Abstract: Five strains of Pleurotus ostreatus were screened for the production of ergosterol as a?vitamin D?precursor in shake flask culture. The optimization of medium components for higher ergosterol content in P. ostreatus mycelia in submerged culture was carried out using response surface methodology. The fruit body resulting from approximately 40-d culture of the mycelia was irradiated with ultraviolet light (280 nm). Then the contents of ergosterol and VD2 were synchronously detected by high performance liquid chromatography (HPLC) and the effect of the cellular structures of different fruit body tissues on ergosterol conversion to VD2 were also explored by field emission scanning electron microscope (FESEM). The results showed that the ergosterol content of strain P831 was up to 3.25 mg/g, higher than those of the other four strains. Therefore, it was used for the following experiments. The Plackett-Burman design indicated that peptone, sucrose, potassium phosphate dihydrogen were key medium constituents affecting the mycelial ergosterol content. Under the optimized conditions of peptone 0.35%, sucrose 3.48% and potassium phosphate dihydrogen 0.35%, the mycelial ergosterol content reached its maximum value of 3.64 mg/g, 12% higher than that before optimization. The contents of ergosterol and VD2 in the cap and lamella were higher than in the stipe. The results of FESEM showed that the cap contained plentiful amounts of mycelia, forming a clamp-like structure; the lamella was covered with large quantities of dense spores; and the stipe was dominated by a fiber-like tubular network structure with no spores being observed. As the mycelial clamp-like structure and spores in the fruit body of Pleurotus edodes are the main tissues with strong cell division ability and metabolism, we speculate that the differences in the contents of ergosterol and VD2 among fruit body tissues may be related to the morphological structure and cellular metabolism.

Key words: cultivation of Pleurotus ostreatus, ergosterol, vitamin D2, strain screening, response surface methodology, light-induced transformation, field emission scanning electron microscope