Research Institute for Sustainable Humanosphere, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, Japan.
School of Biological Sciences, The University of Hong Kong, Pokfulam, Hong Kong, China.
Sci Rep. 2019 Aug 12;9(1):11597. doi: 10.1038/s41598-019-47957-0.
Lignin is a phenylpropanoid polymer produced in the secondary cell walls of vascular plants. Although most eudicot and gymnosperm species generate lignins solely via polymerization of p-hydroxycinnamyl alcohols (monolignols), grasses additionally use a flavone, tricin, as a natural lignin monomer to generate tricin-incorporated lignin polymers in cell walls. We previously found that disruption of a rice 5-HYDROXYCONIFERALDEHYDE O-METHYLTRANSFERASE (OsCAldOMT1) reduced extractable tricin-type metabolites in rice vegetative tissues. This same enzyme has also been implicated in the biosynthesis of sinapyl alcohol, a monolignol that constitutes syringyl lignin polymer units. Here, we further demonstrate through in-depth cell wall structural analyses that OsCAldOMT1-deficient rice plants produce altered lignins largely depleted in both syringyl and tricin units. We also show that recombinant OsCAldOMT1 displayed comparable substrate specificities towards both 5-hydroxyconiferaldehyde and selgin intermediates in the monolignol and tricin biosynthetic pathways, respectively. These data establish OsCAldOMT1 as a bifunctional O-methyltransferase predominantly involved in the two parallel metabolic pathways both dedicated to the biosynthesis of tricin-lignins in rice cell walls. Given that cell wall digestibility was greatly enhanced in the OsCAldOMT1-deficient rice plants, genetic manipulation of CAldOMTs conserved in grasses may serve as a potent strategy to improve biorefinery applications of grass biomass.
木质素是一种苯丙烷类聚合物，在维管植物的次生细胞壁中产生。尽管大多数真双子叶植物和裸子植物物种仅通过对羟基肉桂醇（单体酚）的聚合来产生木质素，但禾本科植物还额外使用黄酮类 tricin 作为天然木质素单体，在细胞壁中生成包含 tricin 的木质素聚合物。我们之前发现，破坏水稻 5-羟松柏醛 O-甲基转移酶（OsCAldOMT1）会减少水稻营养组织中可提取的 tricin 型代谢物。同样的酶也与松柏醇的生物合成有关，松柏醇是构成愈创木基木质素聚合物单元的单体酚。在这里，我们通过深入的细胞壁结构分析进一步证明，OsCAldOMT1 缺陷型水稻植物产生的木质素在很大程度上缺乏愈创木基和 tricin 单元。我们还表明，重组 OsCAldOMT1 对木质素和 tricin 生物合成途径中的 5-羟松柏醛和 selgin 中间体分别表现出相似的底物特异性。这些数据确立了 OsCAldOMT1 作为一种双功能 O-甲基转移酶，主要参与两个平行的代谢途径，这两个途径都专门致力于水稻细胞壁中 tricin-木质素的生物合成。鉴于 OsCAldOMT1 缺陷型水稻植物的细胞壁消化性大大提高，对禾本科植物中保守的 CAldOMTs 的遗传操作可能成为提高草类生物质生物炼制应用的有效策略。
