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

本研究旨在阐明ECM真菌孢子萌发和外生菌根发育早期脂质代谢的机制。首先，我们研究了15属32种55株外生菌根真菌在饱和（棕榈酸）、单不饱和（油酸）、双不饱和（亚油酸）、三不饱和（亚麻酸）脂肪酸和油酸三甘油酯（三油酸）上的生长情况，以阐明其利用脂肪酸和脂质作为生长碳源的能力。根据菌丝生长对葡萄糖的相对利用率（URs, %）表明，属于象科的ECM真菌具有利用棕榈酸盐的能力。另一方面，Laccaria属的ECM真菌具有至少利用棕榈酸盐或油酸盐的能力。此外，水蚤生长在棕榈酸盐、油酸盐和少量三油酸盐上。乳酸菌只生长在棕榈酸盐上。利用脂肪酸和脂质的真菌有可能成为进一步阐明利用脂肪酸和脂质作为碳源的代谢能力的模式真菌。相反，Suillus属真菌几乎不利用脂肪酸和脂质。此外，大多数ECM真菌在亚油酸盐和亚麻酸盐上都不能生长。其次，对外生菌根真菌孢子中乙醛酸循环关键酶、异柠檬酸裂解酶、苹果酸合成酶、异柠檬酸脱氢酶和β-氧化酰基辅酶a脱氢酶的活性进行了检测。第三，我们进行了模拟外生菌根真菌生长的模型研究，在外生菌根发育的早期，Laczko等人提出脂质是由寄主植物提供给真菌的。因此，本文研究了外生菌根真菌紫丝瓜菌（Laccaria amethystea）自由生长过程中脂质代谢酶活性和储存碳水化合物含量对三油酸脂质的影响。将葡萄糖培养的菌丝转移到以三油酸为碳源的培养基中，其酰基辅酶a脱氢酶、异柠檬酸裂解酶和苹果酸合成酶的比活性分别比葡萄糖培养的菌丝提高了340%、780%和340%，但在24 h后逐渐降低。在三油酸培养基上生长时，菌丝中糖原、海藻糖和葡萄糖的含量迅速下降，菌丝数量仅为葡萄糖培养基的29%。此外，在含有三油酸和葡萄糖的培养基中生长的菌丝中，上述三种酶的活性均受到抑制。结果表明，在只有脂质作为碳源的情况下，紫百合仅利用脂质进行短暂生长。综上所述，脂质代谢在ECM真菌有限的生命周期中是重要的，因为代谢受到葡萄糖引起的分解代谢抑制的调节。少

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