concern. However, little is known about its adverse effects on plants, as well as about the role of mycorrhization in protection of plant grown in benzo[a]pyrene-polluted conditions. Hence, to contribute to a better understanding of the adverse effects of polycyclic aromatic hydrocarbons on the partners of mycorrhizal symbiotic association, benzo[a]pyrene-induced oxidative stress was studied in transformed Cichorium intybus roots grown in vitro and colonized or not by Glomus intraradices. The arbuscular mycorrhizal fungus development (colonization, extraradical hyphae length, and spore formation) was significantly reduced in response to increasing concentrations of benzo[a]pyrene (35–280 μM). The higher length of arbuscular mycorrhizal roots, compared to non-arbuscular mycorrhizal roots following benzo[a]pyrene exposure, pointed out a lower toxicity of benzo[a]pyrene in arbuscular mycorrhizal roots, thereby suggesting protection of the roots by mycorrhization. Accordingly, in benzo[a]pyrene-exposed arbuscular mycorrhizal roots, statistically significant decreases were observed in malondialdehyde concentration and 8-hydroxy-2′-desoxyguanosine formation. The higher superoxide dismutase activity detected in mycorrhizal chicory roots could explain the benzo[a]pyrene tolerance of the colonized roots. Taken together, these results support an essential role of mycorrhizal fungi in protecting plants submitted to polycyclic aromatic hydrocarbon, notably by reducing polycyclic aromatic hydrocarbon-induced oxidative stress damage.     

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.     

13C Incorporation into Signature Fatty Acids as an Assay for Carbon Allocation in Arbuscular Mycorrhiza

The ubiquitous arbuscular mycorrhizal fungi consume significant amounts of plant assimilated C, but this C flow has been difficult to quantify. The neutral lipid fatty acid 16:1ω5 is a quantitative signature for most arbuscular mycorrhizal fungi in roots and soil. We measured carbon transfer from four plant species to the arbuscular mycorrhizal fungus Glomus intraradices by estimating ¹³C enrichment of 16:1ω5 and compared it with ¹³C enrichment of total root and mycelial C. Carbon allocation to mycelia was detected within 1 day in monoxenic arbuscular mycorrhizal root cultures labeled with [¹³C]glucose. The ¹³C enrichment of neutral lipid fatty acid 16:1ω5 extracted from roots increased from 0.14% 1 day after labeling to 2.2% 7 days after labeling. The colonized roots usually were more enriched for ¹³C in the arbuscular mycorrhizal fungal neutral lipid fatty acid 16:1ω5 than for the root specific neutral lipid fatty acid 18:2ω6,9. We labeled plant assimilates by using ¹³CO₂ in whole-plant experiments. The extraradical mycelium often was more enriched for ¹³C than was the intraradical mycelium, suggesting rapid translocation of carbon to and more active growth by the extraradical mycelium. Since there was a good correlation between ¹³C enrichment in neutral lipid fatty acid 16:1ω5 and total ¹³C in extraradical mycelia in different systems (r² = 0.94), we propose that the total amount of labeled C in intraradical and extraradical mycelium can be calculated from the ¹³C enrichment of 16:1ω5. The method described enables evaluation of C flow from plants to arbuscular mycorrhizal fungi to be made without extraction, purification and identification of fungal mycelia.     

Interaction of Vesicular-arbuscular Mycorrhizal Fungi and Phosphorus with Meloidogyne incognita on Tomato

