Phenylpropanoid pool

The tightly regulated pathway specifying aromatic amino acid biosynthesis within the plastid compartment implies maintenance of an amino acid pool to mediate regulation. Thus, we have concluded that loss to the cytoplasm of aromatic amino acids synthesized in the chloroplast compartment is unlikely (13). Yet a source of aromatic amino acids is needed in the cytosol to support protein synthesis. Furthermore, since the enzyme systems of the general phenylpropanoid pathway and its specialized branches of secondary metabolism are located in the cytosol (17), aromatic amino acids (especially L-phenylalanine) are also required in the cytosol as initial substrates for secondary metabolism. The simplest possibility would be that a second, complete pathway of aromatic amino acid biosynthesis exists in the cytosol. Ample precedent has been established for duplicate, major biochemical pathways (glycolysis and oxidative pentose phosphate cycle) of higher plants that are separated from one another in the plastid and cytosolic compartments (18). Evidence to support the hypothesis for a cytosolic pathway (1,13) and the various approaches underway to prove or disprove the dual-pathway hypothesis are summarized in this paper. 

It is reasonable to assume from the available evidence that the enzyme acts at a switching point in metabolism and diverts L-phenylalanine from the general pool of amino acids used for protein synthesis to the biosynthesis of phenylpropanoid compounds. Since initial steps are probable sites for overall pathway regulation, it is therefore not surprising that the factors which influence L-phenylalanine ammonia lyase activity have been subject to detailed scrutiny. Thus phytochrome control in dark grown seedlings. 

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