Substrate Specificity of LACCASE8 Facilitates Polymerization of Caffeyl Alcohol for C-Lignin Biosynthesis in the Seed Coat of .

Catechyl lignin (C-lignin) is a linear homopolymer of caffeyl alcohol found in the seed coats of diverse plant species. Its properties make it a natural source of carbon fibers and high-value chemicals, but the mechanism of in planta polymerization of caffeyl alcohol remains unclear. In the ornamental plant , lignin biosynthesis in the seed coat switches from guaiacyl lignin to C-lignin at ∼12 d after pollination. Here we found that the transcript profile of the laccase gene parallels the accumulation of C-lignin during seed coat development. Recombinant ChLAC8 oxidizes caffeyl and sinapyl alcohols, generating their corresponding dimers or trimers in vitro, but cannot oxidize coniferyl alcohol. We propose a basis for this substrate preference based on molecular modeling/docking experiments. Suppression of expression led to significantly reduced C-lignin content in the seed coats of transgenic Cleome plants. Feeding of C-caffeyl alcohol to the Arabidopsis () - mutant resulted in no incorporation of C into C-lignin, but expressing in this genetic background led to appearance of C-lignin with >40% label incorporation. These results indicate that ChLAC8 is required for C-lignin polymerization and determines lignin composition when caffeyl alcohol is available.