PT  - JOURNAL ARTICLE
AU  - Chung-Jui Tsai
AU  - Peng Xu
AU  - Liang-Jiao Xue
AU  - Hao Hu
AU  - Batbayar Nyamdari
AU  - Radnaa Naran
AU  - Xiaohong Zhou
AU  - Geert Goeminne
AU  - Ruili Gao
AU  - Erica Gjersing
AU  - Joseph Dahlen
AU  - Sivakumar Pattathil
AU  - Michael G. Hahn
AU  - Mark F. Davis
AU  - John Ralph
AU  - Wout Boerjan
AU  - Scott A. Harding
TI  - Compensatory guaiacyl lignin biosynthesis at the expense of syringyl lignin in <em>4CL1</em>-knockout poplar
AID  - 10.1101/2019.12.20.885350
DP  - 2019 Jan 01
TA  - bioRxiv
PG  - 2019.12.20.885350
4099  - http://biorxiv.org/content/early/2019/12/20/2019.12.20.885350.short
4100  - http://biorxiv.org/content/early/2019/12/20/2019.12.20.885350.full
AB  - 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 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.One sentence summary Knockout of lignin-associated 4CL1 in Populus reveals a 4CL5-dependent, caffeate-modulated compensatory pathway for lignification with links to thiol redox balance and sulfur assimilation.