The influence of two vesicular-arbuscular mycorrhizal fungi and phosphorus (P) nutrition on penetration, development, and reproduction by Meloidogyne incognita on Walter tomato was studied in the greenhouse. Inoculation with either Gigaspora margarita or Glomus mosseae 2 wk prior to nematode inoculation did not alter infection by M. incognita compared with nonmycorrhizal plants, regardless of soil P level (either 3 μg [low P] or 30 μg [high P] available P/g soil). At a given soil P level, nematode penetration and reproduction did not differ in mycorrhizal and nonmycorrhizal plants. However, plants grown in high P soil had greater root weights, increased nematode penetration and egg production per plant, and decreased colonization by mycorrhizal fungi, compared with plants grown in low P soil. The number of eggs per female nematode on mycorrhizal and nonmycorrhizal plants was not influenced by P treatment. Tomato plants with split root systems grown in double-compartment containers which had either low P soil in both sides or high P in one side and low P in the other, were inoculated at transplanting with G. margarita and 2 wk later one-half of the split root system of each plant was inoculated with M. incognita larvae. Although the mycoorhizal fungus increased the inorganic P content of the root to a level comparable to that in plants grown in high P soil, nematode penetration and reproduction were not altered. In a third series of experiments, the rate of nematode development was not influenced by either the presence of G. margarita or high soil P, compared with control plants grown in low P soil. These data indicate that supplemental P (30 μ/g soil) alters root-knot nematode infection of tomato more than G. mosseae and G. margarita.     

Acidic vesicles in living hyphae of an arbuscular mycorrhizal fungus, Gigaspora margarita

Localization and movement of organelles in living hyphae of an arbuscular mycorrhizal fungus, Gigaspora margarita, were observed using a combination of fluorescent probes and laser-scanning confocal microscopy. Dense, evenly distributed acidic vesicles were visible in germ tubes and extraradical hyphae using DIC with the fluorescent acidotropic probe LysoTracker. These vesicles were distinct from both tubular vacuoles stained with DFFDA and lipid bodies stained with BODIPY 493/503 or Nile Red. Tubular vacuole bundles appeared to be influenced by the bidirectional cytoplasmic streaming of acidic vesicles and lipid bodies. Movement of the acidic vesicles occurred bidrectionally at different rates. The size and distribution of lipid bodies were variable. Based on our observations, the function of these organelles is discussed in relation to nutrient translocation in arbuscular mycorrhizas. Abbreviations: AM – arbuscular mycorrhiza; DAPI – 4′,6-diamidino-2-phenylindole; DIC – differential interference contrast; BODIPY 493/503 – 4,4-difluoro-1,3,5,7,8-pentamethyl-4-bora-3a,4a-diaza-s-indacene; DMSO – dimethyl sulfoxide; FITC – fluorescein isothiocynate; caboxy-DFFDA – Oregon Green 488 carboxylic acid diacetate.     

Real-time RT-PCR profiling of transcription factors including 34 MYBs and signaling components in white lupin reveals their P status dependent and organ-specific expression

Two controlled microcosm experiments aimed at a critical re-assessment of the contributions of divergent arbuscular mycorrhizal (AM) fungi to plant mineral nutrition were established that specifically targeted Plantago lanceolata–Glomus intraradices (B.B/E) and –Gigaspora margarita (BEG 34) symbioses developed in a native, nutrient limited, coastal dune soil. Plant tissue nitrogen (N), phosphorus (P) and potassium (K) status as well as plant growth parameters and levels of mycorrhizal colonization were assessed at harvest. In addition to the general well-established mycorrhizal facilitation of P uptake, the study was able to demonstrate a G. intraradices-specific contribution to improved plant nitrogen and potassium nutrition. In the two respective experiments, G. intraradices-inoculated plants had 27.8% and 40.8% more total N and 55.8% and 23.3% more total K when compared to Gi. margarita inoculated counterparts. Dissimilar overall contribution of the two isolates to plant nutrition was identified in AM-genus specific differences in plant tissue N:P:K ratios. G. intraradices inoculated and non-mycorrhizal plants generally exhibited N:P:K ratios indicative of P limitation whereas for Gi. margarita mycorrhizal plants, corresponding ratios strongly implied either N or K limitation. The study provides further evidence highlighting AM functional biodiversity in respect to plant nutrient limitation experienced by mycorrhizal P. lanceolata in an ecologically relevant soil system.     

