Plant Biology Division, Samuel Roberts Noble Foundation, Ardmore, OK 73401, USA.
Proc Natl Acad Sci U S A. 2012 Jan 31;109(5):1772-7. doi: 10.1073/pnas.1120992109. Epub 2012 Jan 17.
Lignins are complex phenylpropanoid polymers mostly associated with plant secondary cell walls. Lignins arise primarily via oxidative polymerization of the three monolignols, p-coumaryl, coniferyl, and sinapyl alcohols. Of the two hydroxycinnamyl alcohols that represent incompletely methylated biosynthetic products (and are not usually considered to be monolignols), 5-hydroxyconiferyl alcohol is now well established as incorporating into angiosperm lignins, but incorporation of caffeyl alcohol has not been shown. We report here the presence of a homopolymer of caffeyl alcohol in the seed coats of both monocot and dicot plants. This polymer (C-lignin) is deposited to high concentrations in the seed coat during the early stages of seed development in the vanilla orchid (Vanilla planifolia), and in several members of the Cactaceae. The lignin in other parts of the Vanilla plant is conventionally biosynthesized from coniferyl and sinapyl alcohols. Some species of cacti contain only C-lignin in their seeds, whereas others contain only classical guaiacyl/syringyl lignin (derived from coniferyl and sinapyl alcohols). NMR spectroscopic analysis revealed that the Vanilla seed-coat polymer was massively comprised of benzodioxane units and was structurally similar to the polymer synthesized in vitro by peroxidase-catalyzed polymerization of caffeyl alcohol. CD spectroscopy did not detect any optical activity in the seed polymer. These data support the contention that the C-lignin polymer is produced in vivo via combinatorial oxidative radical coupling that is under simple chemical control, a mechanism analogous to that theorized for classical lignin biosynthesis.
木质素是复杂的苯丙烷聚合物，主要与植物次生细胞壁有关。木质素主要通过三种木质素单体，对香豆醇、松柏醇和芥子醇的氧化聚合产生。在两种羟基肉桂醇中，有两种不完全甲基化的生物合成产物（通常不被认为是木质素单体），5-羟基松柏醇现已被证明可以掺入被子植物木质素中，但咖啡醇的掺入尚未得到证实。我们在这里报告了在单子叶植物和双子叶植物的种皮中存在咖啡醇均聚物。这种聚合物（C-木质素）在香草兰（Vanilla planifolia）和仙人掌科的几个成员的种子发育早期被高度沉积在种皮中。香草植物其他部分的木质素通常是由松柏醇和芥子醇生物合成的。一些仙人掌物种的种子中只含有 C-木质素，而另一些则只含有经典的愈创木基/丁香基木质素（由松柏醇和芥子醇衍生而来）。NMR 光谱分析表明，香草种皮聚合物主要由苯并二恶烷单元组成，结构与过氧化物酶催化聚合咖啡醇体外合成的聚合物相似。CD 光谱在种子聚合物中未检测到任何光学活性。这些数据支持了这样一种观点，即 C-木质素聚合物是通过组合氧化自由基偶联在体内产生的，这种偶联受简单的化学控制，类似于经典木质素生物合成理论化的机制。
