Carbon metabolism in spores of the arbuscular mycorrhizal fungus Glomus intraradices as revealed by nuclear magnetic resonance spectroscopy

Arbuscular mycorrhizal (AM) fungi are obligate symbionts that colonize the roots of over 80% of plants in all terrestrial environments. Understanding why AM fungi do not complete their life cycle under free-living conditions has significant implications for the management of one of the world's most important symbioses. We used C-13-labeled substrates and nuclear magnetic r esonance spectroscopy to study carbon fluxes during spore germination and t he metabolic pathways by which these fluxes occur in the AM fungus Glomos i ntraradices. Our results indicate that during asymbiotic growth: (a) sugars are made from stored lipids; (b) trehalose (but not lipid) is synthesized as well as degraded; (c) glucose and fructose, but not mannitol, can be tak en up and utilized; (d) dark fixation of CO2 is substantial; and (e) argini ne and other amino acids are synthesized. The labeling patterns are consist ent with significant carbon fluxes through gluconeogenesis, the glyoxylate cycle, the tricarboxylic acid cycle, glycolysis, non-photosynthetic one-car bon metabolism, the pentose phosphate pathway, and most or all of the urea cycle. We also report the presence of an unidentified betaine-like compound . Carbon metabolism during asymbiotic growth has features in between those presented by intraradical and extraradical hyphae in the symbiotic state.