Ectopic expression of serotonin N-hydroxycinnamoyltransferase and differential production of phenylpropanoid amides in transgenic tomato tissues

Tomato contains high levels of amines such as serotonin and tyramine and is a suitable host to enhance phenylpropanoid amides (PAs), an important class of nutraceuticals with strong antioxidant activity and chemotherapeutic effects, by ectopic expression of the corresponding gene, serotonin N-hydroxycinnamoyltransferase (SHT). To assess whether ectopic overexpression of SHT cDNA under the control of the CaMV 35S promoter would enhance levels of PAs, we generated transgenic tomato plants and analyzed the levels of PAs. Transgenic tomato plants exhibited increased synthesis of PAs such as feruloylserotonin (FS), 4-coumaroylserotonin (CS), feruloyltyramine (FT), 4-coumaroyltyramine (CT), and feruloyloctopamine (FO) in 1-month-old leaves compared to the wild type. The increase and relative levels of PAs were even more apparent in 3-month-old leaves of transgenic tomato. When tomato leaves were challenged by wounding, levels of PAs in the best transgenic line increased by 3- and 10-fold for CS + FS and CT + FT, respectively. In contrast to leaves, tomato fruit only showed enhanced synthesis of CT + FT, whereas CS + FS levels were not enhanced. Regarding amine content, the levels of tyramine were much higher than those of serotonin in tomato leaves and fruits. The high levels of tyramine may contribute to the preferential production of CT + FT rather than CS + FS, although SHT enzyme shows the highest substrate affinity toward serotonin rather than tyramine.