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Ferulate-5-hydroxylase (F5H) is a key rate-limiting enzyme for the conversion of guaiacyl monolignol (G-monolignol) to syringyl monolignol (S-monolignol) in the specific synthetic lignin pathway, through the catalysis of the 5-hydroxylation of S-monolignol precursors ferulic acid, conifer aldehyde, and coniferyl alcohol. In this study, we cloned the F5H gene of Populus tomenta (PtoF5H), whose product has a highly conserved domain of P450-dependent monooxygenase family. Subcellular localization result demonstrated that PtoF5H protein is an endoplasmic reticulum (ER) resident protein. Furthermore, the PtoF5H was transformed into tobacco in the form of sense- and antisense-, showed that the proportion of S-monolignol increased when PtoF5H gene was overexpressed, suggesting PtoF5H could be used as a target gene for modifying lignin composition. These findings provide further insight into the function of PtoF5H. [ABSTRACT FROM AUTHOR]