Short-term response of arbuscular mycorrhizal association to spider mite herbivory

We examined effects of aboveground herbivory by spider mites (Tetranychus urticae) on colonization and activity of arbuscular mycorrhizal fungi (AMF; Gigaspora margarita) using potted plants (Lotus japonicus). We evaluated changes in arbuscular mycorrhizal (AM) association two ways: (1) conventional trypan blue staining of mycorrhizal hyphae to examine AMF biomass in roots (mycorrhizal colonization) and (2) vital staining for a mycorrhizal enzyme (succinate dehydrogenase, SDH) to examine mycorrhizal activity (SDH activity). Mycorrhizal colonization and SDH activity started to increase 4 days after aboveground herbivory, and returned to the initial levels in the absence of mite herbivory in 7 and 12 days, respectively. These results suggest that the change in AM association in response to mite herbivory is a short-term response.     

Functional diversity in arbuscular mycorrhizas: exploitation of soil patches with different phosphate enrichment differs among fungal species

Most terrestrial plant species form associations with arbuscular mycorrhizal fungi (AMF) that transfer soil P to the plant via their external hyphae. The distribution of nutrients in soils is typically patchy (heterogeneous) but little is known about the ability of AMF to exploit P patches in soil. This was studied by growing symbioses of Linum usitatissimum and three AMF (Glomus intraradices, G. mosseae and Gigaspora margarita) in pots with two side-arms, which were accessible to hyphae, but not to roots. Soil in one side-arm was either unamended (P0) or enriched with P; simultaneous labelling of this soil with ³²P revealed that G. intraradices responded to P enrichment both in terms of hyphal proliferation and P uptake, whereas the other AMF did not. Labelling with ³³P of P0 soil in the other side arm revealed that the increased P uptake by G. intraradices from the P-enriched patch was paralleled by decreased P uptake by other parts of the mycelium. This is the first demonstration of variation in growth and nutrient uptake by an AMF as influenced by a localized P enrichment of the soil. The results are discussed in the context of functional diversity of AMF.     

Transmembrane electric potential difference of germ tubes of arbuscular mycorrhizal fungi responds to external stimuli

Measurements of the electric potential difference across the hyphal wall and the cell membrane were made on external hyphae of three species of arbuscular mycorrhizal fungus Gigaspora margarita , Scutellospora calospora and Glomus coronatum and on germ tubes of Gi. margarita . The values of transmembrane electric potential difference recorded (∼–40 mV) are less negative than those previously reported from hyphae of arbuscular mycorrhizal fungi closely associated with roots and from filamentous fungi. The external hyphae of arbuscular mycorrhizal fungi grown in soil had similar values of electric potential difference to those grown in soil-less culture, and to germ tubes. Thermodynamic calculations showed that despite these low values of electric potential difference, efficient high-affinity uptake of phosphate is possible. The transmembrane electric potential difference of germ tubes of Gi. margarita became more negative when plant root extract was added to the medium, showing for the first time that the early stages of interaction between plant and fungus occur via direct effects on the plasma membrane rather than via effects on gene expression. Addition of K⁺ reversibly depolarized the transmembrane electric potential difference of germ tubes of Gi. margarita , indicating that despite the low electric potential difference the fungus has control over the permeability of the plasmamembrane to K⁺.     

The symbiotic recapture of nitrogen from dead mycorrhizal and non-mycorrhizal roots of tomato plants

Aims

The aim was to quantify the nitrogen (N) transferred via the extra-radical mycelium of the arbuscular mycorrhizal fungus Glomus intraradices from both a dead host and a dead non-host donor root to a receiver tomato plant. The effect of a physical disruption of the soil containing donor plant roots and fungal mycelium on the effectiveness of N transfer was also examined.

Methods

The root systems of the donor (wild type tomato plants or the mycorrhiza-defective rmc mutant tomato) and the receiver plants were separated by a 30 μm mesh, penetrable by hyphae but not by the roots. Both donor genotypes produced a similar quantity of biomass and had a similar nutrient status. Two weeks after the supply of ¹⁵?N to a split-root part of donor plants, the shoots were removed to kill the plants. The quantity of N transferred from the dead roots into the receiver plants was measured after a further 2 weeks.

