Xylan in hardwoods is not a simple linear chain. Its backbone of xylose units carries two kinds of decorations that are installed during biosynthesis in the Golgi apparatus: 4-O-methylglucuronic acid, abbreviated mGlcA, and O-acetyl groups. The pattern of these substitutions, rather than their mere abundance, governs how xylan interacts with cellulose and lignin, the other major secondary cell wall polymers. These molecular interactions, in turn, regulate how woody biomass behaves during the enzymatic hydrolysis and extraction steps that a biorefinery relies upon. The Swedish team therefore asked a deceptively simple question: if you want to make wood less recalcitrant, is it better to prevent glucuronic acid side chains from being added to xylan in the first place, or to strip them off after the xylan has already been deposited in the cell wall?

To answer it, the researchers generated transgenic hybrid aspen, Populus tremula x tremuloides, using two distinct approaches. The first, designated Gux1RNAi, targeted xylan biosynthesis directly. By suppressing the GLUCURONIC SUBSTITUTION OF XYLAN1 genes, or GUX1, which encode glucuronosyltransferases acting in the Golgi, this strategy reduced the enzyme activity responsible for attaching mGlcA side chains as the xylan backbone is being built. The second approach, designated WP:GH67, took a post-synthetic route. Instead of interfering with biosynthesis, it expressed a fungal α-glucuronidase from glycoside hydrolase family GH67 in the apoplast of wood-forming tissues, allowing the enzyme to hydrolyze glucuronic acid side chains from xylan that had already been laid down in the secondary cell wall.

The comparison proved technically revealing. Both strategies succeeded in reducing the mGlcA to xylose ratio by approximately 30 percent, without altering the total xylose content recovered in alkali extracts of the wood. Yet the two modifications left distinctly different molecular fingerprints. In the Gux1RNAi lines, the reduction in glucuronidation was concentrated in the major xylan domain characterized by evenly spaced mGlcA substitutions, the domain thought to be most important for xylan’s interactions with cellulose microfibrils. The WP:GH67 lines, by contrast, showed effects on both odd and even glucuronidation patterns, since the fungal enzyme could attack side chains regardless of where they sat along the backbone. Intriguingly, the arrays in which mGlcA appears every second xylosyl unit were untouched by either modification, suggesting that certain xylan domains are either inaccessible to the enzyme or are generated by a GUX1-independent biosynthetic route.

Acetylation emerged as the unexpected protagonist of the story. Xylan acetyl content in subcritical water extracts increased only in the Gux1RNAi lines, not in the WP:GH67 lines. The researchers also detected an increase in xylo-oligosaccharides containing O-2,3 diacetylated xylose specifically in the Gux1RNAi material. This pattern supports the hypothesis that acetylation competes with glucuronidation by GUX1 during biosynthesis: when the glucuronosyltransferase is suppressed, acetyltransferases gain more access to the growing xylan chain, and the result is a more heavily acetylated xylan with different solubility behavior. In other words, the downstream consequences of shutting down GUX1 extend well beyond the removal of glucuronic acid side chains themselves.

When the team tested how these changes translated into biorefinery performance, the findings became even more pointed. Neither modification consistently improved glucose or xylose yields in saccharification assays, whether performed with or without acid pretreatment. A 30 percent reduction in xylan glucuronidation, it turns out, is simply not enough to destabilize the cell wall architecture that keeps enzymes at bay. The authors argue that recalcitrance in the transgenic lines is maintained by xylan domains that neither transgenic strategy affected, particularly the evenly spaced mGlcA arrays every second xylosyl unit, which appear to be critical for anchoring xylan within the wall polymer network.

There was, however, one clear winner on the extraction front. Xylan extractability in subcritical water, a hot-compressed water treatment used to solubilize hemicellulose without harsh chemicals, increased in the Gux1RNAi lines but not in the WP:GH67 lines. Since both line types had comparable reductions in glucuronidation, the difference must lie in the accompanying changes rather than in glucuronidation per se. The increased acetylation caused by GUX1 suppression, rather than the reduced glucuronidation itself, is what improves xylan extractability, according to the study’s central conclusion. This reframing matters because it redirects attention from the side chains that have traditionally been the focus of wood engineering efforts toward the acetyl groups that compete with them during biosynthesis.

The implications for biorefinery applications are substantial. Subcritical water extraction is an attractive pretreatment because it avoids the corrosive chemicals and inhibitor-generating conditions associated with acid pretreatment, and being able to increase xylan solubility through a single genetic modification could reduce energy and chemical inputs in future wood-to-fuel processes. At the same time, the finding that saccharification yields were unaffected cautions against expecting dramatic gains in enzymatic hydrolysis from modest reductions in glucuronidation. If the goal is to unlock more of the wood’s sugar content for fermentation, future engineering efforts may need to target the xylan domains that remained resistant in this study, or to combine glucuronidation reduction with modifications to lignin or cellulose that attack recalcitrance from a different angle.

The study also demonstrates the value of comparing synthetic and post-synthetic strategies as experimental tools. Because the two approaches perturb the same target chemistry through different mechanisms, their divergent downstream effects reveal which changes are intrinsic to glucuronidation and which are artifacts of the biosynthetic perturbation. The Gux1RNAi approach, by acting in the Golgi, reshapes the xylan as it is made and triggers compensatory acetylation. The WP:GH67 approach, by acting in the apoplast, trims side chains from mature xylan without disturbing the biosynthetic machinery. Only by running both experiments in parallel could the team disentangle cause from consequence, and only by doing so did they discover that acetylation, not glucuronidation, is the lever that controls xylan extractability in engineered wood.

As the biorefinery sector matures, the molecular details of wood chemistry are becoming engineering parameters rather than fixed constraints. This work on hybrid aspen shows that a 30 percent reduction in xylan glucuronidation can be achieved by two independent genetic routes, that the routes leave different molecular signatures, and that the most valuable outcome, improved xylan extractability in subcritical water, flows from increased acetylation rather than from the glucuronidation reduction itself. For researchers designing transgenic trees for future biofuel and bioproduct applications, the message is clear: to engineer wood for easier processing, it may be more effective to target the competition between acetyltransferases and glucuronosyltransferases during xylan biosynthesis than to attack the glucuronic acid side chains after the fact.

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Subject of Research: xylan glucuronidation

Article Title: Comparison of synthetic and post-synthetic molecular genetic methods of reducing xylan glucuronidation in hybrid aspen for biorefinery applications

Article References: Comparison of synthetic and post-synthetic molecular genetic methods of reducing xylan glucuronidation in hybrid aspen for biorefinery applications. (n.d.). https://doi.org/10.1186/s13068-026-02830-x

Image Credits: AI Generated

DOI: 10.1186/s13068-026-02830-x

Keywords: xylan, glucuronidation, hybrid aspen, biorefinery, GUX1, xylan acetylation, saccharification, transgenic trees, secondary cell wall, Populus, Comparison, synthetic