The rice (Oryza sativa) OsCYP714D1 gene (also known as EU1) encodes a cytochrome P450 monooxygenase which functions as a gibberellin acid (GA) deactivating enzyme, catalyzing 16α,17-epoxidation of non-13-hydroxylated GAs.To understand whether it would also reduce the production of active GAs and depress the growth rate in transgenic trees, we constitutively expressed OsCYP714D1 in aspen hybrid clone Populus alba x P.berolinensis.Unexpectedly, ectopic expression of OsCYP714D1 in aspen positively regulates the biosynthesis of GAs, including the active GA1 and GA4, leading to promoted growth rate and biomass production in transgenic plants.Transgenic lines which showed significant expression of the introduced OsCYP714D1 gene accumulated higher of GA level and produced more numerous and longer xylem fibers than did the wild type (WT) plants.Quantitative real-time PCR indicated that transcriptions of most homologous PtCYP714 genes were suppressed in these transgenic lines.Therefore, the promoted GA and biomass production in transgenic trees constitutively expressing OsCYP714D1 is probably attributed to the down-regulation of the native PtCYP714 homologs involved in the GA biosynthesis pathway, although their precise functions are yet to be further elucidated.