title: Carbon export from arbuscular mycorrhizal roots involves the translocation of carbohydrate as well as lipid.
abstract: Arbuscular mycorrhizal (AM) fungi take up photosynthetically fixed carbon from plant roots and translocate it to their external mycelium. Previous experiments have shown that fungal lipid synthesized from carbohydrate in the root is one form of exported carbon. In this study, an analysis of the labeling in storage and structural carbohydrates after (13)C(1) glucose was provided to AM roots shows that this is not the only pathway for the flow of carbon from the intraradical to the extraradical mycelium (ERM). Labeling patterns in glycogen, chitin, and trehalose during the development of the symbiosis are consistent with a significant flux of exported glycogen. The identification, among expressed genes, of putative sequences for glycogen synthase, glycogen branching enzyme, chitin synthase, and for the first enzyme in chitin synthesis (glutamine fructose-6-phosphate aminotransferase) is reported. The results of quantifying glycogen synthase gene expression within mycorrhizal roots, germinating spores, and ERM are consistent with labeling observations using (13)C-labeled acetate and glycerol, both of which indicate that glycogen is synthesized by the fungus in germinating spores and during symbiosis. Implications of the labeling analyses and gene sequences for the regulation of carbohydrate metabolism are discussed, and a 4-fold role for glycogen in the AM symbiosis is proposed: sequestration of hexose taken from the host, long-term storage in spores, translocation from intraradical mycelium to ERM, and buffering of intracellular hexose levels throughout the life cycle.
authors.affiliation: Eastern Regional Research Center (US Department of Agriculture/Agricultural Research Service), Wyndmoor, Pennsylvania 19038, USA.
authors.forename: Berta
authors.initials: B
authors.lastname: Bago
authors.forename: Philip E
authors.initials: PE
authors.lastname: Pfeffer
authors.forename: Jehad
authors.initials: J
authors.lastname: Abubaker
authors.forename: Jeongwon
authors.initials: J
authors.lastname: Jun
authors.forename: James W
authors.initials: JW
authors.lastname: Allen
authors.forename: Janine
authors.initials: J
authors.lastname: Brouillette
authors.forename: David D
authors.initials: DD
authors.lastname: Douds
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Lammers
authors.forename: Yair
authors.initials: Y
authors.lastname: Shachar-Hill
mesh_terms: D015061:1,4-alpha-Glucan Branching Enzyme / Q000378:metabolism; D000595:Amino Acid Sequence; D001692:Biological Transport / Q000502:physiology; D050260:Carbohydrate Metabolism; D002244:Carbon / Q000378:metabolism*; D002247:Carbon Isotopes; D002686:Chitin / Q000096:biosynthesis; D002687:Chitin Synthase / Q000378:metabolism; D015971:Gene Expression Regulation, Enzymologic; D005947:Glucose / Q000378:metabolism; D005945:Glutamine-Fructose-6-Phosphate Transaminase (Isomerizing) / Q000378:metabolism; D006003:Glycogen / Q000096:biosynthesis; D006006:Glycogen Synthase / Q000235:genetics / Q000378:metabolism; D050356:Lipid Metabolism; D008969:Molecular Sequence Data; D025282:Mycelium / Q000378:metabolism; D038821:Mycorrhizae / Q000235:genetics / Q000254:growth & development* / Q000378:metabolism; D018517:Plant Roots / Q000378:metabolism* / Q000382:microbiology; D017386:Sequence Homology, Amino Acid; D013559:Symbiosis / Q000502:physiology; D014199:Trehalose / Q000096:biosynthesis
chemical_list: D002247:Carbon Isotopes; D002686:Chitin; D002244:Carbon; D006003:Glycogen; D014199:Trehalose; D006006:Glycogen Synthase; D002687:Chitin Synthase; D015061:1,4-alpha-Glucan Branching Enzyme; D005945:Glutamine-Fructose-6-Phosphate Transaminase (Isomerizing); D005947:Glucose