Aromatic Amino Acids: A Complex Network Ripe for Future Exploration.

In plants, high carbon flux is committed to the biosynthesis of phenylalanine, tyrosine, and tryptophan, owing to their roles not only in the production of proteins, but also as precursors to thousands of primary and specialized metabolites. The core plastidial pathways that supply the majority of aromatic amino acids (AAAs) have previously been described in detail. More recently, the discovery of cytosolic enzymes contributing to overall AAA biosynthesis, as well as the identification of intracellular transporters and the continuing elucidation of transcriptional and post-transcriptional regulatory mechanisms, have revealed the complexity of this intercompartmental metabolic network. Here, we review the latest breakthroughs in AAA production and use the newest findings to highlight both longstanding and newly developed questions. In vivo , in vitro , and in silico analyses have increased our depth of understanding of the plastidial AAA biosynthetic pathway. A newly discovered cytosolic Phe biosynthetic pathway, and the presence of cytosolic Tyr biosynthetic enzymes in some plant species has revealed that AAA biosynthesis is a complex intercompartmental network. Characterization of transporters has answered questions about the physiological importance of the intracellular movement of AAAs. Transcriptional and post-transcriptional regulatory mechanisms are responsible for the tight control of carbon flux distribution into, and within, the AAA network. [ABSTRACT FROM AUTHOR]