Rapin Marie N, Bothwell John H, Fry Stephen C
The Edinburgh Cell Wall Group, Institute of Molecular Plant Sciences, The University of Edinburgh, Edinburgh EH9 3BF, UK.
Department of Biosciences, Durham University, Durham DH1 3LE, UK.
Ann Bot. 2024 Dec 31;134(7):1191-1206. doi: 10.1093/aob/mcae154.
The cell walls of charophytic algae both resemble and differ from those of land plants. Cell walls in early-diverging charophytes (e.g. Klebsormidiophyceae) are particularly distinctive in ways that might enable survival in environments that are incompatible with land-plant polymers. This study therefore investigates the structure of Klebsormidium polysaccharides.
The 'pectin' fraction (defined by extractability) of Klebsormidium fluitans, solubilized by various buffers from alcohol-insoluble residues, was digested with several treatments that (partly) hydrolyse land-plant cell-wall polysaccharides. Products were analysed by gel-permeation and thin-layer chromatography.
The Klebsormidium pectic fraction made up 30-50 % of its alcohol-insoluble residue, was optimally solubilized at pH 3-4 at 100 °C, and contained residues of xylose ≈ galactose > rhamnose > arabinose, fucose, mannose and glucose. Uronic acids were undetectable, and the pectic fraction was more readily solubilized by formate than by oxalate, suggesting a lack of chelation. Some land-plant-targeting hydrolases degraded the Klebsormidium pectic fraction: digestion by α-l-arabinanase, endo-β-(1→4)-d-xylanase and α-d-galactosidase suggests the presence of β-(1→4)-xylan with terminal α-l-arabinose, α-d-galactose and (unexpectedly) rhamnose. 'Driselase' released oligosaccharides of xylose and rhamnose (1:1), and graded acid hydrolysis of these oligosaccharides indicated a 'rhamnoxylan' with rhamnose side-chains. Partial acid hydrolysis of Klebsormidium pectic fraction released rhamnose plus numerous oligosaccharides, one of which comprised xylose and galactose (~1:2 Gal/Xyl), suggesting a galactoxylan. Lichenase was ineffective, as were endo-β-(1→4)-d-galactanase, endo-β-(1→4)-d-mannanase, β-d-xylosidase and β-d-galactosidase.
Klebsormidium pectic fraction possesses many land-plant-like linkages but is unusual in lacking uronic acid residues and in containing rhamnoxylan and galactoxylan domains. Uronic acids allow land-plant and late-diverging charophyte pectins to form Ca2+-bridges, facilitating cell-wall polymer association; their absence from Klebsormidium suggests that neutral heteroxylans rely on alternative cross-linking mechanisms. This lack of dependence on Ca2+-bridges might confer on Klebsormidium the ability to grow in the acidic, metal-rich environments that it tolerates.
轮藻细胞壁与陆地植物细胞壁既有相似之处，也存在差异。早期分化的轮藻（如鞘毛藻科）的细胞壁具有独特之处，这可能使其能够在与陆地植物聚合物不相容的环境中生存。因此，本研究对鞘毛藻多糖的结构进行了研究。
用多种缓冲液从乙醇不溶性残渣中溶解出漂浮鞘毛藻的“果胶”部分（由可提取性定义），并用几种（部分）水解陆地植物细胞壁多糖的处理方法对其进行消化。通过凝胶渗透色谱和薄层色谱对产物进行分析。
鞘毛藻的果胶部分占其乙醇不溶性残渣的约30 - 50%，在100℃、pH 3 - 4条件下溶解度最佳，含有木糖≈半乳糖>鼠李糖>阿拉伯糖、岩藻糖、甘露糖和葡萄糖残基。未检测到糖醛酸，且果胶部分在甲酸盐中的溶解度比草酸盐中更高，这表明缺乏螯合作用。一些针对陆地植物的水解酶能降解鞘毛藻的果胶部分：用α - l -阿拉伯聚糖酶、内切 - β - (1→4) - d -木聚糖酶和α - d -半乳糖苷酶消化表明存在带有末端α - l -阿拉伯糖、α - d -半乳糖和（出乎意料的）鼠李糖的β - (1→4) -木聚糖。“蜗牛酶”释放出木糖和鼠李糖的寡糖（约1:1），对这些寡糖进行分级酸水解表明存在带有鼠李糖侧链的“鼠李糖木聚糖”。鞘毛藻果胶部分的部分酸水解释放出鼠李糖以及大量寡糖，其中一种由木糖和半乳糖组成（约1:2半乳糖/木糖），表明存在半乳木聚糖。地衣酶无效，内切 - β - (1→4) - d -半乳聚糖酶、内切 - β - (1→4) - d -甘露聚糖酶、β - d -木糖苷酶和β - d -半乳糖苷酶也无效。
鞘毛藻果胶部分具有许多与陆地植物相似的连接方式，但不同寻常的是缺乏糖醛酸残基，且含有鼠李糖木聚糖和半乳木聚糖结构域。糖醛酸使陆地植物和晚期分化的轮藻果胶能够形成Ca²⁺桥，促进细胞壁聚合物的缔合；鞘毛藻中缺乏糖醛酸表明中性杂木聚糖依赖于其他交联机制。这种对Ca²⁺桥的不依赖可能赋予鞘毛藻在其耐受的酸性、富含金属的环境中生长的能力。
