Research Institute for Sustainable Humanosphere, Kyoto University, Kyoto 611-0011, Japan.
School of Biological Sciences, The University of Hong Kong, Pokfulam, Hong Kong, China.
Plant Physiol. 2022 Mar 28;188(4):1993-2011. doi: 10.1093/plphys/kiab606.
Lignin is a complex phenylpropanoid polymer deposited in the secondary cell walls of vascular plants. Unlike most gymnosperm and eudicot lignins that are generated via the polymerization of monolignols, grass lignins additionally incorporate the flavonoid tricin as a natural lignin monomer. The biosynthesis and functions of tricin-integrated lignin (tricin-lignin) in grass cell walls and its effects on the utility of grass biomass remain largely unknown. We herein report a comparative analysis of rice (Oryza sativa) mutants deficient in the early flavonoid biosynthetic genes encoding CHALCONE SYNTHASE (CHS), CHALCONE ISOMERASE (CHI), and CHI-LIKE (CHIL), with an emphasis on the analyses of disrupted tricin-lignin formation and the concurrent changes in lignin profiles and cell wall digestibility. All examined CHS-, CHI-, and CHIL-deficient rice mutants were largely depleted of extractable flavones, including tricin, and nearly devoid of tricin-lignin in the cell walls, supporting the crucial roles of CHS and CHI as committed enzymes and CHIL as a noncatalytic enhancer in the conserved biosynthetic pathway leading to flavone and tricin-lignin formation. In-depth cell wall structural analyses further indicated that lignin content and composition, including the monolignol-derived units, were differentially altered in the mutants. However, regardless of the extent of the lignin alterations, cell wall saccharification efficiencies of all tested rice mutants were similar to that of the wild-type controls. Together with earlier studies on other tricin-depleted grass mutant and transgenic plants, our results reflect the complexity in the metabolic consequences of tricin pathway perturbations and the relationships between lignin profiles and cell wall properties.
木质素是一种复杂的苯丙烷类聚合物，沉积在维管植物的次生细胞壁中。与大多数通过单体醇聚合生成的裸子植物和真双子叶植物木质素不同，草类木质素还将类黄酮三嗪作为一种天然木质素单体纳入其中。草细胞壁中整合三嗪木质素（tricln-lignin）的生物合成和功能及其对草类生物质利用的影响在很大程度上仍然未知。本文报道了对水稻（Oryza sativa）突变体中早期类黄酮生物合成基因（编码查尔酮合酶（CHS）、查尔酮异构酶（CHI）和 CHI 样（CHIL）的比较分析，重点分析了破坏的 tricin-lignin 形成以及木质素谱和细胞壁可消化性的变化。所有被检查的 CHS、CHI 和 CHIL 缺陷型水稻突变体都大量缺乏可提取的类黄酮，包括三嗪，并且细胞壁中几乎没有 tricin-lignin，这支持了 CHS 和 CHI 作为关键酶和 CHIL 作为保守生物合成途径中非催化增强剂在类黄酮和 tricin-lignin 形成中的作用。深入的细胞壁结构分析进一步表明，木质素含量和组成，包括单体醇衍生单元，在突变体中发生了差异变化。然而，无论木质素变化的程度如何，所有测试的水稻突变体的细胞壁糖化效率都与野生型对照相似。结合其他三嗪耗尽草突变体和转基因植物的早期研究，我们的结果反映了三嗪途径扰动的代谢后果的复杂性以及木质素谱和细胞壁特性之间的关系。
