The lignin biosynthetic pathway is highly conserved in angiosperms, yet pathway manipulations give rise to a variety of taxon-specific outcomes. Knockout of lignin-associated 4-coumarate:CoA ligases (4CLs) in herbaceous species mainly reduces guaiacyl (G) lignin and enhances cell wall saccharification. Here we show that CRISPR-knockout of 4CL1 in poplar (Populus tremula × alba) preferentially reduced syringyl (S) lignin, with negligible effects on biomass recalcitrance. Concordant with reduced S-lignin was downregulation of ferulate 5-hydroxylases (F5Hs). Lignification was largely sustained by 4CL5, a low-affinity paralog of 4CL1 typically with only minor xylem expression or activity. Levels of caffeate, the preferred substrate of 4CL5, increased in line with significant upregulation of caffeoyl shikimate esterase1 Upregulation of caffeoyl-CoA O-methyltransferase1 and downregulation of F5Hs are consistent with preferential funneling of 4CL5 products toward G-lignin biosynthesis at the expense of S-lignin. Thus, transcriptional and metabolic adaptations to 4CL1-knockout appear to have enabled 4CL5 catalysis at a level sufficient to sustain lignification. Finally, genes involved in sulfur assimilation, the glutathione-ascorbate cycle, and various antioxidant systems were upregulated in the mutants, suggesting cascading responses to perturbed thioesterification in lignin biosynthesis.

中文翻译：

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以4CL1敲除杨树中的丁香基木质素为代价的补偿性瓜酰基酰基木质素生物合成。

木质素的生物合成途径在被子植物中是高度保守的，但是途径的操纵却产生了各种分类单元特异性的结果。木质素相关的4-香豆酸酯：CoA连接酶（4CLs）的敲除在草本物种中主要减少了愈创木脂（G）木质素并增强了细胞壁的糖化作用。在这里，我们显示了在白杨（杨杨×白杨）中4CL1的CRISPR敲除优先降低了丁香基（S）木质素，对生物量顽固性的影响可忽略不计。与降低的S-木质素一致的是下调阿魏酸5-羟化酶（F5Hs）。木质化在很大程度上由4CL5维持，4CL5是低亲和力的4CL1旁系同源物，通常只有少量木质部表达或活性。咖啡因水平（4CL5的首选底物）增加与咖啡酰yl草酸酯酯酶1的显着上调相一致。咖啡酰-CoA O-甲基转移酶1的上调和F5Hs的下调与4CL5产物优先向G-木质素生物合成漏斗相符，而以S-木质素为代价。因此，转录和代谢适应4CL1基因敲除似乎已使4CL5催化水平足以维持木质化。最后，与硫同化，谷胱甘肽-抗坏血酸循环和各种抗氧化剂系统有关的基因在突变体中被上调，表明木质素生物合成中对扰动的硫酯化的级联反应。转录和代谢适应4CL1基因敲除似乎已使4CL5催化水平足以维持木质化。最后，与硫同化，谷胱甘肽-抗坏血酸循环和各种抗氧化剂系统有关的基因在突变体中被上调，表明木质素生物合成中对扰动的硫酯化的级联反应。转录和代谢适应4CL1基因敲除似乎已使4CL5催化水平足以维持木质化。最后，与硫同化，谷胱甘肽-抗坏血酸循环和各种抗氧化剂系统有关的基因在突变体中被上调，表明木质素生物合成中对扰动的硫酯化的级联反应。