Transgenic tobacco (Nicotiana tabacum 1.) plants in which the activity of 4-coumarate:coenzyme A ligase is very low contain a novel lignin in their xylem. Details of changes in hydroxycinnamic acids bound to cell walls and in the structure of the novel lignin were identified by base hydrolysis, alkaline nitrobenzene oxidation, pyrolysis-gas chromatography, and '3C-nuclear magnetic resonance analysis. In the brownish tissue of the transgenic plants, the levels of three hydroxycinnamic acids, p-coumaric, ferulic, and sinapic, which were bound to cell walls, were apparently increased as a result of down-regulation of the expression of the gene for 4-coumarate:coenzyme A ligase. Some of these hydroxycinnamic acids were linked to cell walls via ester and ether linkages. The accumulation of hydroxycinnamic acids also induced an increase in the leve1 of condensed units in the novel lignin of the brownish tissue. Our data indicate that the behavior of some of the incorporated hydroxycinnamic acids resembles lignin monomers in the brownish tissue, and their accumulation results in dramatic changes in the biosynthesis of lignin in transgenic plants.The deposition of lignin in vascular cell walls is a key step in the differentiation of these cells into the functional elements that participate in the transport of water and in the structural support of plants. The biosynthesis of lignin has been studied extensively and the major enzymes involved in the synthesis of precursors to lignin have been identified (Higuchi, 1985; Lewis and Yamamoto, 1990). In the past decade many genes for enzymes related to the biosynthesis of lignin have been cloned from a variety of plant species, and the expression of these genes at various stages of lignification has been characterized (Sederoff et al., 1994; Whetten and Sederoff, 1995). In several recent studies transgenic plants carrying chimeric genes for the enzymes involved in lignin biosynthesis were generated and the altered lignins in these transformants were analyzed (Elkind et al