Cytochrome P450 monooxygenases (P450s) mediate a wide range of oxidative reactions involved in the biosynthesis of plant secondary metabolites including phenylpropanoids and phytoalexins. To investigate the regulation of these P450s in the phenylpropanoid biosynthetic pathway of pea (Pisum safivum), partia1 cDNAs representing four distinct P450s expressed in pea seedlings were cloned using a reverse transcription-polymerase chain reaction strategy. One of the corresponding full-length cDNA clones, designated CYP73A9, encodes pea frans-cinnamic acid 4-hydroxylase, which catalyzes the second core reaction in the phenylpropanoid pathway. As expected from its central role in the production of lignin precursors and defense compounds, northern analysis of poly(A)+ mRNA demonstrates that transcripts encoding CYP73A9 are induced appreciably within 3 h after wounding. A second cDNA clone, designated CYP82, encodes a nove1 P450, whose transcripts are also induced in response to wounding at approximately the same time as CYP73A9 transcripts. Despite the multitude of environmental stimuli known to induce expression of phenylpropanoid pathway enzymes, genomic DNA Southern analysis indicates that each of these P450s is encoded by a low copy number (possibly a single copy) gene family.P450s are heme-dependent, mixed-function oxidase systems that utilize NADPH and/or NADH to reductively cleave dioxygen to produce a functionalized organic substrate and a molecule of water. These modified b-type Cyts range in molecular mass from 45 to 62 kD (average 55 kD) and contain a protoporphyrin IX heme prosthetic group covalently attached to the Cys of the highly conserved F--G-R-C-G motif found near the C terminus of all P450s (Nelson et al., 1993(Nelson et al., , 1996.Many exogenous substrates, including drugs, carcinogens, herbicides, and insecticides, are detoxified by P450-mediated hydroxylations; other natural metabolites, including plant phenylpropanoids, terpenoids, alkaloids, fatty acids, and GA precursors, are biosynthesized by P450-mediated reactions. Reflecting this wide diversity of reactive sites, P450s are encoded by a highly divergent gene superfamily containing more than 450 Cyt P450 (CYP) sequences distributed among a minimum of 65 gene families (Nelson et al., 1993(Nelson et al., , 1996. Within any one organism, this superfamily contains a spectrum of P450s that differ substantially in their primary sequences, substrate speci-