This study investigates the mechanisms by which plants maintain aromatic amino acid homeostasis in the presence of high levels of phenylalanine (Phe). By reducing the expression of all three Phe ammonia lyase (PAL) isoforms in petunia flowers, the researchers achieved a 18-fold increase in internal Phe pools. This hyperaccumulation of Phe led to multifaceted intercompartmental effects on aromatic amino acid metabolism, including a decrease in the flux through the shikimate pathway and a redirection of carbon flux towards tyrosine and tryptophan. The accumulation of Phe did not result in increased flux towards phenylacetaldehyde, suggesting the presence of a metabolically inactive pool of Phe localized in the vacuole. The study identified a vacuolar cationic amino acid transporter (PhCAT2) that contributes to the sequestration of excess Phe in the vacuole. Overexpression of PhCAT2 in yeast confirmed its role as a Phe transporter, and transient downregulation of PhCAT2 expression in PAL-RNAi plants increased phenylacetaldehyde emission, demonstrating its importance in maintaining cytosolic Phe homeostasis. These findings provide insights into how plants maintain aromatic amino acid homeostasis and offer potential for metabolic engineering strategies.This study investigates the mechanisms by which plants maintain aromatic amino acid homeostasis in the presence of high levels of phenylalanine (Phe). By reducing the expression of all three Phe ammonia lyase (PAL) isoforms in petunia flowers, the researchers achieved a 18-fold increase in internal Phe pools. This hyperaccumulation of Phe led to multifaceted intercompartmental effects on aromatic amino acid metabolism, including a decrease in the flux through the shikimate pathway and a redirection of carbon flux towards tyrosine and tryptophan. The accumulation of Phe did not result in increased flux towards phenylacetaldehyde, suggesting the presence of a metabolically inactive pool of Phe localized in the vacuole. The study identified a vacuolar cationic amino acid transporter (PhCAT2) that contributes to the sequestration of excess Phe in the vacuole. Overexpression of PhCAT2 in yeast confirmed its role as a Phe transporter, and transient downregulation of PhCAT2 expression in PAL-RNAi plants increased phenylacetaldehyde emission, demonstrating its importance in maintaining cytosolic Phe homeostasis. These findings provide insights into how plants maintain aromatic amino acid homeostasis and offer potential for metabolic engineering strategies.