Aromatic Amino Acid Network: Biosynthesis, Regulation and Transport

Aromatic amino acids are essential constituents of proteins in all living organisms, while in plants phenylalanine (Phe) and tyrosine (Tyr) additionally serve as precursors for thousands of specialized compounds that have a profound impact on plant growth, development, reproduction and defense. Plants direct 20‐30% of photosynthetically fixed carbon to the production of Phe and Phe‐derived compounds, which constitute approximately 30‐45% of plant organic matter. Humans are incapable of synthesizing Phe and must primarily obtain it directly or indirectly from plants. Phe biosynthesis in plants occurs via two alternative routes with either arogenate or phenylpyruvate as a key intermediate. Using petunia flowers as a model system, we have shown that plants predominantly synthesize Phe in plastids via the arogenate pathway while they can also utilize the microbial‐like phenylpyruvate pathway. Moreover, flux through the phenylpyruvate route is increased when the entry point to the arogenate pathway is limiting. Interestingly, the phenylpyruvate pathway utilizes a cytosolic aminotransferase that strongly favors Tyr as the amino donor. These results demonstrate that Phe biosynthesis is not limited to plastids and that there is an interconnection between aromatic amino acid catabolism and biosynthesis in planta . Another aspect of plant metabolism to be discussed is its compartmentalization and metabolite transport within the cell and how this intracellular transport contributes to the interconnections between and within metabolic networks.