Tryptophan (Trp) and phenylalanine (Phe) are essential aromatic amino acids that animals cannot synthesize and are dependent on plants for their supply. In particular, Trp contributes to the nutritional quality of plant-based foods, and, along with lysine, methionine and threonine, it is used to supplement animal feeds. The other aromatic amino acids, Phe and tyrosine (Tyr), are used for the production of the low-calorie sweetener aspartame and the anti-Parkinson's disease drug L-dopa, respectively. The biosynthetic pathways for aromatic amino acids and their regulation have been extensively explored in bacteria because of their utility in the food and drug industry. In plants, aromatic amino acids are the precursors for a large variety of secondary metabolites including indole alkaloids, phenylpropanoids, flavonoids, and the phenolic polymer, lignin. One of plant growth regulators, indole-3-acetic acid (IAA), has also been shown to be biosynthesized from Trp. Thus, aromatic amino acids also play important roles in plant defense and development.In bacteria, fungi and plants, the three aromatic amino acids, Trp, Phe and Tyr, are synthesized from a common precursor, chorismate that originates from the shikimate pathway ( Figure 1). In bacteria, this pathway is almost exclusively used to produce aromatic amino acids for protein synthesis but, in higher plants, this pathway leads to the production of numerous aromatic secondary metabolites. The importance of this pathway in plants is indicated by the fact that about 20% of the carbon fixed by photosynthesis flow through this pathway. These facts strongly suggest that the modification of the shikimate and its derivative (or downstream) pathways may lead to dramatic changes in secondary metabolism. The enzymatic reactions in the biosynthetic pathway for aromatic amino acids seem to be identical in prokaryotes and eukaryotes but they apparently differ in the regulatory mechanisms for these pathways. For example, 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthase, the enzyme that catalyzes the first reaction in the shikimate pathway, is not feedback inhibited by any of the aromatic amino acids in plants, but, each of the three DAHP synthase isoenzymes are Abstract Aromatic amino acids function as building blocks of proteins and as precursors for secondary metabolism. To obtain plants that accumulate tryptophan (Trp) and phenylalanine (Phe), we modified the biosynthetic pathways for these amino acids in rice and dicot species. By introducing a gene encoding a feedback-insensitive anthranilate synthase (AS) alpha subunit, we successfully obtained transgenic plants that over-accumulated Trp. In addition, we found mutant calli that accumulated Phe and Trp at high concentrations. The causal gene (mtr1-D) encoded an arogenate dehydratase (ADT)/prephenate dehydratase (PDT) that catalyzes the final reaction in Phe biosynthesis. The wild-type enzyme was sensitive to feedback inhibition by Phe, but the mutant enzyme encoded by mtr1-D was relatively insensitive. Further, ...