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.     

Expressed genes in the extraradical hyphae of an arbuscular mycorrhizal fungus, Glomus intraradices, in the symbiotic phase

To collect extraradical hyphae of arbuscular mycorrhizal (AM) fungi for RNA isolation, a PVDF membrane was laid on the hyphal compartment of a two-compartment culture system of transformed carrot hairy roots and Glomus intraradices. Extraradical hyphae free from host tissue were easily collected, and their RNA was rapidly extracted with a modified acid guanidinium thiocyanate-phenol-chloroform method. A 3'-RACE (rapid amplification of cDNA ends) of a known gene indicated that this protocol enabled the isolation of mRNA molecules as small as 2.3 kb. The cDNA libraries of an AM fungus from the aseptic extraradical hyphae in a symbiotic state were constructed for the first time. Three-fourth of 150 ESTs (expressed sequence tags) indicated low or no similarities to known sequences from other organisms.     

Glomus intraradices causes differential changes in amino acid and starch concentrations of in vitro strawberry subjected to water stress

The effect of colonization of tissue-cultured strawberry ( Fragaria × ananassa Duch. cv. Kent) plantlets in vitro by the arbuscular mycorrhizal fungus (AMF) Glomus intraradices on plantlet response to poly(ethylene glycol) (PEG)-8000-induced water stress was investigated. The plantlets were inoculated axenically and co-cultured with the AMF for 4 wk, then transferred to 15% PEG-8000 solutions for 4, 8 and 12 h. Relative water content, water potential, osmotic potential, leaf conductance for water vapour diffusion and photosynthetic efficiency as estimated by chlorophyll a fluorescence were all affected by the PEG treatment and its duration but not by the presence of the intraradical phase of the AMF. However, distinct differences in PEG-induced changes in amino acid content were observed between nonmycorrhizal and mycorrhizal plantlets. In the latter, the treatment with PEG caused a substantial decrease in asparagine levels in leaves that was accompanied by a marked increase in asparagine concentration in roots. The opposite was observed in nonmycorrhizal plantlets. Furthermore, concentrations of aspartic acid, serine, threonine, amino- N -butyric acid, alanine and starch increased in roots of mycorrhizal and decreased in nonmycorrhizal plantlets. Our results suggest the presence of a mobile pool of asparagine that can be translocated from leaves to roots or vice versa in response to PEG-induced water stress, depending on the mycorrhizal status of the plantlets. These opposite patterns suggest different strategies of mycorrhizal and nonmycorrhizal plantlets to water stress, which seem to involve different adjustments in nitrogen and carbon metabolism.     

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.     

The arbuscular mycorrhizal fungus, Glomus intraradices, induces the accumulation of cyclohexenone derivatives in tobacco roots

Tobacco (Nicotiana tabacum L.) plants were grown with and without the arbuscular mycorrhizal fungus, Glomus intraradices Schenk & Smith. High-performance liquid chromatographic analyses of methanolic extracts from mycorrhizal and non-mycorrhizal tobacco roots revealed marked fungus-induced changes in the patterns of UV-detectable products. The UV spectra of these products, obtained from an HPLC photodiode array detector, indicated the presence of several blumenol derivatives. The most predominant compound among these derivatives was spectroscopically identified as 13-hydroxyblumenol C 9-O-gentiobioside (“nicoblumin”), i.e. the 9-O-(6′-O-β-glucopyranosyl)-β-glucopyranoside of 13-hydroxy-6-(3-hydroxybutyl)-1,1,5-trimethyl-4-cyclohexen-3-one, a new natural product. This is the first report on the identification of blumenol derivatives in mycorrhizal roots of a non-gramineous plant. Received: 28 August 1998 / Accepted: 26 October 1998     

Effects of the mycorrhizal fungus Glomus intraradices on uranium uptake and accumulation by Medicago truncatula L. from uranium-contaminated soil

Phytostabilization strategies may be suitable to reduce the dispersion of uranium (U) and the overall environmental risks of U-contaminated soils. The role of Glomus intraradices, an arbuscular mycorrhizal (AM) fungus, in such phytostabilization of U was investigated with a compartmented plant cultivation system facilitating the specific measurement of U uptake by roots, AM roots and extraradical hyphae of AM fungi and the measurement of U partitioning between root and shoot. A soil-filled plastic pot constituted the main root compartment (C_A) which contained a plastic vial filled with U-contaminated soil amended with 0, 50 or 200 mg KH₂PO₄−P kg^–1soil (C_B). The vial was sealed by coarse or fine nylon mesh, permitting the penetration of both roots and hyphae or of just hyphae. Medicago truncatula plants grown in C_A were inoculated with G. intraradices or remained uninoculated. Dry weight of shoots and roots in C_A was significantly increased by G. intraradices, but was unaffected by mesh size or by P application in C_B. The P amendments decreased root colonization in C_B, and increased P content and dry weight of those roots. Glomus intraradices increased root U concentration and content in C_A, but decreased shoot U concentrations. Root U concentrations and contents were significantly higher when only hyphae could access U inside C_B than when roots could also directly access this U pool. The proportion of plant U content partitioned to shoots was decreased by root exclusion from C_B and by mycorrhizas (M) in the order: no M, roots in C_B > no M, no roots in C_B > M, roots in C_B > M, no roots in C_B. Such mycorrhiza-induced retention of U in plant roots may contribute to the phytostabilization of U contaminated environments.     

