Expression of a poplar cDNA encoding a ferulate-5-hydroxylase/coniferaldehyde 5-hydroxylase increases S lignin deposition in Arabidopsis thaliana

Ferulate-5-hydroxylase or coniferaldehyde 5-hydroxylase (F5H or Cald5H; CYP84A1) is the enzyme responsible for the last hydroxylation of the syringyl-type lignin precursors. A cDNA clone highly homologous to the Arabidopsis thaliana F5H/Cald5H cDNA was identified during the random sequencing of a poplar (Populus trichocarpa) differentiating xylem cDNA library. Present as a multigenic family in poplar, this gene (PopF5H) was found to be highly expressed in lignified tissues supporting its role in lignin biosynthesis. When placed under the control of the CaMV 35S promoter, the poplar F5H was able to complement the A. thaliana fah1-2 mutation. Overexpression in wild-type A. thaliana substantially increased the level of syringyl units in lignins. These transgenic lines were also typified by substantial levels of S-units in root lignins, whereas these units are present in low amounts in wild-type A. thaliana. These results support that PopF5H is a functional homolog of the A. thaliana F5H gene.