A functional cascading of lignin modification via repression of caffeic acid O-methyltransferase for bioproduction and anti-oxidation in rice

Crop straws provide substantial biomass resources that are transformable for sustainable biofuels and valuable bioproducts. However, the natural lignocellulose recalcitrance results in an expensive biomass process and secondary waste liberation. As lignin is a major recalcitrant factor, genetic engineering of lignin biosynthesis is increasingly being implemented in bioenergy crops, but much remains unclear about the desired lignocellulose alteration and resulting function. This study attempted to explore the mechanisms of lignin modification responsible for efficient lignocellulose conversion in vitro and an effective plant anti-oxidation response in vivo. We initially selected specific rice mutants by performing modern CRISPR/cas9 editing with caffeic acid O-methyltransferase involved in the synthetic pathways of monolignols (G, S) and ferulic acid (FA), and then explored lignocellulose conversion and plant cadmium (Cd) accumulation using advanced chemical, biochemical and thermal-chemical analyses. Notable lignin modification was achieved from the predominately synergistic down-regulation of S-monomer synthesis in three mutants. This consequently upgraded lignocellulose porosity by up to 1.8 folds to account for significantly enhanced biomass saccharification and bioethanol production by 20 %-26 % relative to the wild-type. The modified lignin also favors the dissection of diverse lignin nanoparticles with dimensions reduced by 1.5-1.9 folds, applicable for thermal-chemical conversion into the carbon quantum dots with increased yields by 15 % and 31 %. The proportions of G-monomers and FA were significantly increased in the mutants, and the lignin extractions were further assayed with higher activities for two standard antioxidants (DPPH and ABTS) in vitro compared to the wild-type, revealing a distinctively enhanced plant antioxidative capacity in the mutants. Water culture showed that young mutant seedlings accumulated more Cd than wild-type did (p < 0.01, n = 3), suggesting effective heavy metal phytoremediation in the mutants. A hypothetical model of characteristic lignin modification for specific S-monomer reduction, accountable for improved lignocellulose recalcitrance, was proposed. It provides a powerful strategy for achieving high-yield biofuels and value-added bioproducts or enhancing plant antioxidative capacity for heavy metal phytoremediation.