The composition of arbuscular mycorrhizal fungal communities differs among the roots,spores and extraradical mycelia associated with five Mediterranean plant species

Arbuscular mycorrhizal fungi (AMF) are essential constituents of most terrestrial ecosystems. AMF species differ in terms of propagation strategies and the major propagules they form. This study compared the AMF community composition of different propagule fractions – colonized roots, spores and extraradical mycelium (ERM) – associated with five Mediterranean plant species in Sierra de Baza Natural Park (Granada, Spain). AMF were identified using 454 pyrosequencing of the SSU rRNA gene. A total of 96 AMF phylogroups [virtual taxa (VT)] were detected in the study site, including 31 novel VT. After per‐sample sequencing depth standardization, 71 VT were recorded from plant roots, and 47 from each of the spore and ERM fractions. AMF communities differed significantly among the propagule fractions, and the root‐colonizing fraction differed among host plant species. Indicator VT were detected for the root (13 Glomus VT), spore (Paraglomus VT281, VT336, Pacispora VT284) and ERM (Diversispora VT62) fractions. This study provides detailed evidence from a natural system that AMF taxa are differentially allocated among soil mycelium, soil spores and colonized root propagules. This has important implications for interpreting AMF diversity surveys and designing applications of AMF in vegetation restoration.     

Growth of micropropagated bananas colonized by root-organ culture produced arbuscular mycorrhizal fungi entrapped in Ca- alginate beads

The effect of root-organ culture (ROC) produced arbuscular mycorrhizal fungi (AMF), i.e. Glomus proliferum, Glomus versiforme and Glomus intraradices, entrapped in Ca-alginate beads on the first stages development of micropropagated bananas (Musa spp. cv. Grande Naine) was investigated. The experimental design consisted of banana plants inoculated with one of the three AMF and two controls, i.e. Control-AL (with empty alginate beads), and Control (no beads). Forty plants were considered per treatment and cultured under greenhouse conditions in a completely randomized design. Eight plants per treatment were harvested 40, 80, 120, 160 and 200 days after inoculation and analysed for root colonization, growth parameters and nutrient concentration. In addition, spores were enumerated in the substrate at the same intervals. Ca-alginate entrapped ROC-produced AMF spores were able (1) to colonize the root system of a micropropagated banana cultivar under nursery conditions, (2) to increase plant P nutrition and biomass, and (3) to proliferate in the commercial nursery substrate, therefore increasing the fungal inoculum biomass. The entrapment of ROC-propagated spores, adaptable to a wide range of Glomeromycetes, represents thus a forthcoming alternative pathogen-free inoculum.     

不同强度盐胁迫下AM真菌对羊草生长的影响

不同浓度NaCl盐处理下,AM真菌对羊草(Leymus chinensis)的侵染能力和对植物生长的影响,从植物形态和离子含量角度探讨了AM真菌提高羊草耐盐性的作用机理。结果表明,在高盐胁迫下,AM真菌显著降低了盐胁迫效应,提高了羊草生物量,菌根效应明显。菌根化羊草的根茎比显著增加,并且N、P浓度较高,Na~+和Cl~-离子浓度较低,表明AM真菌即促进羊草对营养元素的吸收,又减少了离子毒害。菌根化羊草的Ca~(2+)和K~+离子浓度,以及P/Na~+和K~+/Na~+比高于非菌根化羊草,表明AM真菌可通过调节渗透势以避免或减缓盐胁迫造成的生理缺水。随着盐胁迫的增加,菌根化羊草对磷的依赖性逐渐转换为对钾的依赖性。研究结果有助于揭示AM真菌提高植物耐盐能力的作用机理,并对应用菌根技术修复盐化草地具有理论指导意义。     

Time-course study and partial characterization of a protein on hyphae of arbuscular mycorrhizal fungi during active colonization of roots