Results

Up to 10.6 % of donor-root ¹⁵N was recovered in the receiver plants when inoculated with the arbuscular mycorrhizal fungus (AMF). The quantity of ¹⁵N derived from the mycorrhizal wild type roots clearly exceeded that from the only weakly surface-colonised rmc roots. Hyphal length in the donor rmc root compartments was only about half that in the wild type compartments. The disruption of the soil led to a significantly increased AMF-mediated transfer of N to the receiver plants.

Conclusions

The transfer of N from dead roots can be enhanced by AMF, especially when the donor roots have been formerly colonised by AMF. The transfer can be further increased with higher hyphae length densities, and the present data also suggest that a direct link between receiver mycelium and internal fungal structures in dead roots may in addition facilitate N transfer. The mechanical disruption of soil containing dead roots may increase the subsequent availability of nutrients, thus promoting mycorrhizal N uptake. When associated with a living plant, the external mycelium of G. intraradices is readily able to re-establish itself in the soil following disruption and functions as a transfer vessel.     

Fungal Lipid Accumulation and Development of Mycelial Structures by Two Arbuscular Mycorrhizal Fungi

We monitored the development of intraradical and extraradical mycelia of the arbuscular mycorrhizal (AM) fungi Scutellospora calospora and Glomus intraradices when colonizing Plantago lanceolata. The occurrence of arbuscules (branched hyphal structures) and vesicles (lipid storage organs) was compared with the amounts of signature fatty acids. The fatty acid 16:1ω5 was used as a signature for both AM fungal phospholipids (membrane constituents) and neutral lipids (energy storage) in roots (intraradical mycelium) and in soil (extraradical mycelium). The formation of arbuscules and the accumulation of AM fungal phospholipids in intraradical mycelium followed each other closely in both fungal species. In contrast, the neutral lipids of G. intraradices increased continuously in the intraradical mycelium, while vesicle occurrence decreased after initial rapid root colonization by the fungus. S. calospora does not form vesicles and accumulated more neutral lipids in extraradical than in intraradical mycelium, while the opposite pattern was found for G. intraradices. G. intraradices allocated more of its lipids to storage than did S. calospora. Thus, within a species, the fatty acid 16:1ω5 is a good indicator for AM fungal development. The phospholipid fatty acid 16:1ω5 is especially suitable for indicating the frequency of arbuscules in the symbiosis. We propose that the ratio of neutral lipids to phospholipids is more important than is the presence of vesicles in determining the storage status of AM fungi.     

Influence of Glomus intraradices and Soil Phosphorus on Meloidogyne incognita Infecting Cucumis melo

The interaction among Glomus intraradices, Meloidogyne incognita, and cantaloupe was studied at three soil phosphorus (P) levels in a greenhouse. All plants grew poorly in soil not amended with P, regardless of mycorrhizal or nematode status. In soil amended with 50 μg P /g soil, M. incognita suppressed the growth of nonmycorrhizal plants by 84%. In contrast, growth of mycorrhizal plants inoculated with M. incognita was retarded by only 21%. A similar trend occurred in plants grown in soil with 100 μg P /g soil. Mycorrhizal infection had no effect on the degree of root-knot gall formation and did not affect the number of nematode eggs per egg mass. Mineral levels in plant shoots generally declined as soil P levels increased and were not significantly influenced by G. intraradices or M. incognita.     

Effects of Common Mycorrhizal Network on Plant Carbohydrates and Soil Properties in Trifoliate Orange–White Clover Association

