One out of every five photosynthetically fixed carbon atoms is estimated to enter the phenylpropanoid pathway (PPP), ultimately leading to the production of a myriad of secondary metabolites. In plant cells, the initial enzymatic steps of the core PPP are putatively localized to the cytosolic compartment, and can control key features of secondary metabolism. The enzyme PAL (an ammonia-lyase) utilizes the aromatic amino acid phenylalanine to produce (E)-cinnamic acid, and C4H (a monooxygenase) converts (E)-cinnamic acid to p-coumaric acid. These three core chemical compounds are precursors to thousands of secondary metabolites, such as lignins, flavonoids, phenolic volatiles, coumarins, tannins, stilbenes, phytoalexins, and at least one phytohormone (Vogt 2010). As uite ofPAL genes has been identified in most plant species examined; whereas one or two C4H gene copies are common. PAL and C4H can form a heterologous multi-protein association, thereby establishing a metabolic channel that converts phenylalanine to p-coumaric acid without allowing exposure of the biosynthetic intermediate, (E)-cinnamic acid, to other cytosolic constituents. Total PAL activity can be negatively regulated by a feed-back mechanism involving (E)-cinnamic acid, which functions as a critical control point in the metabolic flux of the PPP. The dynamic interplay between PAL and C4H appears to be a key feature for directing carbon flux through the core PPP without negatively affecting overall PAL activity, and thus, total carbon flux subsequent of PAL action. However, a problem then exists for biological situations in which elevated quantities of (E)-cinnamic acid are needed for the production of derived compounds such as volatile benzenoids. A relatively simple plant system that produces considerable levels of benzenoid compounds is Petunia x hybrida cv ‘Mitchell Diploid’ .T his petunia cultivar has relatively large, white flowers that when open, emit large amounts of floral volatile benzenoid/phenylpropanoid (FVBP) compounds (∼100 ug/ gfw/h), and thus can be a suitable biological system for understanding early stages of phenylalanine metabolism. The last common precursor in FVBP biosynthesis is phenylalanine; however, the majority of FVBPs are produced from the PPP intermediates (E)-cinnamic acid and p-coumaric acid (i.e. three metabolites from the core PPP—three branches of the FVBP pathway). Metabolite regulation of the core PPP, whether by enzyme kinetics, allosteric regulation, metabolic channeling, and/or compartmentalization may be crucial to the overall volatile blend these flowers emit; but the regulatory complexities remain scientifically under-explored. In short, the petunia petal limb cell must synthesize enough (E)-cinnamic acid to supply the eventual biosynthesis and emission of volatile benzenoids without stimulating a negative feed-back regulatory mechanism. Writ large, a similar challenge may have resulted in thePAL/C4Hproteincomplex‘solution’forsupplyingp-coumaricacidderived compounds without affecting PAL activity.