Division of Plant Sciences, College of Life Sciences, University of Dundee at the James Hutton Institute, Invergowrie, Dundee, United Kingdom.
Plant Cell. 2011 Dec;23(12):4492-506. doi: 10.1105/tpc.111.089037. Epub 2011 Dec 9.
The manipulation of lignin could, in principle, facilitate efficient biofuel production from plant biomass. Despite intensive study of the lignin pathway, uncertainty exists about the enzyme catalyzing the last step in syringyl (S) monolignol biosynthesis, the reduction of sinapaldehyde to sinapyl alcohol. Traditional schemes of the pathway suggested that both guaiacyl (G) and S monolignols are produced by a single substrate-versatile enzyme, cinnamyl alcohol dehydrogenase (CAD). This was challenged by the discovery of a novel sinapyl alcohol dehydrogenase (SAD) that preferentially uses sinapaldehyde as a substrate and that was claimed to regulate S lignin biosynthesis in angiosperms. Consequently, most pathway schemes now show SAD (or SAD and CAD) at the sinapaldehyde reduction step, although functional evidence is lacking. We cloned SAD from tobacco (Nicotiana tabacum) and suppressed it in transgenic plants using RNA interference-inducing vectors. Characterization of lignin in the woody stems shows no change to content, composition, or structure, and S lignin is normal. By contrast, plants additionally suppressed in CAD have changes to lignin structure and S:G ratio and have increased sinapaldehyde in lignin, similar to plants suppressed in CAD alone. These data demonstrate that CAD, not SAD, is the enzyme responsible for S lignin biosynthesis in woody angiosperm xylem.
木质素的操纵从理论上可以促进植物生物质的高效生物燃料生产。尽管对木质素途径进行了深入研究，但对愈创木基（S）单体木质素生物合成的最后一步（肉桂醛还原为丁香基醇）的酶催化仍存在不确定性。该途径的传统方案表明，均苯型（G）和 S 型单体木质素都是由一种单一的基质多功能酶——肉桂醇脱氢酶（CAD）产生的。这一观点受到了一种新型丁香基醇脱氢酶（SAD）的发现的挑战，该酶优先使用肉桂醛作为底物，并被认为在被子植物中调节 S 木质素生物合成。因此，尽管缺乏功能证据，但大多数途径方案现在都在肉桂醛还原步骤中显示 SAD（或 SAD 和 CAD），而 CAD 的功能证据缺失。我们从烟草（Nicotiana tabacum）中克隆了 SAD，并使用 RNA 干扰诱导载体在转基因植物中抑制它。木质素在木质茎中的特征表明，含量、组成或结构没有变化，S 木质素正常。相比之下，另外在 CAD 被抑制的植物中，木质素结构和 S：G 比值发生了变化，并且木质素中的肉桂醛增加，这与单独在 CAD 被抑制的植物相似。这些数据表明，CAD 而不是 SAD，是木质素生物合成在木本被子植物木质部中的酶。