Common mycorrhizal network (CMN) allows nutrients and signals to pass between two or more plants. In this study, trifoliate orange (Poncirus trifoliata) and white clover (Trifolium repens) were planted in a two-compartmented rootbox, separated by a 37–μm nylon mesh and then inoculated with an arbuscular mycorrhizal fungus (AMF), Diversispora spurca. Inoculation with D. spurca resulted in formation of a CMN between trifoliate orange and white clover, whilst the best AM colonization occurred in the donor trifoliate orange–receptor white clover association. In the trifoliate orange–white clover association, the mycorrhizal colonization of receptor plant by extraradical hyphae originated from the donor plant significantly increased shoot and root fresh weight and chlorophyll concentration of the receptor plant. Enzymatic activity of soil β-glucoside hydrolase, protease, acid and neutral phosphatase, water-stable aggregate percentage at 2–4 and 0.5–1 mm size, and mean weight diameter in the rhizosphere of the receptor plant also increased. The hyphae of CMN released more easily-extractable glomalin-related soil protein and total glomalin-related soil protein into the receptor rhizosphere, which represented a significantly positive correlation with aggregate stability. AMF inoculation exhibited diverse changes in leaf and root sucrose concentration in the donor plant, and AM colonization by CMN conferred a significant increase of root glucose in the receptor plant. These results suggested that CMN formed in the trifoliate orange–white clover association, and root AM colonization by CMN promoted plant growth, root glucose accumulation, and rhizospheric soil properties in the receptor plant.     

Traits related to differences in function among three arbuscular mycorrhizal fungi

Diversity in phosphorus (P) acquisition strategies was assessed among three species of arbuscular mycorrhizal fungi (AMF) isolated from a single field in Switzerland. Medicago truncatula was used as a test plant. It was grown in a compartmented system with root and root-free zones separated by a fine mesh. Dual radioisotope labeling (³²P and ³³P) was employed in the root-free zone as follows: ³³P labeling determined hyphal P uptake from different distances from roots over the entire growth period, whereas ³²P labeling investigated hyphal P uptake close to the roots over the 48 hours immediately prior to harvest. Glomus intraradices, Glomus claroideum and Gigaspora margarita were able to take up and deliver P to the plants from maximal distances of 10, 6 and 1 cm from the roots, respectively. Glomus intraradices most rapidly colonized the available substrate and transported significant amounts of P towards the roots, but provided the same growth benefit as compared to Glomus claroideum, whose mycelium was less efficient in soil exploration and in P uptake and delivery to the roots. These differences are probably related to different carbon requirements by these different Glomus species. Gigaspora margarita provided low P benefits to the plants and formed dense mycelium networks close to the roots where P was probably transiently immobilized. Numerical modeling identified possible mechanisms underlying the observed differences in patterns of mycelium growth. High external hyphal production at the root-fungus interface together with rapid hyphal turnover were pointed out as important factors governing hyphal network development by Gigaspora, whereas nonlinearity in apical branching and hyphal anastomoses were key features for G. intraradices and G. claroideum, respectively.     

Application of in vitro methods to study carbon uptake and transport by AM fungi

Just as multi-compartmented root chambers have advantages over standard plastic pots for the study of nutrient uptake by arbuscular mycorrhizal [AM] fungi in soil, so the split-plate in vitro system has advantages over the standard dual culture system for the study of the physiology of AM fungi. We used the split-plate culture system of Ri T-DNA transformed Daucus carota L. roots and Glomus intraradices Schenck & Smith, in which only the fungus has access to the distal compartment, to study the ability of germ tubes and extraradical and intraradical hyphae to take up ¹³C-labeled substrates. Labeled substrates were added to one side of the plate divider and plates were incubated for 8 weeks while the fungus proliferated on the side from which the root was excluded. Tissues then were recovered from the plate and examined via NMR spectroscopy. Results showed that the morphological phases of the fungus differed in their ability to take up these substrates, most notably that intraradical hyphae take up hexose while extraradical hyphae cannot. In addition, NMR studies indicated that intraradical hyphae actively synthesized lipids while extraradical hyphae did not. These data show that eventual axenic culture of AM fungi is more than a matter of finding the proper substrate for growth. Genetic regulation must be overcome to make extraradical hyphae behave like intraradical hyphae in terms of C uptake and metabolism. This revised version was published online in June 2006 with corrections to the Cover Date.     

