Evolution of p-coumaroylated lignin in eudicots provides new tools for cell wall engineering

Ester-linked p-coumarate (pCA) is a hallmark feature of the secondary cell walls in commelinid monocot plants. It has been shown that pCA groups arise during lignin polymerisation from the participation of monolignol conjugates assembled by p-coumaroyl-CoA:monolignol transferase (PMT) enzymes, members of the BAHD superfamily of acyltransferases. Herein, we report that a eudicot species, kenaf (Hibiscus cannabinus), naturally contains p-coumaroylated lignin in the core tissues of the stems but not in the bast fibres. Moreover, we identified a novel acyltransferase, HcPMT, that shares <30% amino acid identity with known monocot PMT sequences. Recombinant HcPMT showed a preference in enzyme assays for p-coumaroyl-CoA and benzoyl-CoA as acyl donor substrates and sinapyl alcohol as an acyl acceptor. Heterologous expression of HcPMT in hybrid poplar trees led to the incorporation of pCA in lignin, but no improvement in the saccharification potential of the wood. This work illustrates the value in mining diverse plant taxa for new monolignol acyltransferases. Furthermore, the occurrence of pCA outside monocot lineages may represent another example of convergent evolution in lignin structure. This discovery expands textbook views on cell wall biochemistry and provides a new molecular tool for engineering the lignin of biomass feedstock plants.

Biomimetic oxidative coupling of sinapyl acetate by silver oxide: preferential formation of β-O-4 type structures

Biomimetic oxidations of sinapyl alcohol and sinapyl acetate were carried out with Ag2O to better understand the high frequency of β-O-4 structures in highly acylated natural lignins. The major products from the Ag2O oxidation of sinapyl alcohol were sinapyl aldehyde (14% yield), β-O-4-coupled dimer (32% yield), and β-β-coupled dimer (3% yield). In contrast, the Ag2O oxidation of sinapyl acetate produced β-O-4-coupled dimer in 66% yield. Oligomeric products with predominantly β-O-4 structures were also obtained in 18% yield. The yield of the β-O-4-coupled products from sinapyl acetate was much higher than that from sinapyl alcohol. Computational calculations based on density functional theory showed that the negative charge at Cβ was significantly reduced by the γ-acetyl group. The computational calculations suggest that the Coulombic repulsion between Cβ and O4 in sinapyl acetate radicals was significantly reduced by the γ-acetyl group, contributing to the preferential formation of β-O-4 structures from sinapyl acetate.