Department of Botany, University of British Columbia, Vancouver, BC, Canada.
Ann Bot. 2018 May 11;121(6):1107-1125. doi: 10.1093/aob/mcy005.
Secondary cell walls (SCWs) form the architecture of terrestrial plant biomass. They reinforce tracheary elements and strengthen fibres to permit upright growth and the formation of forest canopies. The cells that synthesize a strong, thick SCW around their protoplast must undergo a dramatic commitment to cellulose, hemicellulose and lignin production.
This review puts SCW biosynthesis in a cellular context, with the aim of integrating molecular biology and biochemistry with plant cell biology. While SCWs are deposited in diverse tissue and cellular contexts including in sclerenchyma (fibres and sclereids), phloem (fibres) and xylem (tracheids, fibres and vessels), the focus of this review reflects the fact that protoxylem tracheary elements have proven to be the most amenable experimental system in which to study the cell biology of SCWs.
SCW biosynthesis requires the co-ordination of plasma membrane cellulose synthases, hemicellulose production in the Golgi and lignin polymer deposition in the apoplast. At the plasma membrane where the SCW is deposited under the guidance of cortical microtubules, there is a high density of SCW cellulose synthase complexes producing cellulose microfibrils consisting of 18-24 glucan chains. These microfibrils are extruded into a cell wall matrix rich in SCW-specific hemicelluloses, typically xylan and mannan. The biosynthesis of eudicot SCW glucuronoxylan is taken as an example to illustrate the emerging importance of protein-protein complexes in the Golgi. From the trans-Golgi, trafficking of vesicles carrying hemicelluloses, cellulose synthases and oxidative enzymes is crucial for exocytosis of SCW components at the microtubule-rich cell membrane domains, producing characteristic SCW patterns. The final step of SCW biosynthesis is lignification, with monolignols secreted by the lignifying cell and, in some cases, by neighbouring cells as well. Oxidative enzymes such as laccases and peroxidases, embedded in the polysaccharide cell wall matrix, determine where lignin is deposited.
次生细胞壁（SCWs）构成了陆地植物生物质的结构。它们强化了导管元件并增强了纤维的强度，从而允许直立生长和森林树冠的形成。合成围绕原生质体的强而厚的 SCW 的细胞必须经历对纤维素、半纤维素和木质素生产的巨大承诺。
本文将 SCW 生物合成置于细胞环境中，旨在将分子生物学和生物化学与植物细胞生物学相结合。虽然 SCWs 沉积在不同的组织和细胞环境中，包括厚壁组织（纤维和石细胞）、韧皮部（纤维）和木质部（导管、纤维和导管），但本综述的重点反映了这样一个事实，即原木质部导管元件已被证明是研究 SCW 细胞生物学的最适合的实验系统。
SCW 生物合成需要质膜纤维素合酶、高尔基体中的半纤维素生产和质外体中的木质素聚合物沉积的协调。在 SCW 在皮质微管指导下沉积的质膜处，有高密度的 SCW 纤维素合酶复合物产生由 18-24 个葡聚糖链组成的纤维素微纤维。这些微纤维被挤出到富含 SCW 特异性半纤维素的细胞壁基质中，通常是木聚糖和甘露聚糖。以双子叶植物 SCW 葡萄糖醛酸木聚糖的生物合成为例，说明了蛋白质-蛋白质复合物在高尔基体中的重要性日益增加。从反式高尔基体，携带半纤维素、纤维素合酶和氧化酶的小泡的运输对于 SCW 成分在富含微管的质膜域的胞吐作用至关重要，从而产生特征性的 SCW 模式。SCW 生物合成的最后一步是木质化，木质素单体由木质化细胞分泌，在某些情况下，也由相邻细胞分泌。氧化酶，如漆酶和过氧化物酶，嵌入多糖细胞壁基质中，决定木质素的沉积位置。
