The glyoxylate cycle in an arbuscular mycorrhizal fungus. Carbon flux and gene expression

The arbuscular mycorrhizal (AM) symbiosis is responsible for huge fluxes of photosynthetically fixed carbon from plants to the soil. Lipid, which is t he dominant form of stored carbon in the fungal partner and which fuels spo re germination, is made by the fungus within the root and is exported to th e extraradical mycelium. We tested the hypothesis that the glyoxylate cycle is central to the flow of carbon in the AM symbiosis. The results of C-13 labeling of germinating spores and extraradical mycelium with C-13(2)-aceta te and C-13(2)-glycerol and analysis by nuclear magnetic resonance spectros copy indicate that there are very substantial fluxes through the glyoxylate cycle in the fungal partner. Full-length sequences obtained by polymerase chain reaction from a cDNA library from germinating spores of the AM fungus Glomus intraradices showed strong homology to gene sequences for isocitrat e lyase and malate synthase from plants and other fungal species. Quantitat ive real-time polymerase chain reaction measurements show that these genes are expressed at significant levels during the symbiosis. Glyoxysome-like b odies were observed by electron microscopy in fungal structures where the g lyoxylate cycle is expected to be active, which is consistent with the pres ence in both enzyme sequences of motifs associated with glyoxysomal targeti ng. We also identified among several hundred expressed sequence tags severa l enzymes of primary metabolism whose expression during spore germination i s consistent with previous labeling studies and with fluxes into and out of the glyoxylate cycle.