Great Lakes Bioenergy Research Center, Michigan State University, East Lansing, MI, 4882, USA.
Department of Plant Biology, Michigan State University, East Lansing, MI, 48824, USA.
Plant J. 2018 Mar;93(6):1062-1075. doi: 10.1111/tpj.13830. Epub 2018 Mar 6.
Mixed-linkage (1,3;1,4)-β-glucan (MLG) is a glucose polymer with beneficial effects on human health and high potential for the agricultural industry. MLG is present predominantly in the cell wall of grasses and is synthesized by cellulose synthase-like F or H families of proteins, with CSLF6 being the best-characterized MLG synthase. Although the function of this enzyme in MLG production has been established, the site of MLG synthesis in the cell is debated. It has been proposed that MLG is synthesized at the plasma membrane, as occurs for cellulose and callose; in contrast, it has also been proposed that MLG is synthesized in the Golgi apparatus, as occurs for other matrix polysaccharides of the cell wall. Testing these conflicting possibilities is fundamentally important in the general understanding of the biosynthesis of the plant cell wall. Using immuno-localization analyses with MLG-specific antibody in Brachypodium and in barley, we found MLG present in the Golgi, in post-Golgi structures and in the cell wall. Accordingly, analyses of a functional fluorescent protein fusion of CSLF6 stably expressed in Brachypodium demonstrated that the enzyme is localized in the Golgi. We also established that overproduction of MLG causes developmental and growth defects in Brachypodium as also occur in barley. Our results indicated that MLG production occurs in the Golgi similarly to other cell wall matrix polysaccharides, and supports the broadly applicable model in grasses that tight mechanisms control optimal MLG accumulation in the cell wall during development and growth. This work addresses the fundamental question of where mixed linkage (1,3;1,4)-β-glucan (MLG) is synthesized in plant cells. By analyzing the subcellular localization of MLG and MLG synthase in an endogenous system, we demonstrated that MLG synthesis occurs at the Golgi in Brachypodium and barley. A growth inhibition due to overproduced MLG in Brachypodium supports the general applicability of the model that a tight control of the cell wall polysaccharides accumulation is needed to maintain growth homeostasis during development.
杂合（1,3;1,4）-β-葡聚糖（MLG）是一种对人体健康有益且对农业具有巨大潜力的葡萄糖聚合物。MLG 主要存在于草类植物的细胞壁中，由纤维素合酶样 F 或 H 家族蛋白合成，其中 CSLF6 是研究最为透彻的 MLG 合酶。虽然该酶在 MLG 合成中的功能已经得到确立，但 MLG 在细胞中的合成部位仍存在争议。有人提出，MLG 像纤维素和几丁质一样，在质膜上合成；而另一些人则提出，MLG 像细胞壁中的其他基质多糖一样，在高尔基体中合成。在普遍理解植物细胞壁生物合成的过程中，检验这些相互矛盾的可能性是至关重要的。我们利用 MLG 特异性抗体在拟南芥和大麦中的免疫定位分析发现，MLG 存在于高尔基体、高尔基体后结构和细胞壁中。因此，对在拟南芥中稳定表达的 CSLF6 功能荧光蛋白融合体的分析表明，该酶定位于高尔基体。我们还发现，MLG 的过量生产会导致拟南芥出现发育和生长缺陷，这与大麦中发生的情况相同。我们的研究结果表明，MLG 的合成与其他细胞壁基质多糖一样，发生在高尔基体中，这支持了禾本科植物中广泛适用的模型，即通过严格的机制控制细胞壁中 MLG 的最佳积累，以维持发育和生长过程中的细胞生长。这项工作解决了 MLG 在植物细胞中何处合成的基本问题。通过在一个内源性系统中分析 MLG 和 MLG 合酶的亚细胞定位，我们证明了 MLG 在拟南芥和大麦中的高尔基体中合成。在拟南芥中，由于 MLG 的过量生产导致生长受到抑制，这支持了这样一个普遍适用的模型，即需要严格控制细胞壁多糖的积累，以维持发育过程中的细胞生长平衡。
