Beside their crucial role in the plant's defence mechanisms, several compounds belonging to the plant phenylpropanoids have been demonstrated to be pharmacologically bioactive, as anti-oxidant, anti-inflammatory and anti-tumoral [1]. In this study, a metabolic engineering strategy was used to boost the synthesis of plant bioactive phenylpropanoids, by overexpression of the Osmyb4 transcription factor, isolated from rice [2], in N. tabacum plants and S. sclarea hairy roots. The heterologous TF was correctly expressed in both plants, confirming that the myb transcription factors are highly conserved in plant species. Several genes of the phenylpropanoid pathway were up-regulated in Osmyb4 overxpressing tobacco plants, including the PAL, 4CL1, 4CL2 and HQT genes, indicating that a specific branch of the phenylpropanoid pathway was activated by Osmyb4 TF. Accordingly to the level of expression of this set of genes, a higher content of chlorogenic acid (CGA) was synthesized in tobacco transgenic plants, as revealed by HPLC-DAD, LC-MSn and 1H-NMR analysis. Moreover, the overexpression of this TF in S. sclarea hairy roots caused a significant increase in the content of rosmarinic acid (RA), another biologically active phenylpropanoid [3]. Altogether these data indicate that Osmyb4, as a conserved positive regulator of the biosynthetic pathway of plant phenylpropanoids, is a useful molecular tool to genetically manipulate this pathway in crop and medicinal plants.