Thiols of Cu-treated maize plants inoculated with the arbuscular-mycorrhizal fungus Glomus intraradices

Mycorrhizal colonization of roots, fresh weight, content of cysteine, γ-glutamylcysteine (γEC). glutathione (GSH), thiol groups in Cu-binding peptides (CuBP), and the uptake of Cu were measured in roots and shoots of maize ( Zea mays L., cv. Honeycomb F-1) grown in quartz sand, with Cu at 0, 4.5, 9, 15 and 30 μg g⁻¹ added with or without inoculum of the arbuscular-mycorrhizal fungus (AMF) Glomus intraradices . In control plants (no Cu added) AMF significantly reduced shoot growth, but did not affect root growth. At an external Cu supply of 9 μg (g quartz sand)⁻¹ or higher, both mycorrhizal colonization and growth of roots and shoots of mycorrhizal and non-mycorrhizal plants were significantly reduced.
With up to 9 μg Cu g⁻¹, mycorrhizal colonization increased the content of cysteine, γEC and GSH in the roots. However, the amount of thiols in CuBPs was not increased by mycorrhizal colonization in Cu-treated plants and no differences in Cu uptake were detected between non-mycorrhizal and mycorrhizal plants. A CuBP-complex with a relative molecular mass of 7300 and a SH:Cu ratio of 1.77:1 was separated on a Sephadex G-50 column from both non-inoculated and inoculated roots of Cu-treated plants. HPLC chromatography of the CuBPs of both non-inoculated and inoculated roots resulted in a similar peak pattern, indicating that no additional CuBPs were formed by the fungus. In conclusion, our results do not support the idea that AMF protects maize from Cu-toxicity.     

Cold-storage of mixed inoculum of Glomus intraradices enhances root colonization,phosphorus status and growth of hot pepper

Growth response of hot pepper (Capsicum annuum L.) inoculated with the arbuscular mycorrhizal (AM) fungus, Glomus intraradices Schenck and Smith was evaluated in a greenhouse study. Three treatments in a soil-based medium amended with rock phosphate were: (1) control (CON), (2) inoculation of G. intraradices as a freshly prepared soil mixture of spores, hyphae and colonized roots of Sorghum vulgare (FM), and (3) inoculation of the fungus as cold-stored mixed inoculum (CM). Colonization at 14 weeks after inoculation with CM was 42.5%, but was significantly lower with FM (14.5%). Inoculation with G. intraradices as FM and CM increased growth of pepper, and total phosphorus and nitrogen uptake in shoots and roots compared with the CON treatment. Inoculation with CM resulted in significant increases in plant dry weight and chlorophyll concentration compared to the FM and CON treatments. Acid phosphatase activity in the rhizosphere was generally increased by AM fungal treatments. Highest acid phosphatase activity occurred at 14 weeks after inoculation with CM. Alkaline phosphatase activity in the CM treatment was significantly higher compared to that in CON and FM treatments throughout the growth period. Thus, cold storage of mixed inoculum enhanced colonization and growth-promoting activity of G. intraradices compared to freshly prepared inoculum.     

Alleviation of salt stress in Lotus glaber by Glomus intraradices

Lotus glaber is a glycophytic, perennial legume from Europe that occurs widely in saline habitats. We evaluated the effect of mycorrhizal fungus colonization on the response to salt stress of two genotypes of L. glaber differing in their tolerance to salinity. The experiment consisted of a randomized block design with two factors: (1) mycorrhizal fungus treatments (with or without AM fungus) and (2) two salinity levels of 0 and 200 mM NaCl. Our results indicated that Glomus intraradices established a more efficient symbiosis with the tolerant than with the sensitive genotype. G. intraradices improved growth of L. glaber plants under saline conditions. They showed higher values of net growth, shoot/root and K⁺/Na⁺ ratios, and protein concentrations than controls. Tolerant AM plants also showed higher chlorophyll levels than non-AM ones. Prevention of Na⁺ accumulation in the plant and enhancement of K⁺ concentrations in roots observed in this work could be part of the general mechanism of salt stress alleviation of L. glaber by G. intraradices.     

Interactions of Paecilomyces lilacinus strain 251 with the mycorrhizal fungus Glomus intraradices: Implications for Meloidogyne incognita control on tomato

The interactions of Paecilomyces lilacinus strain 251 with the arbuscular mycorrhizal fungus Glomus intraradices and their significance for the control of Meloidogyne incognita on tomato were investigated in greenhouse experiments. Application of P. lilacinus had no effect on the frequency and intensity of tomato root colonization by G. intraradices. Likewise, the decline of the nematophagous fungus densities after single application in soil was not affected by the presence of the mycorrhizal fungus. Single application of P. lilacinus, as pre-planting soil treatment, resulted in significant reduction of nematode damage. In contrast, mycorrhizal inoculation did not provide sufficient biocontrol. Combined application of the two agents did not enhance root protection compared to single treatments. Double treatment of mycorrhized seedlings with P. lilacinus, as seedling drench and pre-planting soil treatment, 4 and 1 week before transplanting, respectively, resulted in the highest reduction of the nematode damage. These results indicate the potential of the commercial P. lilacinus strain 251 and mycorrhiza for integration in nematode control strategies.