Aromatic amino acids (AAAs) play a crucial role in the synthesis of proteins across all living cells as well as in the generation of various primary and specialized metabolites. In plants, AAA biosynthesis is a critical biochemical pathway on Earth. Plants utilize AAAs to generate specialized metabolites, ranging from electron carriers in photosynthesis to natural products with diverse biological activities. Many of these products have extensive applications in pharmaceutical, nutraceutical, and industrial sectors. However, the mechanisms governing the equilibrium between AAA biosynthesis and metabolism in plants remain elusive. 

A research team led by Prof. ZHAO Qiao from the Shenzhen Institute of Advanced Technology (SIAT) of the Chinese Academy of Sciences has unveiled the role of a cytosolic aromatic aminotransferase, REVERSAL OF SAV3 PHENOTYPE 1 (VAS1), in the recycling of AAAs. This process maintains AAA homeostasis, enabling plants to effectively coordinate the intricate metabolic and biosynthetic pathways of AAAs. The study was published in Science Advances. 

To trace the biosynthetic route of these compounds in planta, isotopic tracing experiments using ¹³C₆-labeled phenylalanine (Phe) and ¹³C₆-labeled tyrosine (Tyr) were conducted, suggesting that 3-carboxy AAAs are derived from pathways other than direct modification of Phe or Tyr. Genetic evidence indicated that the biosynthesis of 3-carboxy AAAs occurs through the isochorismate pathway in the cytosol of plants. These pathways, distinct from general aromatic biosynthesis routes, are specific to certain plant species. Additionally, VAS1 was revealed to be responsible for cytosolic AAA biosynthesis with its enzymatic activity susceptible to inhibition by 3-carboxyphenylalanine. 

Prof. ZHAO underscores the significance of the discovery of 3-carboxy AAAs, providing opportunities for metabolic engineering of natural products.