Carbon uptake and the metabolism and transport of lipids in an arbuscular mycorrhiza

Both the plant and the fungus benefit nutritionally in the arbuscular mycor rhizal symbiosis: The host plant enjoys enhanced mineral uptake and the fun gus receives fixed carbon. In this exchange the uptake, metabolism, and tra nslocation of carbon by the fungal partner are poorly understood. We theref ore analyzed the fate of isotopically labeled substrates in an arbuscular m ycorrhiza (in vitro cultures of Ri T-DNA-transformed carrot [Daucus carota] roots colonized by Glomus intraradices) using nuclear magnetic resonance s pectroscopy. Labeling patterns observed in lipids and carbohydrates after s ubstrates were supplied to the mycorrhizal roots or the extraradical myceli um indicated that: (a) C-13-labeled glucose and fructose (but not mannitol or succinate) are effectively taken up by the fungus within the root and ar e metabolized to yield labeled carbohydrates and lipids; (b) the extraradic al mycelium does not use exogenous sugars for catabolism, storage, or trans fer to the host; (c) the fungus converts sugars taken up in the root compar tment into lipids that are then translocated to the extraradical mycelium ( there being little or no lipid synthesis in the external mycelium); and (d) hexose in fungal tissue undergoes substantially higher fluxes through an o xidative pentose phosphate pathway than does hexose in the host plant.