Enhanced synthesis of feruloyltyramine and 4-coumaroyltyramine is associated with tyramine availability in transgenic rice expressing pepper tyramine N-hydroxycinnamoyltransferase

Tyramine N-hydroxycinnamoyltransferase (THT) is a 28-kDa soluble protein that catalyzes the condensation of hydroxycinnamates and tyramine via the thioesters. The representative products synthesized by THT include feruloyltyramine (FT) and 4-coumaroyltyramine (CT). The THT gene of Capsicum annuum was introduced into the rice genome using Agrobacterium-mediated transformation. Eight independent transgenic rice plants were selected and characterized. The transgenic lines had a single- or two-copy insertion of the THT transgene. Abundant THT mRNA with high THT enzyme activity was detected in transgenic leaves, but not in wild-type leaves. In young leaves, the level of CT + FT was very low, but differed significantly between transgenics and the wild type. However, in old and senesced leaves, a high level of CT + FT was detected in both transgenics and the wild type, at 32 times higher in the best transgenic line than in the wild type. Tyramine applied exogenously to roots was directly coupled with the increased synthesis of CT + FT in young leaves in a dose-dependent manner. Our data suggest that CT and FT synthesis is developmentally regulated and closely associated with the presence of tyramine substrate in rice plants.