Yolande Dalpé

Sterol distribution in arbuscular mycorrhizal fungi

Abstract The sterol composition of spores from 16 species of arbuscular mycorrhizal fungi belonging to the order Glomales were examined by GC–MS. The major compound was found to be 24-ethylcholesterol (up to 85%) followed by cholesterol (up to 15%). Several other sterols such as 24-methylcholesterol, Δ5-avenasterol and 24-ethylcholesta-5,22-dien-3 β -ol were also detected. Significant amounts of α -amyrin, a common vascular plant triterpene, were present in the spores of all the fungal species analyzed. The absence of ergosterol, a classical fungal sterol, is discussed in relation to fungal evolution.

Glomus proliferum sp. nov.: a description based on morphological, biochemical, molecular and monoxenic cultivation data

A new arbuscular mycorrhizal fungus, Glomus proliferum (Glomales, Zygomycetes) is described. The description, based on a monoxenic culture established in association with a Ri T-DNA transformed carrot root, combines sequencing of the small subunit (SSU) rDNA, spore sterols and fatty acid profiles with more classical taxonomic tools such as optical and electron microscopy. The fungus forms clusters containing hundreds of small, hyaline, four-layered spores. The necessity to use different tools for identification of arbuscular mycorrhizal fungi is discussed.

Spore development and nuclear inheritance in arbuscular mycorrhizal fungi

BackgroundA conventional tenet of classical genetics is that progeny inherit half their genome from each parent in sexual reproduction instead of the complete genome transferred to each daughter during asexual reproduction. The transmission of hereditary characteristics from parents to their offspring is therefore predictable, although several exceptions are known. Heredity in microorganisms, however, can be very complex, and even unknown as is the case for coenocytic organisms such as Arbuscular Mycorrhizal Fungi (AMF). This group of fungi are plant-root symbionts, ubiquitous in most ecosystems, which reproduce asexually via multinucleate spores for which sexuality has not yet been observed.ResultsWe examined the number of nuclei per spore of four AMF taxa using high Z-resolution live confocal microscopy and found that the number of nuclei was correlated with spore diameter. We show that AMF have the ability, through the establishment of new symbioses, to pass hundreds of nuclei to subsequent generations of multinucleated spores. More importantly, we observed surprising heterogeneity in the number of nuclei among sister spores and show that massive nuclear migration and mitosis are the mechanisms by which AMF spores are formed. We followed spore development of Glomus irregulare from hyphal swelling to spore maturity and found that the spores reached mature size within 30 to 60 days, and that the number of nuclei per spores increased over time.ConclusionsWe conclude that the spores used for dispersal of AMF contain nuclei with two origins, those that migrate into the spore and those that arise by mitosis in the spore. Therefore, these spores do not represent a stage in the life cycle with a single nucleus, raising the possibility that AMF, unlike all other known eukaryotic organisms, lack the genetic bottleneck of a single-nucleus stage.

Indigenous populations of arbuscular mycorrhizal fungi and soil aggregate stability are major determinants of leek (Allium porrum L.) response to inoculation with Glomus intraradices Schenck & Smith or Glomus versiforme (Karsten) Berch

Abstract Knowledge of physical, chemical and biological soil characteristics influencing plant response to inoculation with arbuscular mycorrhizal (AM) fungi would help to distinguish soils where inoculation could be profitable. The relationship between leek (Allium porrum L.) response to mycorrhizal inoculation with Glomus intraradices Schenck & Smith or G. versiforme (Karsten) Berch and soil texture, bulk density, particle density, porosity, pH, organic matter content, available P, K, Ca, Mg, Fe, Zn, Cu, and Mn, soil structure, soil mycorrhizal potential (SM), preceding crop mycorrhizal potential, composition of indigenous mycorrhizal fungal communities, and the abundance of spores of different species, was studied in 81 agricultural soils using Principal Component Analysis and regression analysis. The nature of the indigenous AM fungi population was an important determinant of leek response to inoculation (RTI). In soils with more than 200 μg available P g–1, SM potential accounted for over 27% of RTI with G. intraradices and G. versiforme, RTI being high in soils with low SM potential. In low P soils, however, a positive relation between the abundance of water stable soil aggregates in the 0.5–2 mm diameter range and RTI was most important. Low soil Zn and high porosity, abundant total mycorrhizal spore as well as scarcity of spores of G. aggregatum and of the group G. etunicatum-rubiforme were also associated to high RTI. The influence of water stable aggregation of soil on RTI was modulated by soil P levels. Abundance of soil aggregates was positively related to RTI at low soil P levels, but negatively related to RTI at high P levels. Different relationships were found between soil variables and spore abundance of different AM fungi species. Some AM species appear to have as yet undefined similarities or complementarities at the biological or ecological levels.

The effect of vesicular-arbuscular mycorrhizae and chilling on two hybrids of Zea mays L.

In order to investigate the effect of vesicular-arbuscular mycorrhizae on the chilling resistance of Zea mays, seeds of two hybrids (Pioneer 3902 and Pride 5) were grown in soil inoculated with Glomus mosseae. Germination tests at 10° C and 25° C showed that Pride 5 was more resistant to chilling than Pioneer 3902. Plants grown at 25° C for 6 weeks were given a 1-week chilling treatment at 10° C and the responses of mycorrhizal and nonmycorrhizal plants of the two hybrids were compared. At 10° C, the mycorrhizal plants had greater biomass, carbohydrate, and protein content than the nonmycorrhizal plants.