Material on the surface of hyphal walls of arbuscular mycorrhizal fungi (AMF) during active colonization of plant roots was detected by a monoclonal antibody. Pot-cultured isolates of Glomus, Acaulospora, Gigaspora, Scutellospora, and Entrophospora had immunofluorescent material (IM) on younger, thinner, intact hyphae, but IM was scant to absent on thicker, melanized or lysing hyphae. Colonization of corn (Zea mays L.), Sudangrass (Sorghum sudanense (Piper) Staph.) or red clover (Trifolium pratense L.) was examined during 5 months of plant growth by removing cores and performing an indirect immunoassay on roots with attached hyphae. Fresh spores of some Glomus spp. had IM on the outer layer of the spore wall. Abundant IM was seen on root hairs of plants colonized by some isolates, and some IM was detected on root surfaces of all plants examined even during early colonization. After cultures were dried, hyphae, roots and spores had little to no IM. Uninoculated control roots had very rare, small patches of IM. An immunoreactive protein was extracted from hyphae of Gigaspora and Glomus isolates by using 20mM citrate (pH 7.0) at 121°C for 90 min. Gel electrophoresis profiles indicated that all isolates tested had the same banding patterns. Lectin-binding of extracted protein is suggestive of a glycoprotein. The immunofluorescence assay can be used to examine root sections for active colonization by AMF, and the potential use of the protein to quantify AMF activity in soil is discussed.     

Differences of arbuscular mycorrhizal fungal diversity and community between a cultivated land,an old field,and a never-cultivated field in a hot and arid ecosystem of southwest China

We investigated the spore density, species composition, and diversity of arbuscular mycorrhizal fungi (AMF) in a cultivated land (CL), an old field (OF), and a never-cultivated field (NCF), which are located adjacently in a slope in the hot and arid ecosystem of southwest China. AMF spores in the rhizosphere soils of representative plants in the three habitats were extracted by wet-sieving and decanting. A total of 47 taxa of AMF including 31 taxa from the genus Glomus, 8 from Acaulospora, 6 from Scutellospora, 1 from Entrophospora, and 1 from Gigaspora were extracted and identified morphologically. The highest spore density occurred in NCF, slightly lower in OF and lowest in CL, and the Shannon–Wiener index of species diversity was reversed. The dominant species of AMF were different in the three habitats. OF resembled NCF more than CL in AMF spore density, species richness, and community composition, which means that AMF community in the OF has been developing from cultivated land to natural habitat. Cluster analysis based on the similarity in AMF community composition indicated that the distribution of AMF was not random over space and that AMF community composition associated with a given plant species was greatly habitat-convergence. Following the cluster analysis, we hypothesized that the effect of habitats on AMF communities were greater than that of the host preference to AMF. L-F. Li and T. Li contributed equally to this work.     

Spatial variability of arbuscular mycorrhizal fungal spores in two natural plant communities

Geostatistical techniques were used to assess the spatial patterns of spores of arbuscular mycorrhizal fungi (AMF) in soils from two contrasting plant communities: a salt marsh containing only arbuscular mycorrhizal and non-mycorrhizal plants in a distinct clumped distribution pattern and a maquis with different types of mycorrhiza where most plants were relatively randomly distributed. Also evaluated was the relationship between the spatial distribution of spores and AM plant distribution and soil properties. A nested sampling scheme was applied in both sites with sample cores taken from nested grids. Spores of AMF and soil characteristics (organic matter and moisture) were quantified in each core, and core sample location was related to plant location. Semivariograms for spore density indicated strong spatial autocorrelation and a patchy distribution within both sites for all AM fungal genera found. However, the patch size differed between the two plant communities and AM fungal genera. In the salt marsh, AM fungal spore distribution was correlated with distance to AM plants and projected stand area of AM plants. In maquis, spatial AM fungal spore distribution was correlated with organic matter. These results suggest that spore distribution of AMF varied between the two plant communities according to plant distribution and soil properties.     

