Lipid metabolism of ectomycorrhizal fungi during their spore germination and the early period of ectomycorrhizal development

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The purpose of the present study was to elucidate the mechanisms for lipid metabolism during spore germination of ECM fungus and early period of ectomycorrhizal development.At first, we have investigated growth of ectomycorrhizal(ECM) fungi, i.e. 55 strains of 32 species in 15 genera, on saturated (palmitate), monounsaturated (oleate), diunsaturated (linoleate), triunsaturated (linolenate) fatty acids and the triacyiglyceride of oleate (triolein) in order to elucidate an ability to utilize fatty acids and lipid as a carbon source for growth. Relative utilization ratios (URs, %) obtained on the basis of mycelial growth on glucose suggest that ECM fungi belonging to the family Thelephoraceae have an ability to utilize palmitate. On the other hand, ECM fungi in the genus Laccaria have an ability to utilize at least either palmitate or oleate. Furthermore, Hygropharus russula grows on palmitate, oleate and slightly on triolein. Lactarius chrysorrheus grows only on palmitate. These fatty ac … More ids- and lipid-utilizing fungi may be promising as model fungi for further elucidation of the metabolic ability to utilize fatty acids and lipid as a carbon source. On the contrary, the fungi in genus Suillus were shown to scarcely utilize fatty acids and lipid. Furthermore, most ECM fungi did not grow on either linoleate or linolenate.Secondly, we have detected the activities of glyoxylate cycle key enzymes, isocitrate lyase, malate synthase, isocitrate dehydrogenase in TCA cycle and acyl-CoA dehydrogenase in β-oxidation from spore of ectomycorrhizal fungi.Thirdly, we have conducted a model study simulating the growth of ectomycorrial fungus in early period of ectomycorrhizal development during which Laczko et al. proposed that lipid is supplied from host plant to the fungus. Thus, the changes in activities of enzymes involved in lipid metabolism and in contents of storage carbohydrates during free-living growth of ectomycorrhizal fungus Laccaria amethystea on triolein lipid were investigated. Transferring glucose-grown mycelia to a medium containing triolein as a carbon source increased the specific activities of acyl-CoA dehydrogenase, isocitrate lyase, and malate synthase up to 340%,780% and 340%, respectively, of those detected in mycelia grown on glucose, but they decreased with in 24 hrs. The contents of glycogen, trehalose and glucose in mycelia decreased rapidly during the growth on triolein and the amount of mycelia reached only 29% of that grown on glucose. Furthermore, the activities of the above three enzymes were repressed in mycelia grown in medium containing both triolein and glucose. The results suggest that L. amethystea merely utilized lipid for temporal growth when only lipid was available as a carbon source.In conclusion, lipid metabolism is suggested to be important in a limited period of life cycle of ECM fungus, because the metabolism is regulated by catabolite repression caused by glucose. Less