Discovery of Cytosolic Phenylalanine Biosynthetic Pathway in Plants

Phenylalanine (Phe) is a proteinogenic aromatic amino acid that also serves as a precursor for numerous primary and secondary metabolites in plants. Phe is synthesized from chorismate, the final product of the shikimate pathway. In plants, Phe is predominantly synthesized in the plastids via the arogenate pathway, while most Phe-derived compounds are produced in the cytoplasm, requiring exportation of Phe from plastids to the cytosol. Here, we provided genetic evidences that a Petunia hybrida plastidial cationic amino acid transporter (PhpCAT) participates in the exportation of Phe from plastids, as well as regulation of carbon flux through Phe biosynthesis.

By using reverse genetics, we demonstrated that a petunia phenylpyruvate aminotransferase (PhPPY-AT) is able to convert phenylpyruvate to Phe in the cytosol in vivo, and that a cytosolic chorismate mutase (CM2), which converts chorismate to prephenate, directs carbon flux from the plastidial Phe biosynthesis pathway towards the cytosolic pathway. Downregulation of PhPPY-AT and PhCM2 resulted in significant decreases in Phe levels and emission of Phe-derived volatiles in petunia flowers, respectively. Metabolic flux analysis showed that the carbon flux through the cytosolic Phe biosynthesis pathway is significantly lower in PhCM2 RNAi petunia flowers relative to wild type control. We also demonstrated that the conversion of prephenate to phenylpyruvate in the cytosol is catalyzed by a cytosolic prephenate dehydratase (PDT) produced from an alternative transcription start site of a known plastidial arogenate dehydratase (ADT). These results suggest that a microbial-like phenylpyruvate pathway for Phe biosynthesis operates in the cytosol of plant cells and the cytosolic pathway splits from the plastidial pathway at chorismate.

To evaluate the metabolic potential of the cytosolic phenylpyruvate pathway, PhCM2 overexpressing transgenic petunia plants were generated. Unexpectedly, Phe levels and emission of Phe-derived volatiles were both reduced, even though the flux through the cytosolic pathway was increased relative to wild type control. Electron microscopy, metabolic profiling and metabolic flux analysis revealed that the number of leucoplasts, starch levels and flux through the plastidial pathway were all reduced in PhCM2 overexpression lines, while the concentrations of auxin and its biosynthetic intermediate, indole-3-pyruvic acid (IPA), were elevated. Overexpression of Arabidopsis aminotransferase VAS1, which converts IPA to Trp, in PhCM2 overexpression petunia background recovered Phe levels and Phe-derived volatiles emission. These results indicate that there exists a metabolic crosstalk between cytosolic Phe production and Trp-dependent auxin biosynthesis .

Our research completed the post-chorismate cytosolic Phe biosynthesis pathway in plants and revealed possible metabolic crosstalk between cytosolic Phe production and auxin biosynthesis in plant cells, providing targets for future genetic modification of metabolites in plants.