Lignin content in natural <i>Populus</i> variants affects sugar release

TLDR

The primary obstacle to producing renewable fuels from lignocellulosic biomass is a plant's recalcitrance to releasing sugars bound in the cell wall. The authors selected 47 extreme phenotypes from 1,100 Populus trichocarpa trees based on lignin content and S/G ratio, then screened their sugar release by enzymatic hydrolysis alone and after hot‑water pretreatment using a high‑throughput method. Sugar yields varied widely, reaching up to 92 % of the theoretical maximum, with a strong negative correlation between release and lignin content only in pretreated samples having S/G < 2.0; higher S/G ratios generally improved release, glucose release correlated with both lignin and S/G while xylose depended only on S/G, and even without pretreatment sugar release rose sharply when lignin fell below 20 %, indicating that factors beyond lignin and S/G ratio affect recalcitrance.

Abstract

The primary obstacle to producing renewable fuels from lignocellulosic biomass is a plant's recalcitrance to releasing sugars bound in the cell wall. From a sample set of wood cores representing 1,100 individual undomesticated Populus trichocarpa trees, 47 extreme phenotypes were selected across measured lignin content and ratio of syringyl and guaiacyl units (S/G ratio). This subset was tested for total sugar release through enzymatic hydrolysis alone as well as through combined hot-water pretreatment and enzymatic hydrolysis using a high-throughput screening method. The total amount of glucan and xylan released varied widely among samples, with total sugar yields of up to 92% of the theoretical maximum. A strong negative correlation between sugar release and lignin content was only found for pretreated samples with an S/G ratio &lt; 2.0. For higher S/G ratios, sugar release was generally higher, and the negative influence of lignin was less pronounced. When examined separately, only glucose release was correlated with lignin content and S/G ratio in this manner, whereas xylose release depended on the S/G ratio alone. For enzymatic hydrolysis without pretreatment, sugar release increased significantly with decreasing lignin content below 20%, irrespective of the S/G ratio. Furthermore, certain samples featuring average lignin content and S/G ratios exhibited exceptional sugar release. These facts suggest that factors beyond lignin and S/G ratio influence recalcitrance to sugar release and point to a critical need for deeper understanding of cell-wall structure before plants can be rationally engineered for reduced recalcitrance and efficient biofuels production.