Summary• To test the response of arbuscular mycorrhizal (AM) fungi to a difference in soil pH, the extraradical mycelium of Scutellospora calospora or Glomus intraradices, in association with Plantago lanceolata, was exposed to two different pH treatments, while the root substrate pH was left unchanged.• Seedlings of P. lanceolata, colonized by one or other of the fungal symbionts, and nonmycorrhizal controls, were grown in mesh bags placed in pots containing pHbuffered sand (pH around 5 or 6). The systems were harvested at approximately 2-wk intervals between 20 and 80 d.• Both fungi formed more extraradical mycelium at the higher pH. Glomus intraradices formed almost no detectable extraradical mycelium at lower pH. The extraradical mycelium of S. calospora had higher acid phosphatase activity than that of G. intraradices. Total AM root colonization decreased for both fungi at the higher pH, and high pH also reduced arbuscule and vesicle formation in G. intraradices.• In conclusion, soil pH influences AM root colonization as well as the growth and phosphatase activities of extraradical mycelium, although the two fungi responded differently.

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 16:15 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 16:15 is a good indicator for AM fungal development. The phospholipid fatty acid 16:15 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.Fungi in the Glomeromycota are obligate symbionts, which colonize host plant roots from spores, extraradical hyphae, or previously colonized roots. Hyphae grow from colonized roots into the soil and form the extraradical mycelium. Lipid droplets in the hyphae accumulate in the developing spores (3, 7). The extraradical mycelium may spread along the root to form new entry points, but usually spreads out from the host root to form an extensive extraradical mycelium (9,25). From the point of mycorrhizal colonization, intercellular (Arum-type colonization) or intracellular (Paris-type colonization) hyphae spread into the root and side branches of hyphae and produce arbuscules, finely branched hyphal structures surrounded by the host plasma membrane (30). In this way, a large area of contact with the host is created, increasing the area of the host plasma membrane up to 10-fold in colonized cells (31). Arbuscules are short-lived structures believed to have a turnover rate of 1 to 2 weeks (1) and probably are a critical site for nutrient transfer between the symbionts (30). At a later stage, the fungus may form vesicles, which are lipid-filled storage structures (7, 20) with a low turnover rate, in intercellular spaces. Typical for Glomus intraradices is that this fungus can also form intraradical spores, which may have a storage function as well. Lipids may, however, also be transported in lipid bodies to the extraradical mycelium within a few days (3, 11).The amount of neutral lipids (for energy storage) is usually higher ...

The influence of external phosphorus (P) on carbon (C) allocation and metabolism as well as processes related to P metabolism was studied in monoxenic arbuscular mycorrhiza cultures of carrot (Daucus carota). Fungal hyphae of Glomus intraradices proliferated from the solid minimal medium containing the colonized roots into C-free liquid minimal medium with different P treatments. The fungus formed around three times higher biomass in P-free liquid medium than in medium with 2.5 mm inorganic P (high-P). Mycelium in the second experiment was harvested at an earlier growth stage to study metabolic processes when the mycelium was actively growing. P treatment influenced the root P content and [13 C]glucose administered to the roots 7 d before harvest gave a negative correlation between root P content and 13 C enrichment in arbuscular mycorrhiza fungal storage lipids in the extraradical hyphae. Eighteen percent of the enriched 13 C in extraradical hyphae was recovered in the fatty acid 16:15 from neutral lipids. Polyphosphate accumulated in hyphae even in P-free medium. No influence of P treatment on fungal acid phosphatase activity was observed, whereas the proportion of alkaline-phosphatase-active hyphae was highest in high-P medium. We demonstrated the presence of a motile tubular vacuolar system in G. intraradices. This system was rarely seen in hyphae subjected to the highest P treatment. We concluded that the direct responses of the extraradical hyphae to the P concentration in the medium are limited. The effects found in hyphae seemed instead to be related to increased availability of P to the host root.Arbuscular mycorrhizal (AM) association is the only way for fungi in the order Glomales to proliferate and reproduce (Bécard and Fortin, 1988; Smith and Read, 1997; Bago et al., 2000). It is well known that C is transferred from colonized plants to AM fungi (Ho and Trappe, 1973), whereas the plants in many cases receive most of their P through hyphal uptake and fungal transfer to the host root (Pearson and Jakobsen, 1993). Colonization by AM fungi increases the C sink strength of roots (Douds et al., 1988). The fungal C demand upon root colonization can constitute a significant cost to the host plant, as indicated by reduced growth at high P levels (Peng et al., 1993). This also implies that there is an important connection between external P supply and the regulation of C allocation to the fungal partner in the symbiosis.C metabolism of the AM fungus Glomus intraradices has been studied using 13 C-NMR in monoxenic cultures (Pfeffer et al., 1999). Although no hexose uptake occurs in the extraradical mycelium, intraradical AM fungal mycelium takes up C supplied as hexoses to the root. Triacylglycerols are synthesized from this C in the fungus and transported to the extraradical mycelium. These triacylglycerols are substantial sinks for C in the AM fungal mycelium (Bago et al., 2000; Olsson and Johansen, 2000).Formation of AM is important for P acquisition in most plants. The adverse effect of high soil P le...

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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 16:15 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 13 C enrichment of 16:15 and compared it with 13 C enrichment of total root and mycelial C. Carbon allocation to mycelia was detected within 1 day in monoxenic arbuscular mycorrhizal root cultures labeled with [13 C]glucose. The 13 C enrichment of neutral lipid fatty acid 16:15 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 13 C in the arbuscular mycorrhizal fungal neutral lipid fatty acid 16:15 than for the root specific neutral lipid fatty acid 18:26,9. We labeled plant assimilates by using 13 CO 2 in whole-plant experiments. The extraradical mycelium often was more enriched for 13 C 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 13 C enrichment in neutral lipid fatty acid 16:15 and total 13 C in extraradical mycelia in different systems (r 2 ‫؍‬ 0.94), we propose that the total amount of labeled C in intraradical and extraradical mycelium can be calculated from the 13 C enrichment of 16:15. 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.Arbuscular mycorrhizal (AM) fungal mycelia acquire hexoses released by the roots of their host (3,39,42,43) and metabolize them to lipids, mainly neutral lipids, such as triacylglycerols (39). Neutral lipids are transported throughout the fungal mycelium (4), are metabolized through the glyoxalate cycle (3,24), and probably provide the major fungal energy source. The mechanisms that regulate C transfer from plant to fungus are not well understood (21). However, AM fungal colonization affects plant C metabolism (13,38,51,52) and the genes that regulate this metabolism (20,40).AM fungal neutral lipids usually are stored in intraradical vesicles or in spores and make up a large proportion of the AM fungal biomass (6,22,36). The fatty acids of these lipids have a characteristic and specific composition (7). In Glomus intraradices, 50 to 70% of the neutral lipids are the fatty acid 16:15 (19, 35), which is uncommon in other groups of fungi (28,32,47) and can be used as an AM fungal signature (31, 37).13 C nuclear magnetic resonance has been used to determine C-metabolic pathways after 13 C labeling of monoxenic root cultures (3). The incorporation and turnover of C in AM fungal mycelium can be difficult to measure because the hyphae are not easily extracted and separated from roots or soil.13 C enrichment or 13 C dilution of signature compounds, such as ...