In the current research, a 60-day experiment was conducted with the purpose of exploring the impacts of methionine on growth performance, muscle nutritive deposition, muscle fibre growth and type I collagen synthesis as well as related signaling pathway. Six diets (iso-nitrogenous) differing in methionine concentrations (2.54, 4.85, 7.43, 10.12, 12.40 and 15.11 g kg-1 diets) were fed to 540 grass carp (178.47 ± 0.36 g). Results showed (P < 0.05) that, compared with methionine deficiency, optimal level of dietary methionine (1) increased feed intake (FI), feed efficiency (FE), specific growth rate (SGR) and percentage weight gain (PWG); (2) increased fish muscle protein, lipid and free amino acid contents, and improved fish muscle fatty acid profile as well as increased protein content in part associated with TORC1/S6K1 signaling pathway; (3) increased the frequency distribution of muscle fibre with >50 µm of diameter; (4) increased type I collagen synthesis partly related to TGF-β1/Smads and CK2/TORC1 signaling pathways. In conclusion, dietary methionine improved muscle growth, which might be due to the regulation of muscle nutritive deposition, and muscle fibre growth and type I collagen synthesis related signal molecules. Finally, according to PWG and muscle collagen content, the methionine requirements for on-growing grass carp (178-626 g) were estimated to be 9.56 g kg-1 diet (33.26 g kg-1 protein of diet) and 9.28 g kg-1 diet (32.29 g kg-1 of dietary protein), respectively.

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