Biosynthesis of phenylpropanoid natural products in tobacco was perturbed by introduction of a heterologous (bean) phenylalanine ammonia-lyase (PAL; L-phenylalanine ammonia-lyase, EC 126.96.36.199) gene, modified by inclusion of cauliflower mosaic virus 35S enhancer sequences in its promoter. These transgenic plants can exhibit a series of unusual phenotypes including localized fluorescent lesions, altered leaf shape and texture, reduced signification in xylem, stunted growth, reduced pollen viability, and altered flower morphology and pigmentation. Genetic analysis of a transformant with severe symptoms showed that symptom development was inherited as a single, partially dominant trait and cosegregated with reduced levels of PAL activity and soluble phenylpropanoid products. Accumulation of transcripts encoded by the endogenous tobacco PAL genes was suppressed. We conclude that the transgene disrupts PAL regulation and that some of the phenotypes reflect interference with putative signals dependent on phenylpropanoid biosynthesis.