Expression of tryptophan decarboxylase and tyrosine decarboxylase genes intobacco results in altered biochemical and physiological phenotypes

The substrate specificity of tryptophan (Trp) decarboxylase (TDC) for Trp a nd tyrosine (Tyr) decarboxylase (TYDC) for Tyr was used to modify the in vi vo pools of these amino acids in transgenic tobacco. Expression of TDC and TYDC was shown to deplete the levels of Trp and Tyr, respectively, during s eedling development. The creation of artificial metabolic sinks for Trp and Tyr also drastically affected the levels of phenylalanine, as well as thos e of the non-aromatic amino acids methionine, valine, and leucine. Transgen ic seedlings also displayed a root-curling phenotype that directly correlat ed with the depletion of the Trp pool. Nontransformed control seedlings cou ld be induced to display this phenotype after treatment with inhibitors of auxin translocation such as 2,3,5-triiodobenzoic acid or N-1-naphthylphthal amic acid. The depletion of aromatic amino acids was also correlated with i ncreases in the activities of the shikimate and phenylpropanoid pathways in older, light-treated transgenic seedlings expressing TDC, TYDC, or both. T hese results provide in vivo confirmation that aromatic amino acids exert r egulatory feedback control over carbon flux through the shikimate pathway, as well as affecting pathways outside of aromatic amino acid biosynthesis.