Enhanced octopamine synthesis through the ectopic expression of tyrosine decarboxylase in rice plants

Tyramine, the decarboxylated product of tyrosine by catalysis via the tyrosine decarboxylase enzyme (TYDC), is employed as an acyl acceptor in the synthesis of hydroxycinnamic acid amides of tyramine, such as feruloyltyramine (FT) and 4-coumaroyltyramine (CT). Ectopic over-expression of TYDC cDNA through a maize ubiquitin promoter led to a massive production of tyramine in transgenic rice plants. Most transgenic rice plants exhibited stunted growth without seed production. From more than 120 transformants, we obtained several lines that were able to produce a few viable seeds, although they were dwarf morphs. Transgenic expression of TYDC was confirmed by Northern blot analysis and TYDC enzyme activity measurements. In parallel with high levels of tyramine, a fourfold increase in the levels of octopamine (β-hydroxytyramine) was observed in transgenic rice leaves relative to wild-type leaves. However, high levels of octopamine synthesis were not observed in transgenic rice seeds, even though the accumulation of tyramine in transgenic rice seeds was more than 80 times that in wild-type seeds. The unaltered levels of octopamine in transgenic seeds were attributed to the low levels of tyramine beta-hydroxylase (TβH) enzyme activity, which catalyzes tyramine to octopamine.