Elemental composition of arbuscular mycorrhizal fungi at high salinity

We investigated the elemental composition of spores and hyphae of arbuscular mycorrhizal fungi (AMF) collected from two saline sites at the desert border in Tunisia, and of Glomus intraradices grown in vitro with or without addition of NaCl to the medium, by proton-induced X-ray emission. We compared the elemental composition of the field AMF to those of the soil and the associated plants. The spores and hyphae from the saline soils showed strongly elevated levels of Ca, Cl, Mg, Fe, Si, and K compared to their growth environment. In contrast, the spores of both the field-derived AMF and the in vitro grown G. intraradices contained lower or not elevated Na levels compared to their growth environment. This resulted in higher K:Na and Ca:Na ratios in spores than in soil, but lower than in the associated plants for the field AMF. The K:Na and Ca:Na ratios of G. intraradices grown in monoxenic cultures were also in the same range as those of the field AMF and did not change even when those ratios in the growth medium were lowered several orders of magnitude by adding NaCl. These results indicate that AMF can selectively take up elements such as K and Ca, which act as osmotic equivalents while they avoid uptake of toxic Na. This could make them important in the alleviation of salinity stress in their plant hosts.     

Toxicity of diesel fuel to germination,growth and colonization of <Emphasis Type="Italic">Glomus intraradices</Emphasis> in soil and <Emphasis Type="Italic">in vitro</Emphasis> transformed carrot root cultures

Phytoremediation is the use of selected plants to decontaminate polluted environments. Arbuscular mycorrhizal fungi (AMF) may potentially be useful for phytoremediation, but it is not known how petroleum hydrocarbons influence AMF spore germination and hyphal growth. To address this question, germination of spores and germ tube growth of Glomus intraradices Schenck and Smith and Glomus aggregatum Schenck and Smith were assessed in soil contaminated with up to 3% (w/v) of F2 diesel oil or HAGO reference oil. Hyphal growth, colonization and progeny spore production were assessed in vitro using transformed root cultures of Daucus carota and G. intraradices spores in a F2 diesel contaminated medium. In addition, extraradical hyphal growth of G. intraradices colonizing Daucus carota in the presence of F2 diesel was studied. Neither F2 diesel nor HAGO reference oil affected spore germination or germ tube growth in soil. However, in the presence of plant roots, germ tube growth of G. intraradices was reduced and delayed in the presence of F2 diesel and root colonization was not detected. Hyphal growth of pre-colonized carrot roots by G. intraradices was reduced and delayed in F2 contaminated medium compared to controls. F2 diesel did not inhibit spore germination of these AMF species but did reduce colonization, germ tube and hyphal growth. These results suggest that AMF inoculum can be established in petroleum-contaminated sites. However, it may prove beneficial to plant pre-colonized plants to increase the probability of sufficient AMF colonization and growth. The likely mechanism(s) of petroleum toxicity in this plant-microbe system was discussed.     

Arbuscular mycorrhizal fungi native from a Mediterranean saline area enhance maize tolerance to salinity through improved ion homeostasis

Soil salinity restricts plant growth and productivity. Na⁺ represents the major ion causing toxicity because it competes with K⁺ for binding sites at the plasma membrane. Inoculation with arbuscular mycorrhizal fungi (AMF) can alleviate salt stress in the host plant through several mechanisms. These may include ion selection during the fungal uptake of nutrients from the soil or during transfer to the host plant. AM benefits could be enhanced when native AMF isolates are used. Thus, we investigated whether native AMF isolated from an area with problems of salinity and desertification can help maize plants to overcome the negative effects of salinity stress better than non‐AM plants or plants inoculated with non‐native AMF. Results showed that plants inoculated with two out the three native AMF had the highest shoot dry biomass at all salinity levels. Plants inoculated with the three native AMF showed significant increase of K⁺ and reduced Na⁺ accumulation as compared to non‐mycorrhizal plants, concomitantly with higher K⁺/Na⁺ ratios in their tissues. For the first time, these effects have been correlated with regulation of ZmAKT2, ZmSOS1 and ZmSKOR genes expression in the roots of maize, contributing to K⁺ and Na⁺ homeostasis in plants colonized by native AMF.     

