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

Abstract

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 ...

Phosphate Treatment Strongly Inhibits New Arbuscule Development But Not the Maintenance of Arbuscule in Mycorrhizal Rice Roots

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Regulation of Arbuscular Mycorrhization by Carbon. The Symbiotic Interaction Cannot Be Improved by Increased Carbon Availability Accomplished by Root-Specifically Enhanced Invertase Activity

“…However, in these experiments with axenic root cultures the authors observed due to feeding of higher Suc amounts an increased carbon transfer from the root to the fungus that was suggested to stimulate P uptake and transfer by the fungus. Here, no indices for a higher carbon transfer from the plant to the AM fungus, deduced from the content of fungus-specific C16:1D 11 and its precursor C16:0 (Trépanier et al, 2005; van Aarle and Olsson, 2005), were found, although levels of available carbon increased. Moreover, the finding of Bü cking and Shachar-Hill (2005) was obtained from experiments with axenic cultures of transformed carrot Figure 5.…”

Sanctions, Cooperation, and the Stability of Plant-Rhizosphere Mutualisms

“…We now know that AMF strains differ dramatically in metabolic strategies such as amount of C extracted from their hosts (Jakobsen et al 1992), amount of lipids allocated to storage (van Aarle & Olsson 2003), the transfer of P to their host plants (Boddington & Dodd 1999), C transport capacities across symbiotic interfaces (Dickson et al 1999), and colonization and hyphal-length allocation (Smith et al 2000, Hart & Reader 2005. Fungal allocation strategies can determine benefits conferred to the host.…”