Current cell-wall models assume no covalent bonding between cellulose and hemicelluloses such as xyloglucan or mixed-linkage β-d-glucan (MLG). However, Equisetum hetero-trans-β-glucanase (HTG) grafts cellulose onto xyloglucan oligosaccharides (XGOs) – and, we now show, xyloglucan polysaccharide – in vitro, thus exhibiting CXE (cellulose:xyloglucan endotransglucosylase) activity. In addition, HTG also catalyzes MLG-to-XGO bonding (MXE activity). In this study, we explored the CXE action of HTG in native plant cell walls and tested whether expansin exposes cellulose to HTG by disrupting hydrogen bonds. To quantify and visualize CXE and MXE action, we assayed the sequential release of HTG products from cell walls pre-labeled with substrate mimics. We demonstrated covalent cellulose–xyloglucan bonding in plant cell walls and showed that CXE and MXE action was up to 15% and 60% of total transglucanase action, respectively, and peaked in aging, strengthening tissues: CXE in xylem and cells bordering intercellular canals and MXE in sclerenchyma. Recombinant bacterial expansin (EXLX1) strongly augmented CXE activity in vitro. CXE and MXE action in living Equisetum structural tissues potentially strengthens stems, while expansin might augment the HTG-catalyzed CXE reaction, thereby allowing efficient CXE action in muro. Our methods will enable surveys for comparable reactions throughout the plant kingdom. Furthermore, engineering similar hetero-polymer formation into angiosperm crop plants may improve certain agronomic traits such as lodging tolerance.

当前的细胞壁模型假设纤维素与半纤维素（如木葡聚糖或混合连接的β- d-葡聚糖（MLG））之间没有共价键。然而，木贼异质反式-β-葡聚糖酶（HTG）将纤维素嫁接到木葡聚糖低聚糖（XGOs）上，我们现在展示木葡聚糖多糖在体外，因此表现出CXE（纤维素：木葡聚糖内转葡糖基化酶）活性。此外，HTG还催化MLG与XGO的键合（MXE活性）。在这项研究中，我们探讨了天然植物细胞壁中HTG的CXE作用，并测试了expansin是否通过破坏氢键将纤维素暴露于HTG。为了量化和可视化CXE和MXE的作用，我们分析了HTG产品从预先用底物模拟物标记的细胞壁中的顺序释放。我们证明了植物细胞壁中纤维素-木葡聚糖的共价键合作用，并表明CXE和MXE的作用分别高达总转葡聚糖酶作用的15％和60％，并在衰老，增强组织中达到顶峰：木质部和细胞间管和细胞边界的CXE巩膜瘤中的MXE。重组细菌扩展蛋白（EXLX1）在体外可大大增强CXE活性。在活的木贼结构组织中，CXE和MXE作用可能会增强茎，而扩展酶可能会增强HTG催化的CXE反应，从而使muro产生有效的CXE作用。我们的方法将有助于调查整个植物界的可比反应。此外，将类似的杂多聚体工程化到被子植物中可以改善某些农艺性状，例如抗倒伏性。