Isolation and analysis of bacteria associated with spores of Gigaspora margarita

Aims:  The aim of this work was to observe bacteria associated with the spores of Gigaspora margarita , an arbuscular mycorrhizal fungus (AMF).
Methods and Results:  First, a direct analysis of DNA from sterilized spores indicated the bacteria belonging to the genus Janthinobacterium . In the second assay, two bacterial strains were isolated by osmosis from protoplasts, which were derived from spores by using two particular enzymes: lysing enzymes and yatalase. After isolation, cultivation and identification by their DNA as performed in the first experiment, the species with the closest relation were Janthinobacterium lividum (KCIGM01) and Paenibacillus polymyxa (KCIGM04) isolated with lysing enzymes and yatalase respectively. Morphologically, J. lividum was Gram negative and oval, while P. polymyxa was also oval, but Gram positive. Both strains had antagonistic effects to the pathogenic fungi Rosellimia necatrix, Pythium ultimum , Fusarium oxysporum and Rhizoctonia solani . In particular, J. lividum was much stronger in this role. However, in phosphorus (P) solubilization P. polymyxa functioned better than J. lividum.
Conclusions:  This experiment had revealed two new bacteria species ( P. polymyxa and J. lividum ), associated with AMF spores, which functioned to suppress diseases and to solubilize P.
Significance and Impact of the Study:  AMF spores could be a useful source for bacterial antagonists to soil-borne diseases and P solubilization.     

Zn uptake by maize under the influence of AM-fungi and Collembola Folsomia candida

Zn uptake by maize plants may be affected by the presence of arbuscular mycorrhizal fungi (AMF). Collembola often play an important controlling role in the inter-relationship between AMF and host plants. The objective of this experiment was to examine whether the presence of Collembola at different densities (0.4 and 1 individuals g⁻¹ dry soil) and their activity have any effect on Zn uptake by maize through the plant–AMF system. The presence of the AMF (Glomus intraradices) and of the Collembola species Folsomia candida was studied in a laboratory microcosm experiment, applying a Zn exposure level of 250 mg kg⁻¹ dry soil. Biomass and water content of the plants were no different when only AMF or when both AMF and Collembola were present. In the presence of AMF the Zn content of the plant shoots and roots was significantly higher than without AMF. This effect was reduced by Collembola at both low and high density. High densities of Collembola reduced the extent of AMF colonization of the plant roots and hyphal length in the soil, but low densities had no effect on either. The results of this experiment reveal that the F. candida–G. intraradices interaction affects Zn uptake by maize, but the mechanisms are still unknown.     

Differential Effects of a <Emphasis Type="Italic">Bacillus megaterium</Emphasis> Strain on <Emphasis Type="Italic">Lactuca sativa</Emphasis> Plant Growth Depending on the Origin of the Arbuscular Mycorrhizal Fungus Coinoculated: Physiologic and Biochemical Traits

Coinoculation with plant growth–promoting rhizobacteria (PGPR) and arbuscular mycorrhizal fungi (AMF) has been proposed as an efficient method to increase plant growth. In this article we investigate how the interaction between three different AMF isolates (Glomus constrictum autochthonous, GcA; G. constrictum from collection, GcC; and commercial Glomus intraradices, Gi) and a Bacillus megaterium strain isolated from a Mediterranean calcareous soil affects Lactuca sativa L. plant growth. Inoculation with B. megaterium increased plant growth when in combination with two of the AMF isolates (GcA and Gi), but decreased it when in combination with GcC. At the same time, plants inoculated with the GcC fungus alone or in combination with B. megaterium (GcC+Bm) showed leaf symptoms of stress injury by accumulating proline and reducing the amount of photosynthetic pigments, whereas the opposite occurred in plants coinoculated with Gi fungus and B. megaterium (Gi+Bm). GcC+Bm leaves also presented the highest glucose-6-phosphate dehydrogenase (G6PDH) and the lowest glutamine synthetase (GS) enzymatic activities, whereas Gi+Bm leaves showed the highest GS activity. Results on these enzymatic activities are further discussed in relation to plant growth and performance.     