Glomales species associated with surface and deep rhizosphere of Faidherbia albida in Senegal

Abstract Five arbuscular mycorrhizal (AM) fungal species were isolated and propagated from surface and deep rhizospheres of Faidherbia albida trees growing in two ecoclimatic zones of West Africa: the semi-arid Sahelian and the more humid Sudano-Guinean areas. Of these species, Glomus aggregatum, Glomus caledonium, and Glomus mosseae were trapped by F. albida roots when cultivated with either surface or deep soils. Glomus fasciculatum was found exclusively at the semi-arid Sahelian sites of Louga and Diokoul and Gigaspora margarita was isolated only from 16.5-m and 34-m-deep samples. Comparable glomalean fungal species richness was identified in deep (1.5–34 m) and surface (0.15 m) samples. The isolation and the propagation of glomalean fungi from F. albida rhizospheres confirmed the presence of viable AM fungal propagules, down to the water table, as deep as 34 m.

Arbuscular mycorrhizal fungal inoculation protects Miscanthus × giganteus against trace element toxicity in a highly metal-contaminated site

Arbuscular mycorrhizal fungus (AMF)-assisted phytoremediation could constitute an ecological and economic method in polluted soil rehabilitation programs. The aim of this work was to characterize the trace element (TE) phytoremediation potential of mycorrhizal Miscanthus × giganteus. To understand the mechanisms involved in arbuscular mycorrhizal symbiosis tolerance to TE toxicity, the fatty acid compositions and several stress oxidative biomarkers were compared in the roots and leaves of Miscanthus × giganteus cultivated under field conditions in either TE-contaminated or control soils. TEs were accumulated in greater amounts in roots, but the leaves were the organ most affected by TE contamination and were characterized by a strong decrease in fatty acid contents. TE-induced oxidative stress in leaves was confirmed by an increase in the lipid peroxidation biomarker malondialdehyde (MDA). TE contamination decreased the GSSG/GSH ratio in the leaves of exposed plants, while peroxidase (PO) and superoxide dismutase (SOD) activities were increased in leaves and in whole plants, respectively. AMF inoculation also increased root colonization in the presence of TE contamination. The mycorrhizal colonization determined a decrease in SOD activity in the whole plant and PO activities in leaves and induced a significant increase in the fatty acid content in leaves and a decrease in MDA formation in whole plants. These results suggested that mycorrhization is able to confer protection against oxidative stress induced by soil pollution. Our findings suggest that mycorrhizal inoculation could be used as a bioaugmentation technique, facilitating Miscanthus cultivation on highly TE-contaminated soil.

Effect of arbuscular mycorrhizal colonization of four species of glomus on physiological responses of maize

Abstract This greenhouse study aimed to analyze the impact of arbuscular mycorrhizal (AM) fungal associations on maize (Zea mays L. hybrid Pioneer 3905) in order to compare their functional compatibility and efficiency. The AM fungus species used for this study were Glomus aggregatum, G. etunicatum, G. mosseae, and G. versiforme. Shoot and leaf masses, chlorophyll, soluble protein, total and reducing sugar, carbon (C), and nitrogen (N) concentrations, and glutamine synthetase (GS) activity in the maize leaves were analyzed. The root colonization ranged from 26% to 72% depending on the AM fungus species. Leaf mass was significantly higher when maize plants were colonized with G. etunicatum in comparison to the non‐AM control. The mycorrhizal effect on dry leaf mass ranged from 15.9% to 23.9% depending on the AM species. However, the total shoot mass did not differ significantly among the treatments. The mycorrhizal treatment had a marginally significant effect on the chlorophyll concentrations in maize lea...

Endomycorrhizae in a newly cultivated acidic meadow: Effects of three years of barley cropping, tillage, lime, and phosphorus on root colonization and soil infectivity

The dynamics of mycorrhizae under disturbance created by crop production is not well understood. A 3-year experiment was undertaken on a nutrient-poor and acidic land that had last been cultivated in the early 1970s. We observed the effects of cropping spring barley (Hordeum vulgare L.) under four P-fertilizer levels and four levels of lime, in a minimum (rototillage), a reduced (chisel), or a conventional tillage system, on the mycorrhizal receptiveness of the host (maximum level of mycorrhizal colonization, as measured at harvest) and soil infectivity most probable number method. The host receptiveness decreased with time, while crop yields and soil infectivity increased simultaneously with time. Liming increased mycorrhizal colonization of barley roots and soil infectivity. P additions decreased root colonization but did not significantly affect the most probable number valuse. Slightly higher soil infectivity estimates were found under reduced tillage.

Development of Acaulospora rehmii spore and hyphal swellings under root-organ culture

A strain of Acaulospora rehmii was, for the first time, successfully grown in vitro on Ri T-DNA transformed carrot roots allowing the in situ observation of Acaulospora spore development and of extraradical thin-walled hyphal swellings. The sporogenous hypha developed intercalarly along thin-walled coenocytic hyphae. The distal part of the sporogenous hypha swelled slightly as a sporiferous saccule primordium followed by the differentiation of a lateral spore primordium along the neck of the sporogenous hypha. Both structures matured simultaneously, and the sporiferous saccule began to collapse after spore maturation and complete differentiation of the spore wall. Several of the in situ observations on in vitro differentiated A. rehmii spores are concordant with previous ontogenic studies done on other Acaulospora species obtained from in vivo cultures. New and original observations on the early developmental stages of sporiferous saccules and spores and on the occurrence of small diameter intercalary hyphal swellings provide additional elements in the study of Acaulospora sporulation process and life cycle.