Compensatory Guaiacyl Lignin Biosynthesis at the Expense of Syringyl Lignin in <i>4CL1</i> -Knockout Poplar

Abstract

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 <i>4</i>-<i>coumarate</i>:<i>CoA ligases</i> (<i>4CLs</i>) in herbaceous species mainly reduces guaiacyl (G) lignin and enhances cell wall saccharification. Here we show that CRISPR-knockout of <i>4CL1</i> in poplar (<i>Populus tremula</i> × <i>alba</i>) preferentially reduced syringyl (S) lignin, with negligible effects on biomass recalcitrance. Concordant with reduced S-lignin was downregulation of <i>ferulate 5</i>-<i>hydroxylases</i> (<i>F5Hs</i>). 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 <i>caffeoyl shikimate esterase1</i> Upregulation of <i>caffeoyl</i>-<i>CoA</i> <i>O</i>-<i>methyltransferase1</i> and downregulation of <i>F5Hs</i> are consistent with preferential funneling of 4CL5 products toward G-lignin biosynthesis at the expense of S-lignin. Thus, transcriptional and metabolic adaptations to <i>4CL1</i>-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.