Priming and filtering of antiherbivore defences among Nicotiana attenuata plants connected by mycorrhizal networks

Arbuscular mycorrhizal fungi (AMF) establish symbiotic associations with a majority of terrestrial plants to form underground common mycorrhizal networks (CMNs) that connect neighbouring plants. Because Nicotiana attenuata plants do not respond to herbivory‐elicited volatiles from neighbours, we used this ecological model system to evaluate if CMNs function in interplant transmission of herbivory‐elicited responses. A mesocosm system was designed to establish and remove CMNs linking N. attenuata plants to examine the herbivory‐elicited metabolic and hormone responses in CMNs‐connected “receiver” plants after the elicitation of “donor” plants by wounding (W) treated with Manduca sexta larval oral secretions (OS). AMF colonization increased constitutive jasmonate (JA and JA‐Ile) levels in N. attenuata roots but did not affect well‐characterized JAs‐regulated defensive metabolites in systemic leaves. Interestingly, larger JAs bursts, and higher levels of several amino acids and particular sectors of hydroxygeranyllinalool diterpene glycoside metabolism were elevated in the leaves of W + OS‐elicited “receivers” with CMN connections with “donors” that had been W + OS‐elicited 6 hr previously. Our results demonstrate that AMF colonization alone does not enhance systemic defence responses but that sectors of systemic responses in leaves can be primed by CMNs, suggesting that CMNs can transmit and even filter defence signalling among connected plants.     

Red list plants: colonization by arbuscular mycorrhizal fungi and dark septate endophytes

Since information concerning the mycorrhization of endangered plants is of major importance for their potential re-establishment, we determined the mycorrhizal status of Serratula tinctoria (Asteraceae), Betonica officinalis (Lamiaceae), Drosera intermedia (Droseraceae) and Lycopodiella inundata (Lycopodiaceae), occurring at one of two wetland sites (fen meadow and peat bog), which differed in soil pH and available P levels. Root colonization by arbuscular mycorrhizal fungi (AMF) and dark septate endophytes (DSE) was quantified. Colonization by AMF appeared to be more frequent in the fen meadow than in the peat bog, and depended on the host plant. Roots of S. tinctoria and B. officinalis were well colonized by AMF in the fen meadow (35–55% root length) and both arbuscules and vesicles were observed to occur in spring as well as in autumn. In the peat bog, L. inundata showed a low level of root colonization in spring, when vesicles were found frequently but no arbuscules. In roots of D. intermedia from the peat bog, arbuscules and vesicles were observed, but AMF colonization was lower than in L. inundata. In contrast, the amount of AMF spores extracted from soil at the peat bog site was higher than from the fen meadow soil. Spore numbers did not differ between spring and autumn in the fen meadow, but they were higher in spring than in autumn in the peat bog. Acaulospora laevis or A. colossica and Glomus etunicatum were identified amongst the AMF spores extracted from soil at the two sites. S. tinctoria and B. officinalis roots were also regularly colonized by DSE (18–40% root length), while L. inundata was only rarely colonized and D. intermedia did not seem to be colonized by DSE at all.     

