Using in situ RNA hybridization and immunohistochemical techniques. We investigated the tissue-specific and light-dependent expression of four genes involved in phenylpropanoid metabolism in various parts of parsley (Petroselinum crispum) seedlings. The genes encode phenylalanine ammonia-lyase (PAL) and 4-coumarate:CoA ligase (4CL), two enzymes of general phenylpropanoid metabolism; chalcone synthase (CHS), the kev enzyme of flavonoid biosynthesis; and bergaptol O-methyltransferase (BMT), a late enzyme of the furanocoumarin pathway. In young leaves, PAL and 4CL genes were expressed predominantly and in a coordinated fashion in epidermal cells, young xylem cells of vascular bundles. and epithelial cells of oil ducts. The gene(s) encoding BMT was active exclusively in the epithelial cells of oil ducts. whereas CHS gene expression was largely confined to the epidermis. In shoot and root apices, PAL, 4CL and CHS mRNAs were detectable at low levels without distinct patterns. The corresponding proteins. however. accumulated preferentially in the protoderm and pith meristem of the shoot apex and in root-cap cells. A gene encoding pathogenesis-related protein 1 (PR 1) was analyzed in roots for comparison and was found to be expressed predominantly in the cortical cells of root tips. The expression of all genes investigated was, to a greater or lesser extent, dependent on light conditions and tissue age, with highest levels occurring in newly differentiated, light-exposed tissue. In the leaf epidermis of seedlings illuminated for 20 h, PAL and CHS mRNAs and proteins, as well as 4CL protein, were particularly abundant.
