Vessel-Specific Reintroduction of CINNAMOYL-COA REDUCTASE1 (CCR1) in Dwarfed <i>ccr1</i> Mutants Restores Vessel and Xylary Fiber Integrity and Increases Biomass

Abstract

Lignocellulosic biomass is recalcitrant toward deconstruction into simple sugars due to the presence of lignin. To render lignocellulosic biomass a suitable feedstock for the bio-based economy, plants can be engineered to have decreased amounts of lignin. However, engineered plants with the lowest amounts of lignin exhibit collapsed vessels and yield penalties. Previous efforts were not able to fully overcome this phenotype without settling in sugar yield upon saccharification. Here, we reintroduced <i>CINNAMOYL-COENZYME A REDUCTASE1</i> (<i>CCR1</i>) expression specifically in the protoxylem and metaxylem vessel cells of Arabidopsis (<i>Arabidopsis thaliana</i>) <i>ccr1</i> mutants. The resulting <i>ccr1 ProSNBE</i>:<i>CCR1</i> lines had overcome the vascular collapse and had a total stem biomass yield that was increased up to 59% as compared with the wild type. Raman analysis showed that monolignols synthesized in the vessels also contribute to the lignification of neighboring xylary fibers. The cell wall composition and metabolome of <i>ccr1 ProSNBE</i>:<i>CCR1</i> still exhibited many similarities to those of <i>ccr1</i> mutants, regardless of their yield increase. In contrast to a recent report, the yield penalty of <i>ccr1</i> mutants was not caused by ferulic acid accumulation but was (largely) the consequence of collapsed vessels. Finally, <i>ccr1 ProSNBE</i>:<i>CCR1</i> plants had a 4-fold increase in total sugar yield when compared with wild-type plants.