连作花魔芋软腐病株与健株根域丛枝菌根真菌群落多样性

【目的】解析不同连作年限花魔芋软腐病株、健株根域的丛枝菌根真菌(arbuscular mycorrhizal fungi,AMF)群落多样性。【方法】使用AMF 18S SSU rRNA基因特异引物AMV4.5NF/AMDGR对正茬及连作2年和3年的软腐病株、健株魔芋根系和根际土壤DNA扩增建库,通过高通量测序和生物信息学分析探究魔芋软腐病与其根域AMF群落多样性的关系。【结果】魔芋根系具有明显的AMF菌丝、泡囊和丛枝等结构。在相同连作年限条件下,健株根系AMF总侵染率、侵染强度和孢子密度均显著高于病株(P<0.05);在不同连作年限条件下,病株根系AMF总侵染率和侵染强度随连作年限延长而降低。从所有样品中共鉴定到9属53种AMF,其中有49个已知种和4个新种。球囊霉属(Glomus)和类球囊霉属(Claroideoglomus)是AMF群落的优势属,其AMF种分别占总AMF种数的41.5%和26.4%;丰度最高的Paraglomus sp.VTX00308是所有样品的共有种。连作、软腐病及二者的交互作用显著影响根系AMF群落的Shannon指数和Simpson指数及根际土壤AMF的Chao1指数(P<0.05)。通过丰度差异分析发现6个在连作软腐病发生后丰度差异显著的AMF种(P<0.05);NMDS分析表明,不同连作年限的魔芋软腐病株与健株之间的根域AMF菌种组成、相对丰度和群落结构存在差异。相关性分析表明,软腐病发病率和病情指数与魔芋根系和根际土壤AMF的Shannon指数、根系AMF的Chao1和Simpson指数以及AMF总侵染率、侵染强度和孢子密度极显著负相关(P<0.01)。【结论】比对健株,连作魔芋软腐病株根际土壤AMF孢子密度以及根系AMF侵染率、种数和多样性均降低,其群落结构显著改变。     

Arbuscular mycorrhizal soil infectivity and spores distribution across plantations of tropical,subtropical and exotic tree species: a case study from the forest reserve of Bandia,Senegal

Several fast‐growing and multipurpose trees such as exotic and valuable native species have been widely used in West Africa to reverse the tendency of massive degradation of plant cover and restore soil productivity. Although benefic effects have been reported on soil stabilization, a lack of information about their impact on soil symbiotic microorganisms still remains. This investigation has been carried out in field trees of 28 years old in a forest reserve at Bandia. To determine the mycorrhizal inoculum potential (MIP) of soils, a mycorrhizal bioassay was conducted using seedlings of Zea mays L. Spores concentration, arbuscular mycorrhizal (AM) fungi morphotypes and mycorrhizal colonization of field plants were examined. Results showed that fungal communities were dominated in all samples by the genus Glomus. Nevertheless, the others genera Gigaspora and Scutellospora occurred preferentially out of the plantations. The number and richness of spores as well as the MIP of soils were decreased in the tree plantations. Accordingly, the amount of annual herbaceous plants kept out of the tree plantations was much greater than those under the tree plantations. The colonization was higher in field root systems of herb plants in comparison with that of the tree plants. Comparisons allowed us to conclude that vegetation type modifies the AM fungal communities, and the results suggest further adoption of management practices that could improve or sustain the development of herbaceous layers and thus promote the AM fungal communities.     

Glomus intraradices dominates arbuscular mycorrhizal communities in a heavy textured agricultural soil

