The PMT-driven p-coumaroylation of poplar lignins impacts lignin structure and improves wood saccharification

Transgenic poplars (Populus tremula x Populus alba, clone INRA 717-1B4) were produced by introducing the Brachypodium distachyon Bradi2g36910 (BdPMT1) gene driven by the Arabidopsis (Arabidopsis thaliana) Cinnamate 4-Hydroxylase (AtC4H) promoter in the wild-type (WT) line and in a line overexpressing the Arabidopsis Ferulate 5-Hydroxylase (AtF5H). BdPMT1 encodes a transferase which catalyzes the acylation of monolignols by p-coumaric acid (CA). Several BdPMT1- OE/WT and BdPMT1-OE/AtF5H-OE transgenic lines were grown in the greenhouse and BdPMT1 expression in xylem was confirmed by RT-PCR. The analysis of the cell walls (CW) of poplar stems and of corresponding purified dioxan lignins (DL) revealed that the BdPMT1-OE lignins were as p-coumaroylated as the lignins of C3 grass straws. For some transformants, CA levels even reached about 11 mg/g CW and 66 mg/g DL, which by far exceeds those of Brachypodium or wheat samples. This unprecedentedly high p-coumaroylation of poplar lignins affected neither the poplar growth, nor the stem lignin content. By contrast, the transgenic lignins were structurally modified, with an increase of terminal units with free phenolic groups. Relative to controls, this increase argues for a reduced polymerization degree of BdPMT1-OE lignins and makes them more soluble in cold NaOH solution. The p-coumaroylation of poplar samples, up to the levels of C3 grasses, improved the saccharification yield of alkali-pretreated poplar CW. These results establish that the genetically-driven p-coumaroylation of lignins is a promising strategy to make wood lignins more susceptible to the alkaline treatments that can be used during the industrial processing of lignocellulosics. One-sentence summaryThe expression of a grass p-coumaroyl-CoA:monolignol transferase induces a high p-coumaroylation of poplar lignins and a better saccharification of alkali-pretreated poplar wood without growth penalty