Localization and speciation of arsenic in Glomus intraradices by synchrotron radiation spectroscopic analysis

The protective mechanisms employed by arbuscular mycorrhizal fungi (AMF) to reduce the toxic effects of arsenic on host plants remain partially unknown. The goal of this research was identifying the in situ localization and speciation of arsenic (As) in the AM fungus Rhizophagus intraradices [formerly named Glomus intraradices] exposed to arsenate [As(V)]. By using a two-compartment in vitro fungal cultures of R. intraradices-transformed carrot roots, microspectroscopic X-ray fluorescence (μ-XRF), and microspectroscopic X-ray absorption near edge structure (μ-XANES), we observed that As(V) is absorbed after 1 h in the hyphae of AMF. Three hours after exposure a decrease in the concentration of As was noticed and after 24 and 72 h no detectable As concentrations were perceived suggesting that As taken up was pumped out from the hyphae. No As was detected within the roots or hyphae in the root compartment zone three or 45 h after exposure. This suggests a dual protective mechanism to the plant by rapidly excluding As from the fungus and preventing As translocation to the plant root. μ-XANES data showed that gradual As(V) reduction occurred in the AM hyphae between 1 and 3 h after arsenic exposure and was completed after 6 h. Principal component analysis (PCA) and linear combination fitting (LCF) of μ-XANES data showed that the dominant species after reduction of As(V) by R. intraradices extra-radical hyphal was As(III) complexed with a reduced iron(II) carbonate compound. The second most abundant As species present was As(V)–iron hydroxides. The remaining As(III) compounds identified by the LCF analyses suggested these molecules were made of reduced As and S. These results increase our knowledge on the mechanism of As transport in AMF and validate our hypotheses that R. intraradices directly participates in arsenic detoxification. These fungal mechanisms may help AMF colonized plants to increase their tolerance to As at contaminated sites.     

AM 真菌影响三叶草根系抗氧化酶活性的系统效应

本文对三叶草接种AM 真菌根内球囊霉, 用盆栽试验和分根试验测定根系的菌根侵染率和抗氧化酶活性, 研究AM 真菌对根系抗氧化酶活性的影响以及该影响的系统性。结果表明, 盆栽试验中接种根内球囊霉显著提高了根系中SOD、POD、CAT 的活性, 表明AM 真菌可以促进根系的抗氧化酶活性; 分根试验中一半根系接种了根内球囊霉的植株, 其另一半未接种的根系SOD、POD 活性也增加, 表明AM 真菌对根系抗氧化酶系统的促进具有系统效应。由于抗氧化酶系统是植物产生抗逆性的生理生化基础, 可以推测, AM 真菌对根系抗氧化酶活性的系统性提高有助于保护根系整体, 而非仅仅保护受侵染根段。     

Symbiont identity matters: carbon and phosphorus fluxes between Medicago truncatula and different arbuscular mycorrhizal fungi

Many studies have scrutinized the nutritional benefits of arbuscular mycorrhizal associations to their host plants, while the carbon (C) balance of the symbiosis has often been neglected. Here, we present quantification of both the C costs and the phosphorus (P) uptake benefits of mycorrhizal association between barrel medic (Medicago truncatula) and three arbuscular mycorrhizal fungal species, namely Glomus intraradices, Glomus claroideum, and Gigaspora margarita. Plant growth, P uptake and C allocation were assessed 7 weeks after sowing by comparing inoculated plants with their non-mycorrhizal counterparts, supplemented with different amounts of P. Isotope tracing (³³P and ¹³C) was used to quantify both the mycorrhizal benefits and the costs, respectively. G. intraradices supported greatest plant P acquisition and incurred high C costs, which lead to similar plant growth benefits as inoculation with G. claroideum, which was less efficient in supporting plant P acquisition, but also required less C. G. margarita imposed large C requirement on the host plant and provided negligible P uptake benefits. However, it did not significantly reduce plant growth due to sink strength stimulation of plant photosynthesis. A simple experimental system such as the one established here should allow quantification of mycorrhizal costs and benefits routinely on a large number of experimental units. This is necessary for rapid progress in assessment of C fluxes between the plants and different mycorrhizal fungi or fungal communities, and for understanding the dynamics between mutualism and parasitism in mycorrhizal symbioses.