Phenylalanine (Phe) biosynthesis in plants is a key process, as Phe serves as a precursor of proteins and phenylpropanoids. The prephenate pathway connects chorismate, the final product of the shikimate pathway, with the biosynthesis of Phe and tyrosine. Two alternative routes of Phe biosynthesis have been reported: one depending on arogenate, and the other on phenylpyruvate. Whereas the arogenate pathway is considered the main route, the role of the phenylpyruvate pathway remains unclear. Here, we report that a deficiency in ADT2, a bifunctional arogenate dehydratase (ADT)/prephenate dehydratase (PDT) enzyme, causes embryo arrest and seed abortion. This result makes a clear distinction between the essential role of ADT2 and the five remaining ADT genes from Arabidopsis, which display mostly overlapping functions. We have found that PHA2, a monofunctional PDT from yeast, restores the adt2 phenotype when it is targeted within the plastids, but not when is expressed in the cytosol. Similar results can be obtained by expressing ADT3, a monofunctional ADT. These results suggest that Phe can be synthesized from phenylpyruvate or arogenate when the bifunctional ADT2 is replaced by other ADT or PDT enzymes during seed formation, highlighting the importance of Phe biosynthesis for embryo development, and providing further insights into the plasticity of Phe biosynthesis.