CCoAOMT suppression modifies lignin composition in Pinus radiata

A cDNA clone encoding the lignin‐related enzyme caffeoyl CoA 3‐O‐methyltransferase (CCoAOMT) was isolated from a Pinus radiata cDNA library derived from differentiating xylem. Suppression of PrCCoAOMT expression in P. radiata tracheary element cultures affected lignin content and composition, resulting in a lignin polymer containing p‐hydroxyphenyl (H), catechyl (C) and guaiacyl (G) units. Acetyl bromide‐soluble lignin assays revealed reductions in lignin content of up to 20% in PrCCoAOMT‐deficient transgenic lines. Pyrolysis‐GC/MS and 2D‐NMR studies demonstrated that these reductions were due to depletion of G‐type lignin. Correspondingly, the proportion of H‐type lignin in PrCCoAOMT‐deficient transgenic lines increased, resulting in up to a 10‐fold increase in the H/G ratio relative to untransformed controls. 2D‐NMR spectra revealed that PrCCoAOMT suppression resulted in formation of benzodioxanes in the lignin polymer. This suggested that phenylpropanoids with an ortho‐diphenyl structure such as caffeyl alcohol are involved in lignin polymerization. To test this hypothesis, synthetic lignins containing methyl caffeate or caffeyl alcohol were generated and analyzed by 2D‐NMR. Comparison of the 2D‐NMR spectra from PrCCoAOMT‐RNAi lines and synthetic lignins identified caffeyl alcohol as the new lignin constituent in PrCCoAOMT‐deficient lines. The incorporation of caffeyl alcohol into lignin created a polymer containing catechyl units, a lignin type that has not been previously identified in recombinant lignin studies. This finding is consistent with the theory that lignin polymerization is based on a radical coupling process that is determined solely by chemical processes.