Translocation and Utilization of Fungal Storage Lipid in the Arbuscular Mycorrhizal Symbiosis

Abstract

The arbuscular mycorrhizal (AM) symbiosis is responsible for huge fluxes of photosynthetically fixed carbon from plants to the soil. Carbon is transferred from the plant to the fungus as hexose, but the main form of carbon stored by the mycobiont at all stages of its life cycle is triacylglycerol. Previous isotopic labeling experiments showed that the fungus exports this storage lipid from the intraradical mycelium (IRM) to the extraradical mycelium (ERM). Here, in vivo multiphoton microscopy was used to observe the movement of lipid bodies through the fungal colony and to determine their sizes, distribution, and velocities. The distribution of lipid bodies along fungal hyphae suggests that they are progressively consumed as they move toward growing tips. We report the isolation and measurements of expression of an AM fungal expressed sequence tag that encodes a putative acyl-coenzyme A dehydrogenase; its deduced amino acid sequence suggests that it may function in the anabolic flux of carbon from lipid to carbohydrate. Time-lapse image sequences show lipid bodies moving in both directions along hyphae and nuclear magnetic resonance analysis of labeling patterns after supplying 13 C-labeled glycerol to either extraradical hyphae or colonized roots shows that there is indeed significant bidirectional translocation between IRM and ERM. We conclude that large amounts of lipid are translocated within the AM fungal colony and that, whereas net movement is from the IRM to the ERM, there is also substantial recirculation throughout the fungus.

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

“…Later on, storage lipids are synthesized. The most abundant form of lipid in AM fungi are triacylglycerols, which also serve as a carbon transport form in the fungus (Bago et al, 2002). In the symbiotic stage, carbohydrates can only be taken up within intraradical structures, no uptake of hexoses could be detected by the extraradical mycelium (Pfeffer et al, 1999;Douds et al, 2000).…”

Dependence of Arbuscular-Mycorrhizal Fungi on Their Plant Host for Palmitic Acid Synthesis

“…This carbon is stored primarily as lipids (5,28), with smaller amounts of carbohydrates stored as glycogen (3,12) and trehalose (1,6). For example, up to 70% of the dry weight of Glomus caledonius is composed of lipids, predominantly triglycerides (9).…”

Symbiotic Status, Phosphate, and Sucrose Regulate the Expression of Two Plasma Membrane H+-ATPase Genes from the Mycorrhizal Fungus Glomus mosseae

“…Several approaches have indicated that glucose may be preferred over fructose or sucrose as carbohydrate imported by the fungus from the apoplastic space (Saito, 1995;Shachar-Hill et al, 1995;Solaiman and Saito, 1997;Pfeffer et al, 1999). This is then converted to lipid for transfer within the external mycelium (Pfeffer et al, 1999;Bago et al, 2002). There, it serves to feed this side of the fungal colony that has limitations for the use of externally supplied carbon sources similar to the early developmental phase (Pfeffer et al, 1999).…”