Overexpression of the maize γ-tocopherol methyltransferase gene (ZmTMT) increases α-tocopherol content in transgenic Arabidopsis and maize seeds

The vitamin E family includes tocopherols and tocotrienols, which are essential lipid-soluble antioxidants necessary for human and livestock health. The seeds of many plant species, including maize, have high gamma (gamma)-tocopherol but low alpha (alpha)-tocopherol contents; however, alpha-tocopherol is the most effective antioxidant. Therefore, it is necessary to optimize the tocopherol composition in plants. alpha-Tocopherol is synthesized from gamma-tocopherol by gamma-tocopherol methyltransferase (gamma-TMT, VTE4) in the final step of the tocopherol biosynthetic pathway. In the present study, the full-length coding sequence (CDS) of gamma-TMT was isolated from Zea mays, named ZmTMT. The ZmTMT CDS was 1059 bp in size, encoding 352 amino acids. Recombinant ZmTMT was expressed in Escherichia coli and the purified protein effectively converted gamma-tocopherol into alpha-tocopherol in vitro. A comparison of enzyme activities showed that the activity of ZmTMT was higher than that of GmTMT2a (Glycine max) and AtTMT (Arabidopsis thaliana). Overexpression of ZmTMT increased the alpha-tocopherol content 4-5-fold in transgenic Arabidopsis and around 6.5-fold in transgenic maize kernels, and increased the alpha-/gamma-tocopherol ratio to approximately 15 and 17, respectively. These results show that it is feasible to overexpress ZmTMT to optimize the tocopherol composition in maize; such a corn product might be useful in the feed industry in the near future.