The metabolism of [1-13 C]glucose in Pisolithus tinctorius cv Coker & Couch, in uninoculated seedlings of Eucalyptus globulus bicostata ex Maiden cv Kirkp., and in the E. globulus-P. tinctorius ectomycorrhiza was studied using nuclear magnetic resonance spectroscopy. In roots of uninoculated seedlings, the 13 C label was mainly incorporated into sucrose and glutamine. The ratio ( 13 C3 ؉ 13 C2)/ 13 C4 of glutamine was approximately 1.0 during the timecourse experiment, indicating equivalent contributions of phosphoenolpyruvate carboxylase and pyruvate dehydrogenase to the production of ␣-ketoglutarate used for synthesis of this amino acid. In free-living P. tinctorius, most of the 13 C label was incorporated into mannitol, trehalose, glutamine, and alanine, whereas arabitol, erythritol, and glutamate were weakly labeled. Amino acid biosynthesis was an important sink of assimilated 13 C (43%), and anaplerotic CO 2 fixation contributed 42% of the C flux entering the Krebs cycle. In ectomycorrhizae, sucrose accumulation was decreased in the colonized roots compared with uninoculated control plants, whereas 13 C incorporation into arabitol and erythritol was nearly 4-fold higher in the symbiotic mycelium than in the free-living fungus. It appears that fungal utilization of glucose in the symbiotic state is altered and oriented toward the synthesis of short-chain polyols.Carbohydrate metabolism in ectomycorrhizae has received considerable attention (for review, see Hampp and Schaeffer, 1995; Smith and Read, 1997). Using carbohydrates for storage, for increasing biomass, and for conversion into metabolic energy, ectomycorrhizal fungi create strong assimilate sinks. Photoassimilates move into the phloem of trees primarily as Suc and reach the ectomycorrhizal tissues in this form (Jakobsen, 1991; Hampp and Schaeffer, 1995). Suc is the main labeled carbohydrate in root cells but is not detected in symbiotic fungal tissues, where mannitol, trehalose, and glycogen are the main labeled carbohydrates (Sö derströ m et al., 1988; Hampp and Schaeffer, 1995; Smith and Read, 1997). Glc resulting from Suc catabolism is thought to be the primary source of carbon for the generation of ATP, reducing power, and carbon skeletons for biosynthetic pathways in ectomycorrhizae (Hampp and Schaeffer, 1995). The metabolic pathways leading to the synthesis of major fungal carbohydrates such as mannitol and trehalose have been characterized in several free-living ectomycorrhizal fungi (Martin et al., 1985(Martin et al., , 1988 Ramstedt et al., 1989). These carbohydrates have also been found in ectomycorrhizae (Ineichen and Wiemken, 1992), but metabolic routes converting Suc to fungal carbohydrates and other metabolites in symbiotic tissues have not been characterized.There is evidence that ectomycorrhizal symbiosis brings about considerable modification of carbon metabolism in the host roots and in the mycobiont forming the association (Martin et al., 1987; Hampp and Schaeffer, 1995). An important question in relation to the physiology of e...