Arbuscular mycorrhizal fungal (AMF) spore communities were surveyed in a long-term field fertilization experiment in Switzerland, where different amounts of phosphorus (P) were applied to soil. Plots receiving no P as well as plots systematically fertilized in excess to plant needs for 31 years were used to test the hypothesis that application of P fertilizer changes the composition and diversity of AMF communities. AMF spores were isolated from the field soil, identified, and counted so as to quantify the effect of P fertilization on AMF spore density, composition, and diversity. Trap cultures were established from field soil with four host plants (sunflower, leek, maize, and Crotalaria grahamiana), and the spore communities were then analyzed in substrate samples from the pots. Altogether, nine AMF species were detected in the soil. No evidence has been acquired for effect of P fertilization on spore density, composition, and diversity of AMF in both the field soil and in trap cultures. On the other hand, we observed strong effect of crop plant species on spore densities in the soil, the values being lowest under rapeseed and highest under Phacelia tanacetifolia covercrop. The identity of plant species in trap pots also significantly affected composition and diversity of associated AMF communities, probably due to preferential establishment of symbiosis between certain plant and AMF species. AMF spore communities under mycorrhizal host plants (wheat and Phacelia in the fields and four host plant species in trap pots) were dominated by a single AMF species, Glomus intraradices. This resulted in exceptionally low AMF spore diversity that seems to be linked to high clay content of the soil.Electronic supplementary material Supplementary material is available for this article at and accessible for authorised users.     

Lunularia cruciata, a potential in vitro host for Glomus proliferum and G. intraradices

A study was conducted to define culture conditions for in vitro growth arbuscular mycorrhizal fungi (AMF) with liverworts as hosts. Lunularia cruciata (L.) Dumortier ex. Lindberg developed in vitro monoxenic mycothalli with both Glomus proliferum Dalpé & Declerck (MUCL 41827) and Glomus intraradices Schenck & Smith (MUCL 43204). AMF inoculated plants were co-cultured in plastic Petri dishes with semi-solidified medium supplemented with sucrose and grown under filtered light. Mycothalli of L. cruciata produced external hyphae and spores in quantities equivalent to those obtained with Ri T-DNA transformed root systems.     

Differential host plant performance as a function of soil arbuscular mycorrhizal fungal communities: experimentally manipulating co-occurring Glomus species

In order to evaluate host plant performance relative to different soil arbuscular mycorrhizal fungal (AMF) communities, Andropogon gerardii seedlings were grown with nine different AMF communities. The communities consisted of 0, 10, or 20 spores of Glomus etunicatum and 0, 10, or 20 spores of Glomus intraradices in all possible combinations. Spores were produced by fungal cultures originating on A. gerardii in a serpentine plant community; seeds of A. gerardii were collected at the same site. The experiment was performed in the greenhouse using a mixture of sterilized serpentine soil and sand to which naturally occurring non-mycorrhizal microbes were added. There was no difference in root AMF colonization rates between single species communities of either G. etunicatum or G. intraradices, but G. intraradices enhanced plant growth and G. etunicatum did not. However, plants grew larger with some combinations of G.␣intraradices plus G. etunicatum than with the same quantity of G. intraradices alone. These results suggest the potential for niche complementarity in the mycorrhizal fungi. That G. etunicatum only increased plant growth in the presence of G. intraradices could be illustrative of why AMF that appear to be parasitic or benign when examined in isolation are maintained within multi-species mycorrhizal communities in nature.     

Colonization potential of in vitro-produced arbuscular mycorrhizal fungus spores compared with a root-segment inoculum from open pot culture

 A reliable inoculum, free from other microorganisms, to produce arbuscular mycorhizal (AM) plants is of the greatest importance when studying the interaction between AM plants and soil microorganisms. We investigated the colonization of leeks from monoxenic in vitro-produced Glomus intraradices spores. The isolated spores were produced using a two-compartment in vitro growth system previously described. A spore suspension was used as inoculum and was compared to the inoculum potential of endomycorrhizal root segments of pot-grown leek (Allium porrum L.) plants. The leeks were grown in a controlled environment and two types of sterilized growth media were tested: calcined montmorillonite clay and a soil mix. Root colonization progressed faster in the soil mix than in the clay. However, in this medium, after an initial delay, root colonization from in vitro-produced spores was essentially the same as that observed with the root-segment inoculum, reaching 44% and 58% respectively, after 16 weeks. Leek roots colonized by the monoxenically-produced spores harbored only the studied AMF fungi while the roots colonized from the root segments were substantially contaminated by other fungi. Accepted: 25 December 1998