Ectopic expression of wax ester synthase under a wood-specific promoter enhances cell wall production and wood hydrophobicity.

Background: Many industrial applications of wood and woody biomass require harsh physicochemical pretreatments to improve the hydrophobicity and durability of the products. Environmentally friendly wood biorefineries necessitate the replacement of chemicals and energy-consuming wood processing. Here, our goal was to increase wood hydrophobicity via the ectopic expression of Jojoba (Simmondsia chinensis) wax ester synthase (ScWS) in poplar (Populus × canescens). We expressed ScWS under a wood-specific promoter (DX15), which naturally controls the expression of FASCICLIN-like ARABINOGALACTAN PROTEIN 15 (FLA15) in the xylem.

Results: In the DX15::ScWS lines, ScWS was highly expressed in wood but not in leaves. The transgenic lines exhibited normal photosynthesis and growth similar to the wild-type poplars. Compared with the wild-type poplars, the DX15::ScWS lines accumulated greater amounts of triacylglycerol in wood and a greater number of lipid droplets in ray parenchyma cells. The composition of the bark cuticle wax esters was unaffected. The wood of the DX15::ScWS lines showed greater water repellency and less swelling than that of the wild-type poplars. Furthermore, the DX15::ScWS lines had an increased expression of FLA15 and increased cell wall deposition in fibers, resulting in increased wood density.

Conclusions: Our results highlight the potential of combining the wood-specific DX15 promoter with ScWS to enhance the technological properties of poplar wood. Reduced wood hydrophilicity represents a significant improvement in wood quality. In addition, our results suggest that the overexpression of the DX15 promoter could be a promising strategy for improving lignocellulose biomass in plants. Since poplars are highly productive species that can be cultivated in short-rotation plantations, our results have high translational potential for advancing sustainable wood utilization for a wider range of applications.